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NGC 3201

How many black holes lurk within the dense environments of globular clusters, and how do these powerful objects shape the properties of the cluster around them? One such cluster, NGC 3201, is now helping us to answer these questions.

Hunting Stellar-Mass Black Holes

Since the detection of merging black-hole binaries by the Laser Interferometer Gravitational-Wave Observatory (LIGO), the dense environments of globular clusters have received increasing attention as potential birthplaces of these compact binary systems.

NGC 3201 center

The central region of the globular star cluster NGC 3201, as viewed by Hubble. The black hole is in orbit with the star marked by the blue circle. [NASA/ESA]

In addition, more and more stellar-mass black-hole candidates have been observed within globular clusters, lurking in binary pairs with luminous, non-compact companions. The most recent of these detections, found in the globular cluster NGC 3201, stands alone as the first stellar-mass black hole candidate discovered via radial velocity observations: the black hole’s main-sequence companion gave away its presence via a telltale wobble.

Now a team of scientists led by Kyle Kremer (CIERA and Northwestern University) is using models of this system to better understand the impact that black holes might have on their host clusters.

A Model Cluster

The relationship between black holes and their host clusters is complicated. Though the cluster environment can determine the dynamical evolution of the black holes, the retention rate of black holes in a globular cluster (i.e., how many remain in the cluster when they are born as supernovae, rather than being kicked out during the explosion) influences how the host cluster evolves.

Kremer and collaborators track this complex relationship by modeling the evolution of a cluster similar to NGC 3201 with a Monte Carlo code. The code incorporates physics relevant to the evolution of black holes and black-hole binaries in globular clusters, such as two-body relaxation, single and binary star evolution, galactic tides, and multi-body encounters. From their grid of models with varying input parameters, the authors then determine which fit best to NGC 3201’s final observational properties.

globular cluster models

Surface brightness profiles for all globular-cluster models at late times compared to observations of NGC 3201 (yellow circles). Blue lines represent models with few retained black holes; black lines represent models with many retained black holes. [Kremer et al. 2018]

Retention Matters

Kremer and collaborators find that the models that best represent NGC 3201 all retain more than 200 black holes at the end of the simulation; models that lost too many black holes due to natal kicks did not match observations of NGC 3201 as well. The models with large numbers of retained black holes also harbored binaries just like the one recently detected in NGC 3201.

Models that retain few black holes, on the other hand, may instead be good descriptions of so-called “core-collapsed” globular clusters observed in the Milky Way. The authors demonstrate that these clusters could contain black holes in binaries with stars known as blue stragglers, which may also be detectable with radial velocity techniques.

Kremer and collaborators’ results suggest that globular clusters similar to NGC 3201 contain hundreds of invisible black holes waiting to be discovered, and they indicate some of the differences in cluster properties caused by hosting such a large population of black holes. We can hope that future observations and modeling will continue to illuminate the complicated relationship between globular clusters and the black holes that live in them.

Citation

Kyle Kremer et al 2018 ApJL 855 L15. doi:10.3847/2041-8213/aab26c

Voyager 1's journey

In 2012, Voyager 1 zipped across the heliopause. Five and a half years later, Voyager 2 still hasn’t followed its twin into interstellar space. Can models of the heliopause location help determine why?

How Far to the Heliopause?

heliosphere

Artist’s conception of the heliosphere with the important structures and boundaries labeled. [NASA/Goddard/Walt Feimer]

As our solar system travels through the galaxy, the solar outflow pushes against the surrounding interstellar medium, forming a bubble called the heliosphere. The edge of this bubble, the heliopause, is the outermost boundary of our solar system, where the solar wind and the interstellar medium meet. Since the solar outflow is highly variable, the heliopause is constantly moving — with the motion driven by changes in the Sun.

NASA’s twin Voyager spacecraft were poised to cross the heliopause after completing their tour of the outer planets in the 1980s. In 2012, Voyager 1 registered a sharp increase in the density of interstellar particles, indicating that the spacecraft had passed out of the heliosphere and into the interstellar medium. The slower-moving Voyager 2 was set to pierce the heliopause along a different trajectory, but so far no measurements have shown that the spacecraft has bid farewell to our solar system.

