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Venus as viewed by Akatsuki

Editor’s note: In these last two weeks of 2019, we’ll be looking at a few selections that we haven’t yet discussed on AAS Nova from among the most-downloaded papers published in AAS journals this year. The usual posting schedule will resume in January.

Long-term Variations of Venus’s 365 nm Albedo Observed by Venus Express, Akatsuki, MESSENGER, and the Hubble Space Telescope

Published August 2019

Main takeaway:

A new study led by Yeon Joo Lee (The University of Tokyo, Japan) has used an entire suite of instruments — Venus Express, Akatsuki, MESSENGER, and the Hubble Space Telescope — to study the long-term reflectivity of Venus’s atmosphere. They find that, over the timescale of roughly a decade, variations in the ultraviolet albedo of the atmosphere cause significant changes in aspects of Venus’s climate, like windspeeds at the planet’s cloud tops.

Why it’s interesting:

Venus vs. Earth

A comparison of Venus and Earth. Though they are nearly the same size and density, the two planets evolved very differently. [NASA]

Often cast as a sibling to Earth, Venus’s similarities to our own planet are striking — yet the two worlds have very different atmospheres. Exploring what drives the weather on Venus helps us to understand what makes these two bodies alike and different.

From Lee and collaborators’ study, we can see that, like Earth, Venus has decade-long climate variations. Unlike on Earth, however, most of the Sun’s energy is absorbed by the planet’s atmosphere, rather than its surface. The decade of observations analyzed in this study provide new insight into Venus’s climate processes, demonstrating that Venusian weather is deeply influenced by ultraviolet absorption in the planet’s atmosphere.

Why these results provide extra intrigue:

Tracking how ultraviolet absorption in Venus’s atmosphere influences the planet’s climate is only part of the puzzle; the other part is understanding what is doing the absorbing. For more than a century, we’ve been aware that Venus has dark patches in its atmosphere that absorb light at ultraviolet to visible wavelengths, peaking at around 360 nm. But what are the patches made of, and why do they absorb ultraviolet light? Lee and collaborators’ analysis may help us to further study these “unknown absorbers” and identify the particles that they’re made up of.

Citation

Yeon Joo Lee et al 2019 AJ 158 126. doi:10.3847/1538-3881/ab3120

quasar

Editor’s note: In these last two weeks of 2019, we’ll be looking at a few selections that we haven’t yet discussed on AAS Nova from among the most-downloaded papers published in AAS journals this year. The usual posting schedule will resume in January.

Discovery of the First Low-luminosity Quasar at z > 7

Published February 2019

Main takeaway:

A team of scientists led by Yoshiki Matsuoka (Ehime University, Japan) announced the discovery of the first low-luminosity quasar — a dim supermassive black hole actively feeding at the center of a galaxy — found at a redshift of z > 7. This redshift represents a time when the universe was still in its infancy, at less than a billion years old.

J1243+0100

False color, composite image around the low-luminosity, high-redshift quasar J1243+0100, marked with the cross-hair. [Matsuoka et al. 2019]

Why it’s interesting:

By exploring distant quasars, we can learn about the conditions in the early universe, helping us to address questions like how the earliest black holes were able to grow to their enormous sizes in such a short time, or what sources were responsible for the majority of our universe’s reionization. Matsuoka and collaborators’ discovery is unique because, before now, the few quasars we’ve found at such high redshifts have been extremely luminous. Since we don’t think those sources are representative of the general population of high-redshift quasars, it’s exciting to now have a distant source to study that’s likely to be a more typical example.

How this quasar was spotted:

It’s hard to find high-redshift quasars — especially low-luminosity ones! — because their emission in observed wavelengths of less than 970 nm is almost entirely absorbed by the intergalactic medium. To discover them, you therefore need wide-field, deep imaging at wavelengths longer than this — which is exactly what the Hyper SuprimeCam on the Subaru Telescope in Hawaii provides. Matsuoka and collaborators conducted a broad survey, the Subaru High-z Exploration of Low-Luminosity Quasars (SHELLQs), which revealed not just the record-setting quasar detailed in this study, but also 82 additional, previously unknown distant quasars.

