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

The stunning substructures of gaps and rings revealed in protoplanetary disks have been attributed to the motions of hidden, newly formed planets. But are we interpreting our observations correctly?

Models of Structure

planet formation

This simulation shows a Jupiter-mass planet forming inside a circumstellar disk. [Frédéric Masset]

When the Atacama Large Millimeter/submillimeter Array (ALMA) came online, one of its first released images was that of HL Tau, a young star surrounded by a protoplanetary disk — a disk that’s structured with a dramatic series of concentric gaps and rings. Since this early image, ALMA has continued to amass observations of disk structure in the inner tens of AU around young stars — and theorists are now left to decide what to make of these.

Multiple explanations for the origin of these structures have been proposed, including snowlines, flows driven by magnetic fields, gravitational instabilities, or dust trapping. But the most popular model suggests that the gaps are driven by the motions of young, invisible planets embedded in the disks.

Challenging Assumptions

Recent studies have suggested that multiple gaps and rings can actually be produced by a single embedded planet. Simulations show that as a planet moves through the disk, it excites multiple spiral density waves. Interactions of these waves with the disk can then carve out several narrow gaps.

But while the basic idea behind these simulations seems sound, two scientists from the Institute for Advanced Study, Ryan Miranda and Roman Rafikov (also of University of Cambridge, U.K.) suggest we need to be a little more careful in how we interpret them.

comparison simulations of planet-disk interactions

Three of the authors’ simulations comparing locally isothermal (left panel of each pair) and non-isothermal disks (right panel of each pair). For low-mass planets (top two pairs), locally isothermal simulations overestimate the contrast of structures. For high-mass planets (bottom pair), locally isothermal simulations also misrepresent the locations of rings and gaps. [Adapted from Miranda & Rafikov 2019]

Because full simulations of disk–planet interactions are computationally inhibitive, numericists make simplifying assumptions in their models. One commonly adopted simplification is to assume that the disk is locally isothermal, i.e., it has a fixed temperature profile. But while this assumption holds in the outer regions of the disk where cooling is efficient, Miranda and Rafikov point out that this isn’t a good model for the poorly cooled inner tens of AU where we observe these ring and gap structures.

Massive Interpretations

What quirks does this assumption introduce? By running a series of comparison simulations of a planet interacting with a locally isothermal and a non-isothermal disk, Miranda and Rafikov show that locally isothermal simulations tend to overestimate the contrast of ring and gap structures produced. This means that using isothermal models to interpret ALMA results would cause us to underestimate the masses of the planets causing the disk structure observed.

What’s more, the authors find that for large planets, the isothermal simulations also misrepresent the locations of the rings. The results in this article suggest a strong need for caution when using locally isothermal simulations to explore the interactions between planets and disks. We’re certainly getting closer to understanding the many complexities of planet formation, but we’ve still got plenty of work to do!

Citation

“On the Planetary Interpretation of Multiple Gaps and Rings in Protoplanetary Disks Seen By ALMA,” Ryan Miranda and Roman R. Rafikov 2019 ApJL 878 L9. doi:10.3847/2041-8213/ab22a7

collapsar

How do we get the heavy elements — elements with atomic mass above iron, like gold, platinum, or uranium — in our universe? A new study suggests that one theorized source, collapsing massive stars, may not be the best option.

Enriching the Universe

The Big Bang produced a universe filled almost exclusively with hydrogen and helium; almost all of the heavier elements in our universe have formed since that time. How and when they formed, however, are still questions we’re working to solve.

element origins

Periodic table showing the origin of each chemical element. Those produced by the r-process are shaded orange and attributed to supernovae in this image; though supernovae are one proposed source of r-process elements, collapsars have been proposed as another. [Cmglee]

We know that the dense, hot cores of stars fuse atoms, producing elements up to iron in mass. But we need more extreme conditions for r-process nucleosynthesis — a set of rapid neutron-capture reactions that we think are responsible for producing about half the atomic nuclei heavier than iron.

Recent research has renewed interest in one potential source of r-process elements: collapsars. Collapsars are massive (>30 solar masses), rapidly rotating stars that suffer catastrophic core-collapses into black holes. In this sudden process, a spinning disk of material accretes onto the core — and conditions in the disk are just right for the r-process. But could collapsars really account for much of the r-process elements in our universe?

