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neutron-star merger

The recent discovery of GW170817 — the first gravitational-wave detection where we also observed electromagnetic signals — has enabled new studies of merging compact objects. What have we since learned about the radiation that emerges from these collisions?

Signs of a Merger

Before GW170817 was observed, our models predicted that when two compact objects — either two neutron stars, or a neutron star and a black hole — merge, a number of observable signals result:

  1. a spike of gravitational waves as the objects conclude their death spiral,
  2. a short gamma-ray burst, an explosion of gamma rays thought to be produced in the relativistic jets launched during the merger,
  3. an afterglow spanning X-rays to radio, caused when the jets slam into the surrounding environment and decelerate, and
  4. an optical/near-infrared transient signal called a kilonova, which occurs when neutron-rich ejected material forms heavy elements that then undergo radioactive decay. 

Last August’s binary-neutron-star merger confirmed this picture beautifully: the discovery of gravitational-wave signal GW170817 was followed by detections of short gamma-ray burst SGRB 170817A and later observations of kilonova AT 2017gfo.

But though this was the first time for all of these signals to come together for a source, it wasn’t our first observation of a short gamma-ray burst or a kilonova! How do past detections compare to this new one, and what can we consequently learn about the bursts of radiation from merging compact objects? A team of scientists led by Ben Gompertz (University of Warwick, UK) have now tackled these questions.

AT 2017gfo

Model fit to the kilonova AT 2017gfo. [Gompertz et al. 2018]

Diversity of Kilonovae

Gompertz and collaborators compared the optical and near-infrared light curves of kilonova AT 2017gfo to equivalent light curves for a sample of a dozen nearby short gamma-ray bursts. What they found was a diversity of signals: some with no evidence of a kilonova, despite observations sensitive enough to pick up a signal several magnitudes fainter than AT 2017gfo; some with confirmed or suspected kilonovae brighter than AT 2017gfo; and some with afterglows so bright that they could be masking kilonovae that are brighter still.

How can we explain these vastly different kilonova outcomes from different mergers? The authors demonstrate that neither line-of-sight dust interference nor viewing angles can explain the differences we see in the optical and near-infrared signals.

It’s All in the Starting Players?

kilonova signals from sGRBs

Magnitudes of the optical and near-infrared emission for a sample of 12 short gamma-ray bursts, as compared to the kilonova AT 2017gfo. [Gompertz et al. 2018]

Instead, Gompertz and collaborators propose that a dichotomy might exist in the short-gamma-ray-burst population, between those created by the merger of two neutron stars, and those created by the merger of a neutron star with a black hole. A neutron-star–black-hole merger can produce as much as ten times more ejecta than a binary-neutron-star merger — perhaps allowing the former to power observable kilonovae, while the latter produce much fainter signals.

Future observations of these events can confirm or deny this picture, depending on whether the magnitude of kilonova emission continues to display a gap in brightness between two populations, or if it instead forms a continuum. Either way, we can look forward to learning more about these explosive collisions soon!

Citation

B. P. Gompertz et al 2018 ApJ 860 62. doi:10.3847/1538-4357/aac206

April 17, 2016 solar flare

Solar flares are often, but not always, associated with coronal mass ejections. Why do coronal mass ejections accompany some solar flares but not others?

Coronal mass ejection

Some solar flares are associated with explosive coronal mass ejections, which can disturb Earth’s protective magnetosphere. The resulting geomagnetic storms generate dazzling auroras, but they can also interrupt radio communications and damage power grids. [SOHO (ESA & NASA)]

To Erupt or Not to Erupt

When a solar flare is blissfully unaccompanied by a violent eruption of plasma from the Sun’s surface, we call it a confined flare — the solar atmosphere remains bound to the surface rather than lashing out into interplanetary space. An eruptive flare, on the other hand, occurs when a coronal mass ejection is released along with the solar flare, exploding material out into the Sun’s surroundings.

It’s not yet clear why some solar flares are confined while others are eruptive. One possibility has to do with the arrangement of the Sun’s complex magnetic field. Twisted and tangled field lines can rearrange themselves through a process called magnetic reconnection, which accelerates charged particles and heats the plasma.