Citation

Yoshiki Matsuoka et al 2019 ApJL 872 L2. doi:10.3847/2041-8213/ab0216

habitable exoplanet

Editor’s note: In these last two weeks of 2019, we’ll be looking at a few selections that we haven’t yet discussed on AAS Nova from among the most-downloaded papers published in AAS journals this year. The usual posting schedule will resume in January.

Rethinking CO Antibiosignatures in the Search for Life Beyond the Solar System

Published March 2019

Main takeaway:

A team of scientists led by Edward Schwieterman (UC Riverside) used computer simulations of planetary ecospheres/atmospheres to show that a number of planet types of interest in the search for life — like planets similar to early Earth, or planets around M-dwarf stars — can maintain an accumulation of carbon monoxide in their atmospheres.

Why it’s interesting:

It was previously thought that the presence of carbon monoxide in a planet’s atmosphere could be considered an antibiosignature — a signature that indicates that there probably isn’t life present. This is because carbon monoxide represents an unexploited source of free energy; its accumulation was thought to indicate that there’s no life available to take advantage of it. Antibiosignatures are valuable because, in the search for life on planets beyond the solar system, they can quickly tell us where we shouldn’t waste our time looking. But Schwieterman and collaborators’ work now suggests we may need to rethink the assumption that carbon monoxide can be used as such an indicator.

What this means for the search for life:

If carbon monoxide can be present in a planet’s atmosphere even when the world is inhabited, we clearly can’t use the accumulation of this gas to unambiguously rule out targets in the search for extraterrestrial life. Instead, our best bet is to continue to develop a framework that relies on the presence or absence of various combinations of gases. We may also be able to use this information together with novel approaches, like searching for seasonal variation that could be caused by the presence of life, or calculating whether the atmosphere and surface of a planet are out of equilibrium.

Citation

Edward W. Schwieterman et al 2019 ApJ 874 9. doi:10.3847/1538-4357/ab05e1

'Oumuamua

Editor’s note: In these last two weeks of 2019, we’ll be looking at a few selections that we haven’t yet discussed on AAS Nova from among the most-downloaded papers published in AAS journals this year. The usual posting schedule will resume in January.

High-drag Interstellar Objects and Galactic Dynamical Streams

Published March 2019

Main takeaway:

Remember the first known interstellar asteroid, 1I/’Oumuamua? A study by scientist Marshall Eubanks (Space Initiatives, Inc.) explores the possibility that the asteroid is a very lightweight, high-drag object that long ago orbited the galaxy, became caught up within dense interstellar gas, and was then released in our direction as part of the Pleiades dynamical stream.

Why it’s interesting:

'Oumuamua velocity

‘Oumuamua’s incoming velocity is consistent with the dynamics of the Pleiades stream. [Eubanks 2019]

This interstellar object visited our solar system for only a brief time, and it displayed a number of perplexing behaviors — like its unexpected acceleration boost as it left the solar system again. Eubanks’ hypothesis provides a plausible natural explanation: if ‘Oumuamua is a body with a large area-to-mass ratio, solar radiation pressure could have provided the acceleration boost (note that this is the same explanation used by Bialy & Loeb to argue that ‘Oumuamua could be a light sail). And this same lightweight, high-drag structure makes it easy for the asteroid to have become entrained in interstellar gas before being sent into our solar system with the Pleiades stream.

Why This Could Provide Cool Future Opportunities:

In Eubanks’ model, ‘Oumuamua may not be unique. Instead, this object could be just one of an entire population of light asteroids with large area-to-mass ratios — all of which are likely to be entrained with dense gas and could be ejected toward us in streams. By searching stellar streams for bodies like this one, we could identify future interstellar visitors like ‘Oumuamua before they arrive, providing us with more time to study them from afar — or even prepare a fly-by mission.

Citation

T. M. Eubanks 2019 ApJL 874 L11. doi:10.3847/2041-8213/ab0f29

KIC 10544976

Editor’s note: In these last two weeks of 2019, we’ll be looking at a few selections that we haven’t yet discussed on AAS Nova from among the most-downloaded papers published in AAS journals this year. The usual posting schedule will resume in January.