Clues from Collapse

abundance ratios

Abundance ratios found by the authors in a sample of low-metallicity stars. The top plot shows good agreement between the collapsar model (red) and observations (black) for the ratio of Mg (not an r-process element) to Fe. But r-process elements Ba, Eu, and Sr show much higher abundances in collapsar models than in the observations. [Macias & Ramirez-Ruiz 2019]

Collapsar r-process nucleosynthesis should leave a visible imprint on the surrounding environment, UC Santa Cruz scientists Phillip Macias and Enrico Ramirez-Ruiz (also University of Copenhagen, Denmark) point out.

In the collapsar model, r-process elements produced in the accreting disk are flung out into the star’s surroundings via disk winds. But collapsars don’t only produce r-process elements — they also create lighter elements like iron, which are spewed from the collapsars via jets. These elements should all then mix, producing a soup of enriched material with a particular ratio of abundances — which will then seed the next generation of stars.

Macias and Ramirez-Ruiz look for signs of this soup imprinted on a sample of 186 very low-metallicity stars that haven’t already been polluted by many additional generations of star formation. If collapsars are the source of most of the r-process material in the universe, then these unpolluted canvases should show the same ratio of r-process elements to iron as the authors calculate from collapsar models.

A Mismatch with the Evidence

Macias and Ramirez-Ruiz find that their stellar sample’s abundance ratios do not match those predicted by the collapsar model — the relative amount of r-process elements would need to be much higher in the observed stars for collapsars to be a good explanation.

Instead, the authors argue that the majority of r-process nucleosynthesis must occur in sources that don’t simultaneously produce iron. One possible source that satisfies this condition is neutron-star mergers, like that observed in the recent gravitational-wave event GW170817. There are challenges to this model as well — but we can hope that future observations will help us to better understand where our universe’s heavy elements come from.

Citation

“Constraining Collapsar r-process Models through Stellar Abundances,” Phillip Macias and Enrico Ramirez-Ruiz 2019 ApJL 877 L24. doi:10.3847/2041-8213/ab2049

gamma-ray bursts

The merger of two compact objects — neutron stars or black holes — could be accompanied by a sudden, immediate flash of radio emission, according to predictions. Can an array of antennas in California help us spot these signals?

neutron-star merger

Artist’s impression of two merging neutron stars producing a gamma-ray burst. [National Science Foundation/LIGO/Sonoma State University/A. Simonnet]

Missing Signal

In recent years, the Laser Interferometer Gravitational-Wave Observatory (LIGO) has opened up a new world with the first detections of gravitational waves from compact-object mergers. In 2017, LIGO announced the detection of a pair of neutron stars colliding — GW170817 — and astronomers around the world rushed to watch the fireworks emitted across the electromagnetic spectrum.

In the time that followed, we saw gamma rays from a short-duration burst emitted during the merger itself; an optical/near-infrared kilonova, powered by the radioactive decay of heavy elements formed in the merger; and a long-lived radio, X-ray, and optical afterglow, caused when ejecta from the merger slammed into the surrounding environment and decelerated. One signal was missing, however: prompt radio emission. 

A Prompt Challenge

OVRO-LWA

A nighttime photo of some of the antennas in the Owens Valley Radio Observatory Long Wavelength Array. [Gregg Hallinan]

According to models, the merger of two neutron stars should produce an immediate flash of radio emission. Unlike belated afterglow radio emission, prompt radio emission should emerge immediately after the merger; models predict that it should arrive within as little as a minute of the gravitational-wave signal.

Catching this signal could provide us with information about the magnetic environment immediately around the binary, the properties of the intergalactic medium, and more. But it’s a tricky prospect — there’s very little lead time between the gravitational-wave detection and the radio signal, and the gravitational-wave source is poorly localized. How can we know where to point, quickly enough to capture the prompt radio emission from a merger?

GW170104 localization

The localization of GW170104 is shown in blue, plotted over the OVRA-LWA’s field of view at the time. The gray area was outside of the array’s field of view. [Callister et al. 2019]

Radio Eyes on the Sky

One approach: all-sky radio monitoring, like that provided by the Owens Valley Radio Observatory Long Wavelength Array (OVRO-LWA). This array of 288 antennas in California continuously scans the sky overhead, taking 13-s integrations that it stores for 24 hours.

This broad coverage and data storage means that even if the alert for a gravitational-wave detection comes hours later, scientists still have access to relevant OVRO-LWA data. We just have to get lucky to have the source in the field of view of the antennas — something that didn’t work out for GW170817, but likely will eventually!