If, after reconnection, the magnetic field is either in a low-energy configuration or it’s sufficiently constricted by stronger magnetic fields farther from the Sun’s surface, no eruption will occur and the flare is confined. But how can we figure out what actually happens during confined flares?

Out on a Limb

Ning et al. 2018 Figure 2

The July 24, 2016, solar flare as seen in three extreme ultraviolet channels by Solar Dynamics Observatory. Panels a, b, and c show the progression of the flare from the beginning to the peak to the end. Click to enlarge. [Ning et al. 2018]

To better understand what makes solar flares tick, researchers often trace the movement of magnetic field loops before, during, and after the event. However, many flares are observed against the background of the bright solar disk, which can make it difficult to track the behavior of the magnetic loops.

To sidestep this issue, Hao Ning (Shandong University, China) and collaborators investigated the magnetic configuration of a flare extending from the edge of the Sun’s disk — the solar limb — where the bright arcs of plasma are clearly silhouetted against the dark background of the sky.

Ning and collaborators combined observations of a confined flare from the space-based observatories Solar Dynamics Observatory (SDO) and Ramaty High Energy Solar Spectroscopic Imager (RHESSI), as well as the Nobeyama Radioheliograph on the ground.

Time Evolution of a Solar Flare

Light curves extracted from the flaring region showed not one peak but two, which is thought to signal the presence of magnetic reconnection. Why two peaks? Magnetic reconnection in the corona accelerates charged particles, which then interact with the relatively dense chromospheric plasma. This interaction produces high-energy X-rays (the first peak) and heats the plasma, causing it to emit thermally at lower-energy X-ray wavelengths after a short delay (the second peak).

Ning et al. 2018 Figure 1

Soft X-ray light curves from RHESSI and hard X-ray light curves from the Geostationary Operational Environmental Satellite (GOES). The two peaks are labeled Stage A and B. The double-peaked behavior is evident at almost all energies. Click to enlarge. [Ning et al. 2018]

What makes this particular confined flare interesting is the range of wavelengths that exhibit the double peak — reaching energies as high as 50 keV (30 million K). This is a departure from previous observations of double-peaked flares, which have a lower-energy secondary peak.

Because of the unusually high energy, Ning and collaborators conclude that the plasma must be heated not only indirectly, by collisions with particles accelerated by reconnection, but also directly, by reconnection itself. This observation of a super-hot plasma component in the second stage of a confined flare is the first of its kind — and surely further observations will reveal new details of how confined flares evolve, continuing to build our picture of what determines confinement vs. eruption!

Citation

Hao Ning et al 2018 ApJ 854 178. doi:10.3847/1538-4357/aaaa69

Pillars of Creation

On 1 April 1995, Hubble captured one of its most well-known images: a stunning photo of towering features known as the Pillars of Creation, located in the Eagle Nebula just 7,000 light-years away. A new study explores how these iconic columns are influenced by the magnetic fields within them.

Pillars from Shocks

Pillars of Creation magnetic fields

An illustrative figure of the BISTRO magnetic-field vectors observed in the Pillars of Creation, overlaid on a Hubble composite of the pillars. [Pattle et al. 2018]

In the Hubble image, we see the result of young, hot stars that have driven a photoionization shock into the cloud around them, forming complex structures in the dense gas at the shock interfaces. These structures — in this case, dense columns of neutral gas and dust — are then bombarded with hot radiation from the young stars, giving the structures a misty, ethereal look as they photoevaporate.

Though we have a rough picture, the specifics of how the Pillars of Creation were formed and how they evolve in this harsh radiation environment aren’t yet fully understood. In particular, the role of magnetic fields in shaping and sustaining these pillars is poorly constrained, both observationally and theoretically.

To address this problem, a team of scientists led by Kate Pattle (University of Central Lancashire, UK and National Tsing Hua University, Taiwan), has now made the first direct observations of the magnetic-field morphology within the Pillars of Creation.

Pillars of Creation formation schematic

The authors’ proposed formation scenario: a) an ionization front approaches an overdensity in the molecular gas, b) the front is slowed at the overdensity, causing the magnetic field lines to bend, c) the compressed magnetic field supports the pillar against radial collapse, but can’t support against longitudinal erosion. [Adapted from Pattle et al. 2018]

Observing Fields

Pattle and collaborators imaged the pillars as a part of the B-Fields in Star-Forming Region Observations (BISTRO) project, which uses a camera and polarimeter mounted on the James Clerk Maxwell Telescope in Hawaii. The high-resolution, submillimeter-wavelength polarimetric observations allowed the team to measure the orientations of the magnetic fields within the pillars.