Orbital Period Variation of KIC 10544976: Applegate Mechanism versus Light Travel Time Effect

Published March 2019

Main takeaway:

In a study led by Leonardo Almeida (Federal University of Rio Grande do Norte and University of São Paulo, Brazil), scientists announce evidence for a 13-Jupiter-mass planet around an evolved binary system, KIC 10544976, that consists of a white dwarf and a red dwarf star orbiting each other once every 0.35 days.

Why it’s interesting:

This is the first planet found orbiting an evolved binary like this one, and it raises questions as to how it formed. Was the planet born at the same time as the stars, and somehow survived the end of life of the binary member that evolved into a white dwarf? Or was the planet instead born later, out of the gas ejected by this star as it died? By studying the KIC 10544976 planet with next generation telescopes, we should be able to answer this question.

How the planet was discovered:

Observations of the eclipsing binary stars show timing variations in the eclipses. This change in orbit could be caused by one of two things: either the gravitational tug of an additional unseen, massive body, or period fluctuations in the magnetic field of the red dwarf. By studying the magnetic activity cycle for the red dwarf using years of flare and starspot data, Almeida and collaborators were able to rule out the hypothesis that magnetic activity caused the eclipse timing variations. This made the presence of a giant planet the most likely explanation.

Citation

L. A. Almeida et al 2019 AJ 157 150. doi:10.3847/1538-3881/ab0963

ANTARES

Editor’s note: In these last two weeks of 2019, we’ll be looking at a few selections that we haven’t yet discussed on AAS Nova from among the most-downloaded papers published in AAS journals this year. The usual posting schedule will resume in January.

Search for Multimessenger Sources of Gravitational Waves and High-energy Neutrinos with Advanced LIGO during Its First Observing Run, ANTARES, and IceCube

Published January 2019

Main takeaway:

No significant detections of high-energy neutrinos and gravitational waves coming from the same astrophysical source were found during the Laser Interferometer Gravitational-Wave Observatory’s (LIGO’s) first observing run. This was established from detailed analysis and comparison of IceCube and Antares neutrino candidates and LIGO gravitational-wave candidates over the ~130-day observing period.

Why it’s interesting:

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]

Null results matter too! Core-collapse supernovae, binary neutron star mergers, and neutron star–black hole mergers are all among the astrophysical sources that we expect to produce both gravitational waves (which come from the source shedding angular momentum) and neutrinos (which come from astrophysical outflows like jets). By searching for joint neutrino/gravitational-wave signals from these sources, non-detections help us set limits on the rates and energies for these cosmic catastrophes.

Why this is only the start:

IceCube and Antares are already remarkable neutrino detectors located beneath kilometers of ice at the South Pole and water in the Mediterranean Sea, respectively. But the future holds hope for a significant upgrade to IceCube (IceCube-Gen2), work on an expanded neutrino observatory in the Mediterranean Sea is already underway (KM3NeT), and we’re even constructing a neutrino observatory in the deepest lake in the world, Lake Baikal in Russia (Baikal-GVD). Observations from these new detectors, combined with data from the upgraded LIGO/Virgo observatories, should place even further constraints on our understanding of astrophysical neutrino and gravitational-wave sources in the future.

Citation

A. Albert et al 2019 ApJ 870 134. doi:10.3847/1538-4357/aaf21d

black widow pulsar

One of the fastest spinning radio pulsars known has now been detected to pulse in gamma rays, too. What can we learn about this extreme pulsar from new observations?

Pushing the Record for Spin

pulsar

Artist’s illustration of a pulsar, a fast-spinning, magnetized neutron star. [NASA]

Pulsars are rapidly spinning, magnetized neutron stars left behind at the end of a star’s lifetime. Pulsar J0952-0607, a pulsar in a binary orbit with a very low-mass companion star, has the second-fastest known pulsar spin, rotating 707 times each second. For comparison, that’s about 70 times faster spin than the fastest helicopter rotors — and it’s an object that’s 10 km across and weighs more than the Sun!

As it spins, PSR J0952-0607 flashes a beam of radio waves across the path of the Earth, radiating from a hot spot on its surface. In a recent study, a team of scientists led by Lars Nieder (Albert Einstein Institute and Leibniz University Hannover, Germany) have now hunted through years of data from the Fermi Gamma-ray Space Telescope to see if we can spot pulsations from a gamma-ray beam as well.