A Successful Nondetection

OVRO-LWA view

The OVRO-LWA view of the radio sky (from 27 to 84 MHz) at the time of GW170104. The contours show the localization of the gravitational-wave source. [Callister et al. 2019]

As a proof of concept, a team of scientists led by Thomas Callister (California Institute of Technology) recently used the OVRO-LWA to perform a search for prompt radio emission from GW170104, a gravitational-wave event for which the majority of the localization region was within the array’s field of view.

Callister and collaborators didn’t spot a signal — but this was expected, since GW170104 was a binary black-hole merger, making it unlikely to produce electromagnetic radiation. The team did find, however, that they were able to place useful constraints on the amount of radio emission the merger could have produced, demonstrating the power of the OVRO-LWA for future observations.

The OVRO-LWA is clearly up to the task of detecting prompt emission from upcoming neutron-star mergers. Now we’ll just have to wait and see if it spots something in the third LIGO observing run, currently underway!

Citation

“A First Search for Prompt Radio Emission from a Gravitational-wave Event,” Thomas A. Callister et al 2019 ApJL 877 L39. doi:10.3847/2041-8213/ab2248

exomoon candidate Kepler-1625b-i

Last October, the first discovery of a potential exomoon was announced. But is Kepler-1625b-i an actual moon in another solar system? Or just an artifact of data reduction?

A Tricky Business

exomoon

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

Moons are a useful diagnostic — they can provide all kinds of information about their host planets, like clues to formation history, evolution, and even whether the planet might be habitable. What’s more, exomoons themselves have been indicated as potential targets in the search for life: while a habitable-zone gas-giant planet might not be an ideal host, for example, such a planet could have moons that are.

Given all we stand to learn from exomoons, it’d be great to find some! But for all that our solar system is chock full of moons (at last count, Jupiter alone hosts 79!), we’ve yet to find any sign of exomoons orbiting planets beyond the solar system. 

Jupiter moons

A montage of Jupiter and its four largest moons. [NASA/JPL]

This may well be because exomoon signals are difficult to spot. Not only would an exomoon’s signal be tiny compared to that of its host planet, but we also would need to separate that signal from the host’s — a tricky business. Throw in some instrument systematics to obscure all the data, and exomoon identification becomes even more of a challenge.

For these reasons, it was a pretty exciting announcement last fall when Columbia University astronomers Alex Teachey and David Kipping presented Kepler-1625b-i, a signal that they argued represented an exomoon around the gas giant Kepler-1625b. But a healthy dose of scientific caution has sent other teams scrambling to explore these data and draw their own conclusions — and one of these groups is calling the exomoon discovery into question.

Waiting for Consensus

Kepler 1625 light curve models

Best-fit models for the Kepler 1625 light curve assuming a planet and no moon (top) or moon (bottom). Data as analyzed by Kreidberg et al. are on the left (blue); data as analyzed by Teachey&Kipping are on the right (red). Kreidberg et al. find that the best fit is given by the no-moon model. Click to enlarge. [Kreidberg et al. 2019]

Led by Laura Kreidberg (Harvard-Smithsonian Center for Astrophysics), a team of astronomers has independently analyzed the same Hubble transit data that Teachey and Kipping used to identify their exomoon candidate. Unlike the other group, however, Kreidberg and collaborators found that the data are best fit by a simple planet transit model — the presence of an exomoon isn’t necessary or indicated.

According to Kreidberg and collaborators, the discrepancy between their results and Teachey and Kipping’s is most likely due to differences in data reduction. Teachey and Kipping have responded to this work with additional analysis in a recent paper submitted to AAS journals, but the debate is far from settled.

So is there an exomoon, or isn’t there? We don’t know yet, but that’s okay!

The case of Kepler 1625 is a beautiful illustration of the messy reality of the scientific process: sometimes the data don’t immediately spell out an answer, and it takes more time, more analysis, and likely more observations before the scientific community reaches a consensus. This isn’t a bad thing, though — this is science being done right! Keep an eye on the story of Kepler 1625b-i going forward; we’re bound to continue to learn about this maybe/maybe-not exomoon.

Citation

“No Evidence for Lunar Transit in New Analysis of Hubble Space Telescope Observations of the Kepler-1625 System,” Laura Kreidberg et al 2019 ApJL 877 L15. doi:10.3847/2041-8213/ab20c8

What happens to binary stars under the influence of a nearby supermassive black hole? A new study shows that things can go one of two ways — being torn apart or becoming closer than ever.