In a recent study, a team of scientists led by Haruichi Washimi (Kyushu University, Japan and CSPAR, University of Alabama-Huntsville) argues that models of the heliosphere can help explain this behavior. Because the heliopause location is controlled by factors that vary on many spatial and temporal scales, Washimi and collaborators turn to three-dimensional, time-dependent magnetohydrodynamics simulations of the heliosphere. In particular, they investigate how the position of the heliopause along the trajectories of Voyager 1 and Voyager 2 changes over time.

Modeled location of the heliopause

Modeled location of the heliopause along the paths of Voyagers 1 (blue) and 2 (orange). Click for a closer look. The red star indicates the location at which Voyager 1 crossed the heliopause. The current location of Voyager 2 is marked with a red circle. [Washimi et al. 2017]

A Time-Varying Barrier

The authors consider the impact that solar flares, coronal mass ejections, and other disturbances in the solar outflow have on the heliopause distance. These solar disturbances intermingle as they travel outward to form what the authors call global merged interaction regions.

Using their hydrodynamical simulations, Washimi and collaborators capture the complex behavior of the global merged interaction regions as they propagate through the termination shock and collide with the heliopause. Part of the shock is transmitted into the local interstellar medium, while part of it is reflected back toward and collides with the termination shock, which is pushed toward the Sun. This complex interplay of transmitted and reflected shocks — combined with the nonuniformity of the local interstellar medium — causes the heliopause location to vary dramatically in time as well as space.

What Does this Mean for Voyager 2?

Washimi and collaborators find that the location of the heliopause along the trajectories of Voyagers 1 and 2 has changed considerably over the past decade. In particular, they find that the heliopause has been pushed outward over the past few years due to an increase in the solar wind ram pressure. According to their simulations, Voyager 2 is currently traveling outward faster than the heliopause is advancing, which means that the spacecraft should soon cross the boundary — perhaps even this year — to become Earth’s second interstellar messenger.

Citation

Haruichi Washimi et al 2017 ApJL 846 L9. doi:10.3847/2041-8213/aa8556

NICER

What happens to a neutron star’s accretion disk when its surface briefly explodes? A new instrument recently deployed at the International Space Station (ISS) is now watching bursts from neutron stars and reporting back.

Deploying a New X-Ray Mission

NICER launch

Launch of NICER aboard a Falcon 9 rocket in June 2017. [NASA/Tony Gray]

In early June of 2017, a SpaceX Dragon capsule on a Falcon 9 rocket launched on a resupply mission to the ISS. The pressurized interior of the Dragon contained the usual manifest of crew supplies, spacewalk equipment, and vehicle hardware. But the unpressurized trunk of the capsule held something a little different: the Neutron star Interior Composition Explorer (NICER).

In the two weeks following launch, NICER was extracted from the SpaceX Dragon capsule and installed on the ISS. And by the end of the month, the instrument was already collecting its first data set: observations of a bright X-ray burst from Aql X-1, a neutron star accreting matter from a low-mass binary companion.

Impact of Bursts

NICER’s goal is to provide a new view of neutron-star physics at X-ray energies of 0.2–12 keV — a window that allows us to explore bursts of energy that neutron stars sometimes emit from their surfaces.

X-ray binary

Artist’s impression of an X-ray binary, in which a compact object accretes material from a companion star. [ESA/NASA/Felix Mirabel]

In X-ray burster systems, hydrogen- and helium-rich material from a low-mass companion star piles up in an accretion disk around the neutron star. This material slowly funnels onto the neutron star’s surface, forming a layer that gravitationally compresses and eventually becomes so dense and hot that runaway nuclear fusion ignites.

Within seconds, the layer of material is burned up, producing a burst of emission from the neutron star that outshines even the inner regions of the hot accretion disk. Then more material funnels onto the neutron star and the process begins again.