Fermi

An artist’s illustration of the Fermi Gamma-ray Space Telescope. [NASA/General Dynamics]

Finding High-Energy Pulses

The radio observations of PSR J0952-0607’s pulsations span only 100 days, which isn’t long enough to precisely constrain its properties. The Fermi Gamma-ray Space Telescope launched in 2008, and its Large Area Telescope (LAT) has been providing all-sky images on a regular basis since then. Nieder and collaborators reasoned that if they could spot PSR J0952-0607 in gamma rays in the Fermi LAT data, then they’d be able to observe the pulsar over a much longer baseline than its radio observations provide.

The catch? PSR J0952-0607 is very faint in gamma-rays — which is why its pulsations weren’t previously detected. Nieder and collaborators had to develop novel search and timing methods with greater sensitivity, ultimately using the computational equivalent of 24 years on a single-core computer to search for a signal. Their efforts paid off, however — they managed to detect faint gamma-ray pulsations from PSR J0952-0607 spanning from July 2011 to the end of the dataset in January 2017.

Some Answers and Some New Puzzles

P–Pdot diagram

Plot of the spin-down rate vs. the spin for the known pulsar population outside of globular clusters. PSR J0952-0607 is marked by an orange star. [Nieder et al. 2019]

From the gamma-ray observations, Nieder and collaborators were able to measure a precise spin-down rate for the pulsar (it slows by less than 4.6 x 10-21 seconds each second), as well as other properties. PSR J0952-0607’s inferred magnetic field is among the 10 lowest magnetic fields measured for pulsars — an extreme that is predicted by theory based on this pulsar’s remarkably fast spin.

Though we’ve gained a lot of information about PSR J0952-0607 from its gamma-ray pulsations, new mysteries have also been introduced. The fact that its pulsations are undetectable before July 2011 is one of these — could the pulsar’s flux have changed? Or its orbit around its companion star? We’ll need more data to be able to solve this puzzle.

We still have more to learn about PSR J0952-0607, but the newly discovered gamma-ray pulsations have provided us with unique insight into the extremes that arise when compact astrophysical bodies spin at such high speeds. With luck, future observations of this pulsar — and others like it — will help us to further probe the physics of these unusual sources.

Citation

“Detection and Timing of Gamma-Ray Pulsations from the 707 Hz Pulsar J0952−0607,” L. Nieder et al 2019 ApJ 883 42. doi:10.3847/1538-4357/ab357e

exoplanet system

One of the goals of the Transiting Exoplanet Survey Satellite (TESS) is to identify exoplanets whose atmospheres can be characterized by other telescopes. Part of this process entails measuring planetary masses to some degree of precision. So just how well do we need to know an exoplanet’s mass to understand its atmosphere?

The Use of Transmission Spectra

One way to study the atmosphere of an exoplanet is to observe the light from its host star that passes through the planet’s atmosphere. Comparing the resulting spectrum — called a transmission spectrum — with the spectrum of the host star alone can tell us about what’s in the planet’s atmosphere.

A planet’s mass plays an important role in how far its atmosphere extends. This has prompted studies on whether a planet’s mass could be inferred from its transmission spectrum alone. In some cases, this approach works. But in other cases, the transmission spectra of very different planets can appear to be alike.

exoplanet mass uncertainty vs mass

The precision of exoplanet mass measurements versus their most likely mass. The seven planets used in this study are highlighted. [Adapted from Batalha et al. 2019]

So should we know the mass of a planet before trying to characterize its atmosphere? If so, how well? And how do these answers change for different types of planets? Natasha Batalha (University of California, Santa Cruz) and collaborators attempt to tackle these questions with simulated James Webb Space Telescope (JWST) transmission spectra.

Seven Special Planets

For their study, Batalha and collaborators chose seven known planets that span the gamut of exoplanets we’ve observed. Their sample included three hot Jupiters (WASP-17b, HAT-P-1b, WASP-12b), three Neptune-like planets (HAT-P-26 b, GJ 436b, GJ 1214b), and one Earth-like planet (TRAPPIST-1e). 

To simulate transmission spectra, the authors started with models that are consistent with Hubble spectroscopy of their chosen planets. They then used these models to simulate the analogous JWST spectra.