An artist’s impression of an X-ray binary, many of which are seen in the galactic center. [ESA/NASA/Felix Mirabel]

Life in Galactic Downtown

There’s a lot going on in the galactic center. The crowded stellar environs near the Milky Way’s central black hole have long been studied in order to understand how the presence of a nearby black hole can shape stellar populations in our galaxy and others.

A quick peek at the center of the Milky Way reveals that it plays host to an unusually large number of interesting astrophysical phenomena like stellar mergers, hyper-velocity stars, and X-ray sources that could point to cataclysmic variables or binary systems with a neutron-star or stellar-mass-black-hole component.

How do these various unusual objects form in the galactic center? One possibility is that ordinary, garden-variety binary stars do strange things when they evolve in the extreme environment near a supermassive black hole

A histogram of the binary mergers as a function of time in the simulation. [Stephan et al. 2019]

Binary Systems Through the Ages

To explore this scenario, a team of astronomers led by Alexander Stephan (University of California, Los Angeles) simulated the evolution of main-sequence binaries near a supermassive black hole. Specifically, they used Monte Carlo simulations of binary systems in the inner 0.33 light-years of a Milky-Way-like galaxy with a four-million-solar-mass black hole at its center.

Stephan and coauthors considered how the dynamics of binary systems with a range of starting masses would change over time as close encounters with other stars, gentle nudges from more distant objects, and the looming gravitational influence of the black hole warp and disturb their orbits.

While the orbits of the binary systems change over time due to gravitational interactions, the individual stars are busy evolving as well — a process that the authors captured by using a single-star stellar evolution code. Post-main-sequence evolution is important for the dynamics of a binary system since it can lead to mass transfer between the stars or even stellar mergers.

A flowchart of the simulated outcomes of binary evolution. Click to enlarge. [Stephan et al. 2019]

So Long, Partner

If you belong to a binary system and you’re hoping to spend the rest of your days with your companion, the news isn’t good: the authors found that after a few hundred million years, 75% of binary systems have been torn apart by gravitational interactions.

Of the remaining 25%, about half end up merging due to dynamical perturbations or because one star has swelled into a red giant and engulfed the other. The other half have even more exotic outcomes, becoming close binary systems containing white dwarfs, neutron stars, or black holes.

Clearly, the presence of a nearby supermassive black hole shakes up the evolution of otherwise ordinary binary stars: these rare systems are the precursors to Type Ia supernovae, cataclysmic variables, and compact object mergers that can generate gravitational waves.

Citation

“The Fate of Binaries in the Galactic Center: The Mundane and the Exotic,” Alexander Stephan et al 2019 ApJ 878 58. doi:10.3847/1538-4357/ab1e4d

habitable-zone planets

As of last week, the count of confirmed exoplanets officially exceeds 4,000 — and while we’ve learned a lot about planet formation from this wealth of data, it’s also prompted new questions. Could the recent detection of two intriguing new planets shed light on one of these open puzzles?

radius gap

In 2017, a team of scientists led by B.J. Fulton identified a gap in the distribution of radii of small Kepler-discovered planets. [NASA/Ames/Caltech/University of Hawaii (B. J. Fulton]

Mind the Gap

Our growing exoplanet statistics recently revealed a curious trait: there’s a gap in the radius distribution of planets slightly larger than Earth. Rocky super-Earth planets of up to ~1.5 Earth radii are relatively common, as are gaseous mini-Neptunes in the range of ~2–4 Earth radii. But we’ve detected very few planets in between these sizes.

What’s the cause of this odd deficit? One theory is that high-energy radiation emitted by stars early in their lifetimes erodes the atmospheres of planets that are too close in, stripping them of their expansive shells of gas and leaving behind only their dense, rocky cores. Planets that lie further out or start with a thicker shell may be spared this fate, retaining some of their gas for a significantly larger, fluffier construction.

Disentangling Factors

HD 15337 lightcurve

Folded light curves for HD 15337 showing the transits of planets b (top) and c (bottom). [Gandolfi et al. 2019]

This theory can be difficult to test, however, due to the large number of intertwined variables. The super-Earths and mini-Neptunes we’ve observed lie at varying distances from their host stars — but they also orbit around different types of stars with very different radiation histories. It’s hard to tell what role these various factors play in the planets’ evolution.

But a recent discovery from the Transiting Exoplanet Survey Satellite (TESS) may help simplify this picture. With more than 750 planet-candidate detections so far, TESS is rapidly adding to our exoplanet statistics — and two TESS-discovered planets around HD 15337 may be especially useful for better understanding the radius gap.