Pattle and collaborators found that the magnetic fields inside the Pillars of Creation are actually quite organized: they generally run along the length of the pillars, perpendicular to and decoupled from the field in the surrounding cloud. The authors use their observations to estimate the strength of the fields: roughly 170–320 µG in the pillars.

Magnetic Support

What do these results tell us? First, the strength of the fields is consistent with a formation scenario in which very weakly magnetized gas was compressed to form columns. The authors propose that the Pillars of Creation were formed when an ionization front — driven by radiation from nearby young, hot stars — encountered a dense clump as it moved through the cloud of molecular gas. The overdensity slowed the front, causing the magnetic field to bend as the surrounding gas moved. The compressed magnetic field then supported the resulting column from collapse.

Pattle and collaborators argue that the magnetic fields in the Pillars of Creation are supporting the pillars radially against collapse even now. They may also be preventing the pillar ends from breaking off into disconnected clumps known as cometary globules, a process that could eventually disintegrate the pillars.

So what’s BISTRO up to now? The project is continuing to survey magnetic fields in the dense gas of other nearby high-mass star-forming regions. This may help confirm the results found for the Pillars of Creation, bringing us another step closer to understanding how magnetic fields influence the some of the striking features that Hubble and other telescopes have revealed in our astronomical backyard.

Citation

Kate Pattle et al 2018 ApJL 860 L6. doi:10.3847/2041-8213/aac771

Earth from space

There’s no hiding — changes in Earth’s atmosphere over the seasons are a dead giveaway to the fact that Earth hosts life. Now a new study explores whether we might use atmospheric seasonality like Earth’s to detect life on other planets.

Looking for Change

Most of the searches for life beyond our planet focus on identifying static biosignatures, like the presence of methane or large amounts of oxygen in an exoplanetary atmosphere. This approach suffers from many ambiguities, however — including a high likelihood of false positives (processes that chemically mimic life signatures but aren’t life) and false negatives (non-detections despite the presence of life).

CO2 and CH4 levels

Earth’s atmospheric carbon dioxide (top) and methane (bottom) levels vary seasonally, as seen in these data from NOAA’s Earth System Research Laboratory. [Olson et al. 2018]

In a new study led by Stephanie Olson (UC Riverside and NASA Astrobiology Institute Alternative Earths and Virtual Planetary Laboratory Teams), a team of scientists has proposed an alternative approach: to search for distinctive variability of exoplanet atmospheres that indicates the presence of life.

Seasons and Life

if you’re like me, you probably haven’t spent a lot of time thinking about interactions between the Earth’s biosphere and its axial tilt. Nonetheless, this interplay is responsible for detectable and distinctive seasonal changes in our planet’s atmosphere!

seasonal variations in gases

This schematic shows how oxygen and carbon dioxide levels in the atmosphere vary in opposing phase seasonally, with the increased sunlight in summer driving greater conversion of carbon dioxide into oxygen. [Olson et al. 2018]

Since so much of our globe is covered by photosynthesizing life, the seasonal availability of sunlight regulates the conversion of carbon dioxide to oxygen, providing a signature in our atmosphere that varies over the course of the year. And photosynthesis isn’t the only culprit! Other biological products evolve seasonally as well — as the surface temperature on our globe changes throughout the year, biological rates, gas solubility, precipitation patterns, and more all respond accordingly.

Olson and collaborators ask a simple question: if our atmosphere varies distinctively in a way that reveals the presence of life on Earth, can we search for similar variation on other planets?