Aside from mass, the sample planets also varied in composition. Their host stars are also different, meaning that in real life the JWST would have to adopt different observing strategies to get quality transmission spectra.

The accuracy with which different atmospheric properties are recovered from the simulated transmission spectra. From top left, clockwise: temperature, metallicity (the abundance of elements that are not hydrogen or helium), radius, and mass. The shaded regions correspond to mass being known and the unfilled regions correspond to mass not being known. The colors of the curves indicate different planets. Click to enlarge. [Batalha et al. 2019]

A Matter of Caution

To test what role mass played in the usefulness of transmission spectra, the authors attempted to measure atmospheric properties from their modeled spectra. They tried different precisions on mass (how far off the assumed mass could be from the true mass) as well as not knowing a planet’s mass at all.

The authors found that transmission spectra alone could not reliably characterize a planet’s atmosphere. Hot Jupiters required the loosest mass constraints to infer atmospheric properties, though cloud cover  — such as in the case of WASP-12b — could make that untrue. For the other Neptunes and the Earth-like planet, mass had to be known with at least a 50% precision to get accurate atmospheric properties.

A recurring theme was that a mass measurement is necessary to distinguish one planet from others with similar transmission spectra. To this end, the authors recommend that any planets selected for atmospheric characterization have their mass known to at least 50% precision.

One of TESS’s goals is to measure the mass of fifty Earth-sized planets, and Batalha and collaborators have set a benchmark for those measurements. This sort of groundwork is critical to exoplanet science and should contribute to great results not too long from now!

Citation

“The Precision of Mass Measurements Required for Robust Atmospheric Characterization of Transiting Exoplanets,” Natasha E. Batalha et al 2019 ApJL 885 L25. https://doi.org/10.3847/2041-8213/ab4909

tidal disruption event

Close encounters between stars and supermassive black holes generally don’t end well for the stars. Under the influence of a black hole’s strong gravitational forces, an unsuspecting passing star can be completely shredded, resulting in a spectacular tidal disruption flare. But what happens when the star is only partially destroyed?

Unfortunate Encounters

Tidal disruption event

This simulated TDE shows the looping tidal stream caused when a star is pulled apart by the gravitational forces of a black hole. When this stream intersects with itself, material collides and rains onto the black hole, causing it to light up. [NASA/S. Gezari (JHU)/J. Guillochon (UCSC)]

Stars don’t often wander close to supermassive black holes — which means that tidal disruption events (TDEs) are relatively infrequent. Nonetheless, we’ve spotted around 40 of these destructive encounters so far, and they’re being discovered at an ever-increasing rate.

During a TDE, a passing star is torn apart and stretched into a stream of debris by the tidal forces of the supermassive black hole. Part of this stellar material escapes the black hole’s pull and scatters; the rest collides with itself during looping orbits, eventually raining down on the black hole and accreting. This accretion emits radiation, causing the black hole to briefly flare, producing a characteristic light curve that we might observe.

A Universal Decay?

The shape of the light curve is what makes TDEs distinctive. When we search for tidal flares, we hunt for transient signals that feature a sharp rise in the light curve followed by a long, decaying tail with a shape that’s governed by the physics of the fallback and accretion of the stellar material. For TDEs, that decay asymptotes to a characteristic power-law slope — a slope that was thought to be universal for all such stellar destruction.

But is it truly? A new study by scientists Eric Coughlin (Princeton University; Columbia Astrophysics Laboratory) and Chris Nixon (University of Leicester) explores whether we can expect to see differences when a star is only partially destroyed in its encounter with a supermassive black hole.

Survival Under Stress

fallback rates

Fallback rate onto a million-solar-mass supermassive black hole as a function of time for a disrupted Sun-like star. Different color curves represent different masses of surviving stellar cores. The curve representing a fully destroyed star (µ=0, red) asymptotes to a shallower slope (~t-5/3). The other curves, which all represent partial disruptions of varying degrees, all asymptote to a more steeply decaying slope (~t-9/4). Click to enlarge. [Coughlin & Nixon 2019]

In a partial stellar destruction, material is stripped from the star, but some fraction remains bound together as a stellar core. This core then orbits around the black hole along with the stream of colliding, accreting debris. Coughlin and Nixon show that the gravitational pull of this surviving stellar core affects the rate at which material falls back onto the black hole, causing different behavior than if the star had been fully destroyed.