A Non-Identical Pair

In a publication led by Davide Gandolfi (University of Turin, Italy), a team of scientists carefully analyzes the TESS light curves for HD 15337, as well as archival spectroscopic data from the High Accuracy Radial velocity Planet Searcher. They show that there is evidence for the presence of two planets — HD 15337 b and c — that have similar masses: ~7.5 and ~8.1 Earth masses, respectively.

But while HD 15337 b appears to be a close-in (period of 4.8 days), rocky super-Earth with radius of 1.6 Earth radii and density of 9.3 g/cm3, HD 15337 c lies further out (period of 17.2 days) and is a fluffy mini-Neptune, with a radius of 2.4 Earth radii and a density of 3.2 g/cm3.

atmospheric erosion

Artist’s impression of a star’s high-energy radiation evaporating the atmosphere of its planet. [NASA’s Goddard SFC]

Since these two planets orbit the same star, it seems likely that their different orbital radii are what led to their places on either side of the radius gap. Using a planet atmospheric evolution algorithm, Gandolfi and collaborators show that the properties of the two planets can be produced by high-energy radiation from HD 15337 early in the system’s lifetime.

As our observational statistics for exoplanets continue to grow, it’s exciting to see how these continued discoveries can both raise and address new questions of planet formation and evolution. Who knows what else we’ll learn as detections continue to pile up!

Citation

“The Transiting Multi-planet System HD15337: Two Nearly Equal-mass Planets Straddling the Radius Gap,” Davide Gandolfi et al 2019 ApJL 876 L24. doi:10.3847/2041-8213/ab17d9

NGC 6397

Unusually blue and bright stars may not have only themselves to thank for their uniqueness. A new study looks at one way these unconventional objects might form in clusters … with a little help from a friend. 

Cluster Stand-Outs

blue stragglers on an HR diagram

Sketch of a Hertzsprung–Russell diagram for a star cluster. Blue stragglers lie above the main-sequence turnoff point for the rest of the stars in the cluster. [RicHard-59]

A stellar cluster is a group of stars that were all born together and should evolve in a consistent way. According to stellar evolution theory, for a given cluster, the stars of the cluster should fall onto a well-defined track on a Herzsprung–Russell (H–R) diagram — a plot of stellar brightness vs. color — with the stars’ location on the track dependent only on their initial mass.

But a few stars defy this logic. These so-called “blue stragglers” seem to have been left behind as their fellow cluster inhabitants evolved without them; on the H–R diagram, blue stragglers lie alone above the main-sequence turnoff point, shining brighter and bluer than they should be.

Just a Little Boost?

What causes these unorthodox stars? The simplest explanation is that they are main-sequence stars that belatedly received a bump in their mass. Theorists favor two possible formation channels:

  1. Mass transfer from an evolved donor onto a main-sequence star in a binary, which increases the main-sequence star’s mass and consequently causes it to become brighter and hotter.
  2. Collision and merger of two main-sequence stars, which forms a new, more massive main-sequence star that is brighter and hotter than usual.

But these two channels can only explain some observed blue stragglers; other systems — like WOCS ID 7782, a binary consisting of two blue stragglers in a 10-day orbit — are unlikely to have formed in either of these ways.

formation channel for blue stragglers

Schematic detailing the authors’ proposed scenario for the formation of WOCS 7782, in which a binary pair of main-sequence stars have material fed onto them by an evolved outer tertiary companion. [Portegies Zwart & Leigh 2019]

With WOCS ID 7782 in mind, scientists Simon Portegies Zwart (Leiden University) and Nathan Leigh (American Museum of Natural History; Stony Brook University; and University of Concepción, Chile) have now proposed an alternative formation channel.

A Third Star in the Mix

Portegies Zwart and Leigh’s model relies on one important element: a third star. In their proposed scenario, two main-sequence stars in a close binary are orbited by a giant, evolved companion star. As this evolved star ages and overflows its Roche lobe, gas flows from it onto the main-sequence binary, increasing the masses of the two inner stars.

Snapshot from one of the authors’ simulated triple systems. The binary system at left is being fed by gas from the outer tertiary companion on the right. [Portegies Zwart & Leigh 2019]

The authors use simulations to show that the final result of this process can be a close binary with two similar-mass blue stragglers, just as seen in WOCS ID 7782. In this scenario, the outer companion eventually evolves into a hard-to-spot white dwarf on a wide orbit with a period of more than ~5.8 years.