Gaseous Signatures

To answer this question, Olson and collaborators examine the potential for seasonal variation of several atmospheric gases: carbon dioxide, methane, molecular oxygen, and ozone. For a weakly oxygenated planet (like early Earth), the authors find that a detectable indicator of life may be seasonal variations in the strength of ozone spectral bands at ultraviolet wavelengths. This variation serves as a tracer of the seasonality of molecular oxygen.

seasonal O3 spectral line

On a planet with the right conditions, seasonal oxygen oscillations could create an observable difference in the depth of the ozone spectral line, as shown here. [Adapted from Olson et al. 2018]

To discover such a signature in the atmospheres of distant planets, we’ll likely need extended direct imaging; transit spectroscopy, such as that expected from the James Webb Space Telescope, will probe planets at only one point in their orbits, precluding the detection of seasonal changes. Olson and collaborators therefore advocate that upcoming direct-imaging missions, like LUVOIR and HabEx, include ultraviolet observing capabilities.

What is the likelihood that we’ll actually be able to detect seasonal changes in the atmospheric gases of distant exoplanets? More detailed modeling will need to be performed to say for certain — but in the meantime, this study presents an interesting additional technique we can add to our arsenal and explore further in the future!

Citation

Stephanie L. Olson et al 2018 ApJL 858 L14. doi:10.3847/2041-8213/aac171

NGC 4261

What are the feeding — and burping — habits of the supermassive black holes peppering the universe? In a new study, observations of one such monster reveal more about the behavior of its powerful jets.

Beams from Behemoths

Across the universe, supermassive black holes of millions to billions of solar masses lie at the centers of galaxies, gobbling up surrounding material. But not all of the gas and dust that spirals in toward a black hole is ultimately swallowed! A large fraction of it can instead be flung out into space again, in the form of enormous, powerful jets that extend for thousands or even millions of light-years in opposite directions.

M87

M87, shown in this Hubble image, is a classic example of a nearby (55 million light-years distant) supermassive black hole with a visible, collimated jet. Its counter-jet isn’t seen because relativistic effects make the receding jet appear less bright. [The Hubble Heritage Team (STScI/AURA) and NASA/ESA]

What causes these outflows to be tightly beamed — collimated — in the form of jets, rather than sprayed out in all directions? Does the pressure of the ambient medium — the surrounding gas and dust that the jet is injected into — play an important role? In what regions do these jets accelerate and decelerate? There are many open questions that scientists hope to understand by studying some of the active black holes with jets that live closest to us.

Eyes on a Nearby Giant

In a new study led by Satomi Nakahara (The Graduate University for Advanced Studies in Japan), a team of scientists has used multifrequency Very Long Baseline Array (VLBA) and Very Long Array (VLA) images to explore jets emitted from a galaxy just 100 million light-years away: NGC 4261.

This galaxy’s (relatively) close distance — as well as the fact that we’re viewing it largely from the side, so we can clearly see both of its polar jets — allows us to observe in detail the structure and intensity of its jets as a function of their distance from the black hole. Nakahara and collaborators’ observations span the enormous radial distance of a thousand to a billion times the radius of the black hole, or about 54 light-days to more than a million light-years.

Scale for Change

width vs. radius of jet

The width of the jet as a function of radial distance from the black hole, for NGC 4261 (red) compared to the few other jets from nearby supermassive black holes that we’ve measured. NGC 4261’s jets transition from parabolic to conical at around 10,000 times the radius of the black hole (RS). [Nakahara et al. 2018]

The authors’ observations of NGC 4261’s jets indicate that a transition occurs at ~10,000 times the radius of the black hole (that’s a little over a light-year from the black hole). At this point, the jets’ structures change from parabolic (becoming more tightly beamed) to conical (expanding freely). Around the same location, Nakahara and collaborators also see the radiation profile of one of the jets change, suggesting the physical conditions in the jets transition here as well.

This is the first time we’ve been able to examine jet width this closely for both of the jets emitted from a supermassive black hole. The fact that the structure changes at the same distance for both jets indicates that the shape of these powerful streams is likely governed by global properties of the environment surrounding the galaxy’s nucleus, or properties of the jets themselves, rather than by a local condition.

The authors next hope to pin down velocities inside NGC 4261’s jets to determine where the jets accelerate and decelerate. This nearby powerhouse is clearly going to be a useful laboratory in the future, helping to unveil the secrets of more distant, feeding monsters.