Though we have a good picture of the physics that causes these bursts, we don’t yet understand the impact that these X-ray flashes have on the accretion disk and the environment surrounding the neutron star. In a new study led by Laurens Keek (University of Maryland), a team of scientists now details what NICER has learned on this subject.

Extra X-Rays

Aql X-1 burst

Light curve (top) and hardness ratio (bottom) for the X-ray burst from Aql X-1 captured by NICER on 3 July 2017. [Keek et al. 2018]

In addition to thermal emission from the neutron star, NICER revealed an excess of soft X-ray photons below 1 keV during Aql X-1’s burst. The authors propose two possible models for this emission:

  1. The burst radiation from the neutron star’s surface was reprocessed — i.e., either scattered or absorbed and re-emitted — by the accretion disk.
  2. The persistent, usual accretion flow was enhanced as a result of the burst’s radiation drag on the disk, briefly bumping up the disk’s X-ray flux.

While we can’t yet conclusively state which mechanism dominates, NICER’s observations do show that bursts have a substantial impact on their accretion environment. And, as there are over 100 such X-ray burster systems in our galaxy, we can expect that NICER will allow us to better explore the effect of X-ray bursts on neutron-star disks and their surroundings in many different systems in the future.

Bonus

Check out the awesome gif below, provided by NASA, which shows NICER being extracted from the Dragon capsule’s trunk by a robotic arm.

Citation

L. Keek et al 2018 ApJL 855 L4. doi:10.3847/2041-8213/aab104

Cassiopeia A supernova remnant

Supernova explosions enrich the interstellar medium and can even briefly outshine their host galaxies. However, the mechanism behind these massive explosions still isn’t fully understood. Could probing the asymmetry of supernova remnants help us better understand what drives these explosions?

SN1987a

Hubble image of the remnant of supernova 1987A, one of the first remnants discovered to be asymmetrical. [ESA/Hubble, NASA]

Stellar Send-Offs

High-mass stars end their lives spectacularly. Each supernova explosion churns the interstellar medium and unleashes high-energy radiation and swarms of neutrinos. Supernovae also suffuse the surrounding interstellar medium with heavy elements that are incorporated into later generations of stars and the planets that form around them.

The bubbles of expanding gas these explosions leave behind often appear roughly spherical, but mounting evidence suggests that many supernova remnants are asymmetrical. While asymmetry in supernova remnants can arise when the expanding material plows into the non-uniform interstellar medium, it can also be an intrinsic feature of the explosion itself.

Single-lobe explosion

Simulation results clockwise from top left: Mass density, calcium mass fraction, oxygen mass fraction, nickel-56 mass fraction. Click to enlarge. [Adapted from Wollaeger et al. 2017]

Coding Explosions

The presence — or absence — of asymmetry in a supernova remnant can hold clues as to what drove the explosion. But how can we best observe asymmetry in a supernova remnant? Modeling lets us explore different observational approaches.

A team of scientists led by Ryan T. Wollaeger (Los Alamos National Laboratory) used radiative transfer and radiative hydrodynamics simulations to model the explosion of a core-collapse supernova. Wollaeger and collaborators introduced asymmetry into the explosion by creating a single-lobed, fast-moving outflow along one axis.

Their simulations showed that while some chemical elements lingered near the origin of the explosion or were distributed evenly throughout the remnant, calcium was isolated to the asymmetrical region, hinting that spectral lines of calcium may be good tracers of asymmetry.

Synthetic light curves

Bolometric (top) and gamma-ray (bottom) synthetic light curves for the authors’ model for a range of simulated viewing angles. [Adapted from Wollaeger et al. 2017]

Synthesizing Spectra

Wollaeger and collaborators then generated synthetic light curves and spectra from their models to determine which spectral features or characteristics indicated the presence of the asymmetric outflow lobe. They found that when an asymmetric outflow lobe is present, the peak luminosity of the explosion depends on the angle at which you view it; the highest luminosity occurs when the lobe is viewed from the side, while the lowest luminosity — nearly 40% dimmer — is seen when the explosion is viewed “down the barrel” of the lobe. The dense outflow shades the central radioactive source from view, lowering the luminosity.