In addition to potentially explaining WOCS ID 7782, Portegies Zwart and Leigh’s model can produce a number of other masses, geometries, and configurations for blue-straggler systems, depending on the initial masses and separations of the binary and the outer companion. This formation scenario — which relies on just a little help from a friend — may therefore go a long way toward explaining the formation of the blue-straggler systems that have stumped us before now.

Citation

“A Triple Origin for Twin Blue Stragglers in Close Binaries,” Simon Portegies Zwart and Nathan W. C. Leigh 2019 ApJL 876 L33. doi:10.3847/2041-8213/ab1b75

active galactic nucleus

How does life arise on exoplanets? What environments are necessary for it to survive? What conditions pose threats to life? These are some of the many questions of astrobiology, the study of life beyond our solar system.

While much research explores how stellar radiation influences whether planets can form or sustain life, fewer studies examine other sources of radiation. Today’s study explores extreme sources: the violent and bright centers of active galaxies.

Extreme Sources

active galactic nuclei

Active galactic nuclei emit powerful high-energy radiation as they accrete gas and dust. [ESA/NASA, the AVO project and Paolo Padovani]

Active galactic nuclei, or AGN, contain enormous supermassive black holes that rapidly accrete gas and dust, emitting harsh high-energy radiation in the process. Could this radiation seriously hamper the formation and evolution of extraterrestrial life on nearby planets?

A recent study by Harvard University scientists Manasvi Lingam, Idan Ginsburg, and Shmuel Bialy suggests the opposite may be true: the radiation from AGN has the potential to aid (Earth-like) life’s chances of formation and survival.

The Cons

Let’s start with the downside of having an enormous source of high-energy radiation lurking nearby: as we’ve all experienced via the occasional sunburn, too much ultraviolet (UV) radiation can have harmful effects for life. On the extreme end, an excess of UV radiation can inhibit photosynthesis — a process on which the vast majority of Earth-based life relies for survival — and can damage DNA and other biomolecules.

But how much is too much? Lingam and collaborators conduct a series of calculations to show that an AGN doesn’t have much of an impact on the vast majority of planets in a galaxy. If the Milky Way had an active nucleus, the danger zone for potential extinction via UV radiation would extend to just ~100 light-years from the AGN — a tiny distance on the scale of our 100,000-light-year galaxy.

astrobiology

Artist’s impression of radiation as a driver of the chemistry of early life. [NASA]

The Pros

But UV light isn’t all bad! In fact, UV radiation is a necessary ingredient for the prebiotic chemical reactions that led to the synthesis of biomolecular building blocks on Earth. Lingam and collaborators show that, within a certain distance of the AGN (out to ~150 light-years in our Milky-Way-size active-galaxy example), UV radiation from the accreting black hole could actually jump-start these chemical reactions and eventually lead to the formation of life.

What’s more, the authors demonstrate that the visible light from AGN can power photosynthesis on nearby planets — which could be particularly useful for free-floating planets that don’t have a host star to provide that light. The zone for which this holds true is broad: out to more than 1,100 light-years, for a Milky-Way-size active galaxy. 

The Upshot

AGN influence

Author estimates for the distances at which an AGN could enable prebiotic chemistry (dO, blue line), facilitate potential extinction events (dB, red line), and permit photosynthesis (dP, green line) as a function of the black hole mass. Solid and dashed lines indicate two different levels of AGN power. [Lingam et al. 2019]

Ultimately, Lingam and collaborators find that AGN won’t influence life formation and survival either way for the vast majority of planets in a Milky-Way-size galaxy; beyond ~3,000 light-years from the galactic center, planets won’t notice the radiation from the AGN.

But in other types of galaxies — like those with especially large supermassive black holes, or compact dwarf galaxies with high stellar densities at their center — AGN could play a significant role in sparking life and helping it to stay alive.

Citation

“Active Galactic Nuclei: Boon or Bane for Biota?,” Manasvi Lingam et al 2019 ApJ 877 62. doi:10.3847/1538-4357/ab1b2f

MAVEN orbiting Mars

When solar ultraviolet and X-ray photons collide with atoms and molecules in Mars’s atmosphere, they form a layer of plasma called an ionosphere. That’s what happens on the sunlit side, at least. What’s going on in Mars’s shadow? 