Bonus

Curious what these hungry supermassive black holes look like? Check out this artist’s imagining of NGC 4261, which shows how it feeds from a large, swirling accretion disk and emits fast-moving, collimated jets. [Original video credit to Dana Berry, Space Telescope Science Institute]

Citation

Satomi Nakahara et al 2018 ApJ 854 148. doi:10.3847/1538-4357/aaa45e

Milky Way

Weighing galaxies is a tricky business — especially when that galaxy is our own! In a recent study, scientists have tackled this problem by harnessing incredibly precise measurements of the motions of Milky-Way satellites.

A Challenging Measurement

satellites of the Milky Way

Locations of some of the ~50 satellite galaxies known around the Milky Way. [AndrewRT]

Our spot in the middle of our galaxy’s disk makes it difficult for us to assess the total mass of gas, dust, stars, and dark matter surrounding us; estimates for the Milky Way’s mass span from 700 billion to 2 trillion solar masses! Pinning down this number is critical for better understanding the structure and dynamics of our local universe.

So what’s the key to precisely weighing the Milky Way? A new study led by Ekta Patel (University of Arizona) — presented at the American Astronomical Society meeting two weeks ago — suggests it may be the barely preceptible motions of the small satellite galaxies that orbit around the Milky Way. Around 50 Milky-Way satellites are currently known, and simulations suggest that there may be up to 100–200 in total. By watching the motions of these satellites, we can trace the potential of their host — the Milky Way — and estimate its mass.

Illustris cosmological simulation

The Illustris-Dark simulation evolves our universe to the present day, providing a view of how dark matter organizes itself into galaxy halos over time. [Illustris Collaboration]

Tiny Motions of Tiny Galaxies

In this era of precision astronomy, remarkable measurements are becoming possible. In their study, Patel and collaborators use years of proper-motion observations from the Hubble Space Telescope for nine satellite galaxies of the Milky Way. The precision needed for measurements like these is insane: watching these satellites move is roughly like watching a human hair grow at the distance of the Moon.

Rather than using the instantaneous position and velocity measured for a satellite — which changes over time during the satellite’s orbit — Patel and collaborators demonstrate that the satellite’s specific angular momentum is a more useful parameter when attempting to estimate its host galaxy’s mass.

For each of the nine individual satellite galaxies, the authors compare its measured momentum to that of ~90,000 simulated satellite galaxies from the Illustris-Dark cosmological simulation. This matching is used to build a probability distribution for the mass of the host galaxy most likely to be orbited by such a satellite. The probability distributions for the nine satellite galaxies are then combined to find the best overall estimate for the Milky Way’s mass.

Tipping the Scale

Milky-Way mass estimates

Top: summary of the most likely Milky-Way mass estimated from each of the 9 satellite galaxies, using the instantaneous positions and velocities (left) and the momentum (right) of the satellites. The momentum method shows less scatter in the host masses. Bottom: probability distributions for the most likely Milky-Way mass for each of the satellites (colored curves) and combined (grey curve). Click for a better look. [Patel et al. 2018]

Using this technique, Patel and collaborators find a mass of 0.96 trillion solar masses for the Milky Way. The error bars for their measurement are around 30% — and while this is more confined than the broad range of past estimates, it’s not yet extremely precise. The beauty of Patel and collaborators’ method, however, is that it is both extendable and generalizable.

The authors only had access to precise proper motions for nine satellite galaxies when they conducted their study — but since then, the Gaia mission has provided measurements for 30 satellites, with more expected in the future. Including these additional satellites and using improved, higher-resolution cosmological simulations for comparison will continue to increase the precision of Patel and collaborators’ estimate in the future.

In addition, this approach can also be used to weigh our neighboring Andromeda galaxy, or any other galaxy for which we’re able to get precise proper-motion measurements for its satellites. Keep an eye out in the future, as techniques like this continue to reveal more properties of our local universe.

Citation

Ekta Patel et al 2018 ApJ 857 78. doi:10.3847/1538-4357/aab78f

KELT-9b

Move over, hot Jupiters — there’s an even stranger kind of giant planet in the universe! Ultra-hot Jupiters are so strongly irradiated that the molecules in their atmospheres split apart. What does this mean for heat transport on these planets?