This effect also plays out in the gamma-ray light curves; when viewed down the barrel, the shading of the central source by a high-density lobe slows the rise of the gamma-ray luminosity and changes the shape of the light curve compared to views from other vantage points.

Another promising avenue for exploring asymmetry is a near-infrared band encompassing an emission line of singly-ionized calcium near 815 nm. Since calcium is confined within the outflow lobe in the simulation, its emission lines are blueshifted when the lobe points toward the observer.

The authors point out that there is much more to be done in their models, such as including the effects of shock heating of circumstellar material, which can contribute strongly to the light curve, but these simulations bring us a step closer to understanding the nature of asymmetrical supernova remnants — and the explosions that create them.

Citation

Ryan T. Wollaeger et al 2017 ApJ 845 168. doi:10.3847/1538-4357/aa82bd

exomoon

close-encounter outcomes

Four examples of close-encounter outcomes: a) the moon stays in orbit around its host, b) the moon is captured into orbit around its perturber, c) and d) the moon is ejected from the system from two different starting configurations. [Adapted from Hong et al. 2018]

Planet interactions are thought to be common as solar systems are first forming and settling down. A new study suggests that these close encounters could have a significant impact on the moons of giant exoplanets — and they may generate a large population of free-floating exomoons.

Chaos in the System

In the planet–planet scattering model of solar-system formation, planets are thought to initially form in closely packed systems. Over time, planets in a system perturb each other, eventually entering an instability phase during which their orbits cross and the planets experience close encounters.

During this “scattering” process, any exomoons that are orbiting giant planets can be knocked into unstable orbits directly by close encounters with perturbing planets. Exomoons can also be disturbed if their host planets’ properties or orbits change as a consequence of scattering.

Led by Yu-Cian Hong (Cornell University), a team of scientists has now explored the fate of exomoons in planet–planet scattering situations using a suite of N-body numerical simulations.

Chances for Survival

Hong and collaborators find that the vast majority — roughly 80 to 90% — of exomoons around giant planets are destabilized during scattering and don’t survive in their original place in the solar system. Fates of these destabilized exomoons include:

  • moon collision with the star or a planet,
  • moon capture by the perturbing planet,
  • moon ejection from the solar system,
  • ejection of the entire planet–moon system from the solar system, and
  • moon perturbation onto a new heliocentric orbit as a “planet”.

Unsurprisingly, exomoons that have close-in orbits and those that orbit larger planets are the most likely to survive close encounters; as an example, exomoons on orbits similar to Jupiter’s Galilean satellites (i.e., orbiting at a distance of less than 4% of their host planet’s Hill radius) have a ~20–40% chance of survival.

moon survival rates

Moon initial semimajor axis vs. moon survival rate. Three of Jupiter’s Galilean moons are shown for reference. [Hong et al. 2018]

Free-Floating Moons

An intriguing consequence of Hong and collaborators’ results is the prediction of a population of free-floating exomoons that were ejected from solar systems during planet–planet scattering and now wander through the universe alone. According to the authors’ models, there may be as many of these free-floating exomoons as there are stars in the universe!

Future surveys that search for objects using gravitational microlensing — like that planned with the Wide-Field Infrared Survey Telescope (WFIRST) — may be able to detect such objects down to masses of a tenth of an Earth mass. In the meantime, we’re a little closer to understanding the complex dynamics of early solar systems.

Citation

Yu-Cian Hong et al 2018 ApJ 852 85. doi:10.3847/1538-4357/aaa0db

Cygnus-X

How do you spot very young, newly formed stars? One giveaway is the presence of jets and outflows that interact with the stars’ environments. In a new study, scientists have now discovered an unprecedented number of these outflows in a nearby star-forming region of our galaxy.

Young Stars Hard at Work

map of outflows in Cygnus-X

CO map of the Cygnus-X region of the galactic plane, with the grid showing the UWISH2 coverage and the black triangles showing the positions of the detected outflows. [Makin & Froebrich 2018]

The birth and evolution of young stars is a dynamic, energetic process. As new stars form, material falls inward from the accretion disks surrounding young stellar objects, or YSOs. This material can power collimated streams of gas and dust that flow out along the stars’ rotation axes, plowing through the surrounding material. Where the outflows collide with the outside environment, shocks form that can be spotted in near-infrared hydrogen emission.