Mars ionosphere cartoon

A cartoon depicting the interaction of the solar wind with Mars’s atmosphere, as well as likely regions for atmospheric escape. [NASA/GSFC]

Planetary Plasma

Even though there are no solar photons striking Mars’s atmosphere at night, plasma is still present — but it’s not immediately clear where it comes from. Does it come from bombardment by galactic cosmic rays or trapped solar-wind particles, or is it transported from the sunlit side by winds?

And once the plasma has been produced, what happens to it? Is it lost when electrons and ions reunite to form neutrals, or does it escape the planet’s atmosphere entirely?

One way to assess the sources and sinks of plasma is by calculating the rates of production by electron-impact ionization — when energetic electrons ionize neutrals through collisions — and loss by dissociative recombination — when molecular ions capture an electron and are split apart. If the rates are equal, those two processes dominate. If not, other processes must play a role.

Cui et al. 2019 Fig. 1

From left to right, the densities of the major ion and neutral species, neutral (black) and electron (red) temperatures, and the average electron intensity. Click to enlarge. [Adapted from Cui et al. 2019]

MAVEN on a Mission 

Evaluating whether or not the two rates are equal requires neutral and ion densities, electron temperatures, and a spectrum of incident energetic electrons. Luckily, NASA’s Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft, which has been orbiting Mars since 2014, collects all that information and more.

Normally, MAVEN comes within 150 km of Mars’s surface, but it occasionally drops its closest approach to 125 km. These so-called Deep Dip campaigns, of which there have been nine, give scientists a close look at the densest plasma in the ionosphere. In this study, a team led by Jun Cui (Sun Yat-sen University, Chinese Academy of Sciences, and National Astronomical Observatories, China) analyzed data from two Deep Dip orbits in 2015 and 2016.

Using the in-situ measurements made along each orbit, Cui and collaborators calculated the rate at which CO2 — the dominant neutral species — is ionized by electron impacts and the rate at which O2+, NO+, and HCO+ — the three dominant ion species — dissociatively recombine.

Cui et al. 2019 Fig. 4

Comparison of the dissociative recombination and electron-impact ionization rates for the two orbits. Open circles represent calculations made with individual measurements, while closed squares indicate average values for each altitude bin. The starred points have been corrected for instrumental effects. Click to enlarge. [Cui et al. 2019]

A Complex Nightside Picture

At low altitudes (below 140 km for the midnight orbit and 180 km for the dawn orbit), the authors found that the electron-impact ionization rate agrees with the dissociative recombination rate, which indicates that sources of plasma other than electron-impact ionization don’t play a major role at these altitudes.

At high altitudes, however, the rate of electron-impact ionization is higher than the rate of dissociative recombination, which is a sign that there is another important plasma loss process happening at those altitudes. It’s possible that magnetic pressure gradients at those altitudes encourage ions to escape down Mars’s magnetotail.

Last month, MAVEN finished its two-month aerobraking campaign, during which the spacecraft altitude dipped as low as ~125 km to use atmospheric drag to change its orbit, giving scientists a long look at Mars’s ionosphere. Expect more atmospheric news from MAVEN in the future!

Citation

“Evaluating Local Ionization Balance in the Nightside Martian Upper Atmosphere during MAVEN Deep Dip Campaigns,” J. Cui et al 2019 ApJL 876 L12. doi:10.3847/2041-8213/ab1b34

M82

In December, AAS Nova Editor Susanna Kohler had the opportunity to fly aboard the NASA/DLR Stratospheric Observatory for Infrared Astronomy (SOFIA). This week we’re taking a look at that flight, as well as some of the recent science the observatory produced and published in an ApJ Letters Focus Issue.

One of SOFIA’s great strengths is that the instruments mounted on this flying telescope can be easily swapped out, allowing for a broad range of infrared observations. Three of SOFIA’s instruments are featured in science recently published in the ApJ Letters Focus Issue: the Far Infrared Field-Imaging Line Spectrometer (FIFI-LS), the High-Resolution Airborne Wideband Camera Plus (HAWC+), and the Echelon-Cross-Echelle Spectrograph (EXES).

HAWC+

The HAWC+ instrument mounted on the SOFIA telescope. [NASA]

Meet HAWC+

HAWC+ is a one-of-a-kind instrument: it’s the only currently operating astronomical camera that takes images in far-infrared light. HAWC+ observes in the 50-μm to 240-μm range at high angular resolution, affording us a detailed look at low-temperature phenomena, like the early stages of star and planet formation.