Atmospheres of Exotic Planets

Ultra-hot Jupiter diagram

A diagram showing the orbit of an ultra-hot Jupiter and the longitudes at which dissociation and recombination occur. [Bell & Cowan 2018]

Similar to hot Jupiters, ultra-hot Jupiters are gas giants with atmospheres dominated by molecular hydrogen. What makes them interesting is that their dayside atmospheres are so hot that the molecules dissociate into individual hydrogen atoms — more like the atmospheres of stars than planets.

Because of the intense stellar irradiation, there is also an extreme temperature difference between the day and night sides of these planets — potentially more than 1,000 K! As the stellar irradiation increases, the dayside atmosphere becomes hotter and hotter and the temperature difference between the day and night sides increases.

When hot atomic hydrogen is transported into cooler regions (by winds, for instance), it recombines to form H2 molecules and heats the gas, effectively transporting heat from one location to another. This is similar to how the condensation of water redistributes heat in Earth’s atmosphere — but what effect does this phenomenon have on the atmospheres of ultra-hot Jupiters?

Planetary maps

Maps of atmospheric temperature of molecular hydrogen dissociation fraction for three wind speeds. Click to enlarge. [Bell & Cowan 2018]

Modeling Heat Redistribution

Taylor Bell and Nicolas Cowan (McGill University) used an energy-balance model to estimate the effects of H2 dissociation and recombination on heat transport in ultra-hot Jupiter atmospheres. In particular, they explored the redistribution of heat and how it affects the resultant phase curve — the curve that describes the combination of reflected and thermally emitted light from the planet, observed as a function of its phase angle.

For reasonable eastward wind speeds, Bell and Cowan found that the recombination of atomic hydrogen shifts the peak of the phase curve in the eastward direction, with the shift becoming more pronounced with increasing eastward wind speed. Additionally, because heat is distributed more evenly across the planet, including this process decreases the amplitude of the phase variations.

A Bright Future for Ultra-hot Jupiters

Modeled phase curves

Theoretical phase curves for three wind speeds. Transits and eclipses have been neglected. [Bell & Cowan 2018]

While this simple model doesn’t include potentially important effects such as the changing atmospheric opacity as a function of longitude or formation of clouds on the planet’s nightside, this result indicates that caution is required when interpreting phase curves of ultra-hot Jupiters. For example, neglecting recombination means assuming a lower heat transport efficiency, which will require artifically high wind speeds to match observed phase curves.

Only a few ultra-hot Jupiters are currently known, but that will soon change. The Transiting Exoplanet Survey Satellite (TESS) mission, which is set to begin its first science observations on June 17, 2018, will search for exoplanets around bright stars, including nearby cool stars and more distant hot stars. The hot stars may play host to these exotic exoplanets, and upcoming observations of ultra-hot Jupiters like KELT-9b will put this theory of heat redistribution to the test.

Citation

Taylor J. Bell & Nicolas B. Cowan 2018 ApJL 857 L20. doi:10.3847/2041-8213/aabcc8

nascent planets

Occasionally, science comes together beautifully for a discovery — and sometimes this happens for more than one team at once! Today we explore how two independent collaborations of scientists simultaneously found the very first kinematic evidence for young planets forming in a protoplanetary disk. Though they explored the same disk, the two teams in fact discovered different planets.

Evidence for Planets

HD 163296

ALMA’s view of the dust in the protoplanetary disk surrounding the young star HD 163296. Today’s studies explore not the dust, but the gas of this disk. [ALMA (ESO/NAOJ/NRAO); A. Isella; B. Saxton (NRAO/AUI/NSF)]

Over the past three decades, we’ve detected around 4,000 fully formed exoplanets. Much more elusive, however, are the young planets still in the early stages of formation; only a handful of these have been discovered. More observations of early-stage exoplanets are needed in order to understand how these worlds are born in dusty protoplanetary-disk environments, how they grow their atmospheres, and how they evolve.

Recent observations by the Atacama Large Millimeter/submillimeter Array (ALMA) have produced stunning images of protoplanetary disks. The unprecedented resolution of these images reveals substructure in the form of gaps and rings, hinting at the presence of planets that orbit within the disk and clear out their paths as they move. But there are also non-planet mechanisms that could produce such substructure, like grain growth around ice lines, or hydrodynamic instabilities in the disk.