What does this mean for observations? The authors argue that we should expect to see two types of TDEs: those representing complete stellar disruptions, whose light curves asymptote to a shallower slope, and those representing partial stellar disruptions, whose light curves asymptote to a steeper slope.

Coughlin and Nixon estimate that, for stars that undergo tidal disruptions, just under half of low-mass stars and around 70% of high-mass stars will be only partially disrupted. They therefore expect that a substantial fraction of TDEs detected by future facilities — like the Large Synoptic Survey Telescope (LSST) coming online in 2020 — will represent stars that partially survived their close encounter with a supermassive black hole … though perhaps a little the worse for wear.

Citation

“Partial Stellar Disruption by a Supermassive Black Hole: Is the Light Curve Really Proportional to t −9/4?,” Eric R. Coughlin and C. J. Nixon 2019 ApJL 883 L17. doi:10.3847/2041-8213/ab412d

M-dwarf planets

What determines the climate of an Earth-like planet orbiting its host star? And how is that climate affected by the type of star the planet orbits? A new study explores how distant terrestrial worlds are shaped by their hosts.

Radiation In, Radiation Out

The climate for a planet like Earth is largely set by the delicate balance between incoming radiation from the planet’s star, and outgoing radiation in the form of heat emitted into space. The amount of energy absorbed, reflected, and emitted by a planet’s surface and atmosphere dictate how this balance plays out.

mean energy budget

Diagram describing the annual mean energy budget for a planet orbiting a G dwarf star. Click to enlarge. [Adapted from Shields et al. 2019]

The pathways that govern this global energy budget for our own planet have been worked out through many decades of modeling and analysis of observations — to the point where we can identify sources of imbalance in the Earth’s system, like those currently caused by anthropogenic CO2 emissions.

But these climate models don’t apply directly to other planets, because the factors that determine a planet’s global energy budget all depend on the wavelength distribution of incoming light. Since stars of different temperatures emit varying amounts of radiation at different wavelengths, models that describe the energy budget for a planet around a Sun-like G dwarf won’t accurately describe a planet around a cooler M dwarf or hotter F dwarf.

So how do the climates of distant, Earth-like worlds change when orbiting a different type of host star? A team of scientists led by Aomawa Shields (University of California, Irvine) has now used detailed 3D global climate models to find out.

A Difference of Hosts

Shields and collaborators’ models of terrestrial planets take into account details like the interaction between the incoming host star’s radiation and gases like CO2 and H2O in the planet’s atmosphere, as well as with icy and snowy surfaces on the ground.

global mean surface temp

Plot of the global mean surface temperature as a function of the amount of incoming stellar radiation at the top of the planet’s atmosphere, shown for a planet orbiting an F dwarf (blue triangles), a G dwarf (black plus symobls), and an M dwarf (red x symbols). [Adapted from Shields et al. 2019]

The authors show that M-dwarf planets absorb more of their hosts’ radiation, both in their atmospheres and their surfaces, whereas F-dwarf planets absorb less. As a result, a planet can have a climate similar to that of modern-day Earth if it’s receiving current solar amounts of incoming radiation from a G-dwarf star — but to achieve the same climate around an M-dwarf star, it would need to receive 12% less incoming radiation. Around an F-dwarf star, it would need to receive 8% more.

What about rotation? The above models assumed that the planets all had 24-hour rotation rates, but Shields and collaborators also test how this compares to a tidally locked planet that always shows the same face to its host. For an M-dwarf host, a tidally locked planet has lower minimum and maximum dayside temperatures when compared with a planet with a 24-hour rotation period; the average dayside temperature is around 37 K colder on the tidally locked planet.

As we continue to discover more planets around a variety of stars, a constant question is whether these distant worlds have the potential to support life. Understanding how these planets’ global climates are shaped by their host stars is an important part of this exploration!

Citation

“Energy Budgets for Terrestrial Extrasolar Planets,” Aomawa L. Shields et al 2019 ApJL 884 L2. doi:10.3847/2041-8213/ab44ce

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