In addition to the camera, HAWC+ also includes a polarimeter, which allows the instrument to measure the alignment of incoming light waves produced by dust emission. By observing this far-infrared polarization, HAWC+ can produce detailed maps of otherwise invisible celestial magnetic fields. The insight gained with HAWC+ spans an incredible range of astronomical sources, from nearby star-forming regions to the large-scale environments surrounding other galaxies.

Some Recent HAWC+ Science

Cygnus A core

Artist’s conception of Cygnus A, surrounded by the torus of dust and debris with jets launching from its center. Magnetic fields are illustrated trapping dust near the supermassive black hole at the galaxy’s core. [NASA/SOFIA/Lynette Cook]

Cygnus A is the closest and most powerful radio-loud active galactic nucleus. At its heart, a supermassive black hole is actively accreting material, producing enormous jets — but this core is difficult to learn about, because it is heavily shrouded by dust.

In a recent study led by Enrique Lopez-Rodriguez (SOFIA Science Center; National Astronomical Observatory of Japan), a team of scientists has used HAWC+ to observe the polarized infrared emission from aligned dust grains in the dusty torus surrounding Cygnus A’s core. Lopez-Rodriguez and collaborators find that a coherent dusty and magnetic field structure dominates the infrared emission around the nucleus, suggesting that magnetic fields confine the torus and funnel the dust in to accrete onto the supermassive black hole.

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Messier 82 and NGC 253 are two nearby starburst galaxies — galaxies with a high rate of star formation. Such galaxies often have strong outflowing galactic winds, which are thought to contribute to the enrichment of the intergalactic medium with both heavy elements and magnetic fields.

A study led by Terry Jay Jones (University of Minnesota) uses HAWC+ to map out the magnetic field geometry in the disk and central regions of these two galaxies. M82 shows the most spectacular results, revealing clear evidence for a massive polar outflow that drags the magnetic field vertically away from the disk along with entrained gas and dust.

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lensed starburst galaxy

SOFIA/HAWC+ 89 μm detection of the gravitationally lensed starburst galaxy J1429-0028. Right: false-color composite image of J1429-0028 from Hubble and Keck. [Ma et al. 2018]

A study led by Jingzhe Ma (University of California, Irvine) presents the HAWC+ detection of the distant, gravitationally lensed starburst galaxy HATLAS J1429-0028. This beautiful system consists of an edge-on foreground disk galaxy and a nearly complete Einstein ring of an ultraluminous infrared background galaxy. What causes this background galaxy to shine so brightly in infrared wavelengths? The HAWC+ observations suggest it’s not due to emission from an active galactic nucleus; instead, this galaxy is likely powered purely by star formation.

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G 9 region

The G 9 region, as represented by the Digital Palomar Observatory Sky Survey. The cyan polygon represents the SOFIA HAWC+ coverage of the filamentary dark cloud GF 9. The yellow diamond marks the YSO GF 9-2. [Clemens et al. 2018]

In a recent study examining the geometry of magnetic fields surrounding sites of massive star formation, Dan Clemens (Boston University) and collaborators obtained HAWC+ observations of a young stellar object (YSO) embedded in a molecular cloud. The polarimetric measurements of HAWC+ revealed the magnetic field configuration around the YSO, the dense core that hosts it, and the clumpy filamentary dark cloud that surrounds it, GF 9.

Surprisingly, the observations show a remarkably uniform magnetic field threading the entire region, from the outer, diffuse cloud edge all the way down to the smallest scales of the YSO surroundings. These results contradict some models of how cores and YSOs form, providing important information that will help us better understand this process.

Citation

ApJL Focus issue:
Focus on New Results from SOFIA

HAWC+ articles:
“The Highly Polarized Dusty Emission Core of Cygnus A,” Enrique Lopez-Rodriguez et al. 2018 ApJL 861 L23. doi:10.3847/2041-8213/aacff5
“SOFIA Far-infrared Imaging Polarimetry of M82 and NGC 253: Exploring the Supergalactic Wind,” Terry Jay Jones et al. 2019 ApJL 870 L9. doi:10.3847/2041-8213/aaf8b9
“SOFIA/HAWC+ Detection of a Gravitationally Lensed Starburst Galaxy at z = 1.03,” Jingzhe Ma et al. 2018 ApJ 864 60. doi:10.3847/1538-4357/aad4a0
“Magnetic Field Uniformity Across the GF 9-2 YSO, L1082C Dense Core, and GF 9 Filamentary Dark Cloud,” Dan P. Clemens et al. 2018 ApJ 867 79. doi:10.3847/1538-4357/aae2af

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