How can we definitively determine whether there are nascent planets embedded in these disks? Direct direction of a point source in a dust gap would be a strong confirmation, but now we have the next best thing: kinematic evidence for planets, from the motion of a disk’s gas.

velocity kink

Observations of carbon monoxide line emission at +1km/s from the systemic velocity (left) vs. the outcome of a computer simulation (right) in the Pinte et al. study. A visible kink occurs in the flow, which can be reproduced by the presence of a 2-Jupiter-mass planet at 260 AU. [Pinte et al. 2018]

Watching Gas Move

In two papers published today in ApJL — one led by Richard Teague (University of Michigan) and the other led by Christophe Pinte (Monash University in Australia and Grenoble Alpes University in France) — astronomers have announced the detection of distinctive signs of planets in the gas motion of the disk surrounding HD 163296. This young star, located about 330 light-years away, is only ~4 million years old.

Unlike studies that hinge on observations of a disk’s dust — which only makes up ~1% of the disk’s mass! — both studies here took a new approach: they used detailed ALMA observations revealing the dynamics of the disk’s carbon monoxide gas. By studying the gas’s motion, the teams found deviations from the Keplerian velocity that would be expected if there were no planets present. The authors then ran simulations to demonstrate that the deviations are consistent with local pressure perturbations caused by the passage of giant planets.

velocity deviations caused by planets

Rotational velocity deviations due to changes in the local pressure, caused in this simulation by the presence of planets. [Teague et al. 2018]

Giants Found

What did they find? Teague and collaborators, whose technique to identify velocity variations is best suited to explore the inner regions of the disk, discovered evidence for two separate Jupiter-mass planets orbiting at distances of 83 AU and 137 AU in the disk. Pinte and collaborators, whose velocity-measurement technique better explores the outer regions of the disk, found evidence for a two-Jupiter-mass planet orbiting at 260 AU.

These results will rely on additional imaging in the coming years to confirm the presence of these newly born planets — and a detection of point sources at these radii remains a hopeful goal for the future. Nonetheless, the new techniques explored here by Teague, Pinte, and collaborators are a promising route for young exoplanet discovery and characterization in other disks imaged by ALMA and future instruments.

Citation

Richard Teague et al 2018 ApJL 860 L12. doi:10.3847/2041-8213/aac6d7
C. Pinte et al 2018 ApJL 860 L13. doi:10.3847/2041-8213/aac6dc

neutron-star merger

When two neutron stars merged in August of last year, leading to the first simultaneous detection of gravitational waves and electromagnetic signals, we knew this event was going to shed new light on compact-object mergers.

A team of scientists says we now have an answer to one of the biggest mysteries of GW170817: after the neutron stars collided, what object was formed?

black hole

Artist’s illustration of the black hole that resulted from GW170817. Some of the material accreting onto the black hole is flung out in a tightly collimated jet. [NASA/CXC/M.Weiss]

A Fuzzy Division

Based on gravitational-wave observations, we know that two neutron stars of about 1.48 and 1.26 solar masses merged in GW170817. But the result — an object of ~2.7 solar masses — doesn’t have a definitive identity; the remnant formed in the merger is either the most massive neutron star known or the least massive black hole known.

The theoretical mass division between neutron stars and black holes is fuzzy, depending strongly on what model you use to describe the physics of these objects. Observations fall short as well: the most massive neutron star known is perhaps 2.3 solar masses, and the least massive black hole is perhaps 4 or 5, leaving the location of the dividing line unclear. For this reason, determining the nature of GW170817’s remnant is an important target as we analyze past observations of the remnant and continue to make new ones.

Chandra GW170817

Chandra images of the field of GW170817 during three separate epochs. Each image is 30” x 30”. [Adapted from Pooley et al. 2018]

Luckily, we may not have long to wait! Led by David Pooley (Trinity University and Eureka Scientific, Inc.), a team of scientists has obtained new Chandra X-ray observations of the remnant of GW170817. By combining this new data with previous observations, the authors have drawn conclusions about what object was left behind after this fateful merger.

X-Rays Provide Answers

X-ray radiation is generated in a merger of two neutron stars when the merger’s shock wave expands and slams into the surrounding interstellar medium. The earliest X-ray detection from GW170817 — around 9 days after the merger — likely indicated the moment when that interaction began. GW170817’s X-ray emission continued to grow over the first ~100 days post-merger, expected as the shock continues to expand.

If the merger had produced a neutron star, however, there should be an additional source of X-ray radiation besides the shock: the neutron star itself. This emission should, by now, have started to dominate over the emission from the propagating shock. Instead, Pooley and collaborators find that the observed X-ray flux from GW170817 falls significantly short of what’s needed to justify the presence of a highly magnetized, spinning neutron star. For this reason, the authors conclude that GW170817 likely produced a black hole.

Future Confirmation

How can we be sure? Pooley and collaborators point out that we can confirm this theory just by observing GW170817 for another year. Around this time, energy released from the spin-down of a central neutron star would catch up to the decelerating shock front, causing a dramatic brightening in GW170817’s X-ray flux. 

If we don’t see this brightening, the authors argue that we can conclude with certainty that GW170817’s remnant is a black hole. Either way, continued observations of this remnant are sure to provide a wealth of information about the physics of mergers, shocks, and outflows that we can hope to mine for years to come.

Citation

David Pooley et al 2018 ApJL 859 L23. doi:10.3847/2041-8213/aac3d6

Mars atmosphere

In the search for life elsewhere in the universe, Mars has always represented an important target for exploration. What can this planet’s evolution — from a potentially habitable world to its current inhospitable state — tell us about the possibility of life on habitable exoplanets beyond our solar system?

A Past of Contrast

Mars ocean

Artist’s illustration of an ancient ocean on Mars. [NASA/GSFC]

Life on Mars has been a topic of speculation for well over a century. Today, scientists no longer consider it very likely that inhospitable Mars is inhabited by alien life — but the red planet wasn’t always so unwelcoming.

While today’s Mars is an arid desert, mounting evidence suggests that ancient Mars — the Mars of 4 billion years ago — not only had aqueous environments, but possibly even global oceans. Evidence for minerals, biogenic elements, and suitable energy sources for prebiotic chemistry increase the prospects for ancient Mars’s habitability.

So what caused the difference between this welcoming ancient Mars and the inhospitable Mars of the current epoch? Ancient Mars had one more thing: a thick protective atmosphere. Today’s Mars has only a thin, tenuous atmosphere remaining between its surface and the hostile conditions of space.

In a new theoretical study led by Chuanfei Dong (Princeton University), a team of scientists explore how Mars may have lost its atmosphere, causing the planet’s transformation to its current state.

atmospheric escape rates

Results from the authors’ simulations showing the ion (total ion shown in blue) and photochemical (heated atomic oxygen shown in magenta) escape rates over the Martian history. Ion escape rates were much higher 4 billion years ago. [Dong et al. 2018]

Loss of an Atmosphere

Dong and collaborators use sophisticated global 3D simulations of a Mars-like body to explore different types of atmospheric loss over time. By including the evolution of the ultraviolet radiation levels and solar wind strength, the authors can explore how atmospheric escape rates for ions and atoms have changed over Mars’s history.

Dong and collaborators find that Mars’s atmospheric ion escape rate, in particular, was more than two orders of magnitude higher 4 billion years ago compared to the present-day level. This is a result of the stronger solar wind and higher ultraviolet fluxes that came from the young Sun.

These high ion escape rates are more than enough to explain the rapid loss of Mars’s thick atmosphere very early in its lifetime — which leads to the depletion of any surface water Mars might have once had. Dong and collaborators argue that Mars might once have had a global ocean at least 2.6 m deep; according to the authors’ simulations, this water would have been depleted within 4 billion years, resulting in the arid Mars we observe today.

Beyond Our Solar System

exoplanet atmosphere

Artist’s impression of a rocky exoplanet with a thick water atmosphere. [MPIA]

What implications do these discoveries have on our search for life beyond our solar system? Early Mars may well be a prototype for small, rocky, potentially habitable planets orbiting solar-type stars. In this case, the authors’ results suggest that early atmospheric escape may be common among such planets, preventing life from easily persisting. The time-dependence of atmospheric loss is therefore an important element to keep in mind as we choose our targets and explore exoplanet candidates in the future.

Citation

Chuanfei Dong et al 2018 ApJL 859 L14. doi:10.1088/0004-637X/810/2/136

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