Though we’ve learned a lot about these outflows, there remain a number of open questions. What factors govern their properties, such as their lengths, luminosities, and orientations? What is the origin of the emission features we see within the jets, known as knots? What roles do the driving sources and the environments play in the behavior and appearance of the jets?

examples of outflows found in Cygnus-X

A selection of previously unknown outflows discovered as a result of this survey. Click for a closer look. [Makin & Froebrich 2018]

To answer these questions, we need to build a large, unbiased statistical sample of YSOs from across the galactic plane. Now, a large infrared survey — known as the UKIRT Widefield Infrared Survey for H2 (UWISH2) — is working toward that goal.

Jackpot in Cygnus-X

In a recent publication, Sally Makin and Dirk Froebrich (University of Kent, UK), present results from UWISH2’s latest release: a survey segment targeting a 42-square-degree region in the galactic plane known as the Cygnus-X star-forming region.

The team’s search for shock-excited emission in Cygnus-X yielded spectacular results. They found a treasure trove of outflows — a remarkable 572 in total, representing a huge increase over the 107 known previously.

Makin and Froebrich then measured properties of the outflows themselves — such as length, orientation, and flux — as well as properties of the sources that appear to drive them.

Pinning Down Properties

low-mass bright-rimmed cloud near IRAS 20294+4255

This low-mass bright-rimmed cloud near IRAS 20294+4255 contains a number of stellar outflows. It may warrant further study as a classical example of triggered star formation. [Makin & Froebrich 2018]

Of the 572 outflows, the authors found that 27% are one-sided jets and 46% are bipolar. The bipolar outflows are typically ~1.5 light-years in total length, and they are frequently asymmetric, with the shorter jet lobe averaging only 70% the length of the longer one. The flux from the two sides of bipolar jets is also often asymmetric: typically one side is brighter by about 50%.

Exploring the knots of bright emission within the outflows, the authors found that they are typically closely spaced, suggesting that the material generating them is ejected every 900–1,400 years. This rapid production — faster than what has been found in YSO outflows in other regions — rules out some models of how these knots are produced.

Based on the fraction of UWISH2 data analyzed so far, the authors estimate that the entire UWISH2 survey will uncover a total of ~2,000 jets and outflows from YSOs. This large, unbiased new sample is finally allowing astronomers to build out the statistics of YSO outflows to better understand them.

Citation

S. V. Makin and D. Froebrich 2018 ApJS 234 8. doi:10.3847/1538-4365/aa8862

binary neutron star

Got any plans in 46 million years? If not, you should keep an eye out for PSR J1946+2052 around that time — this upcoming merger of two neutron stars promises to be an exciting show!

Survey Success

PSR J1946+2052 profile

Average profile for PSR J1946+2052 at 1.43 GHz from a 2 hr observation from the Arecibo Observatory. [Stovall et al. 2018]

It seems like we just wrote about the dearth of known double-neutron-star systems, and about how new surveys are doing their best to find more of these compact binaries. Observing these systems improves our knowledge of how pairs of evolved stars behave before they eventually spiral in, merge, and emit gravitational waves that detectors like the Laser Interferometer Gravitational-wave Observatory might observe.

Today’s study, led by Kevin Stovall (National Radio Astronomy Observatory), goes to show that these surveys are doing a great job so far! Yet another double-neutron-star binary, PSR J1946+2052, has now been discovered as part of the Arecibo L-Band Feed Array pulsar (PALFA) survey. This one is especially unique due to the incredible speed with which these neutron stars orbit each other and their correspondingly (relatively!) short timescale for merger.

An Extreme Example

The PALFA survey, conducted with the enormous 305-meter radio dish at Arecibo, has thus far resulted in the discovery of 180 pulsars — including two double-neutron-star systems. The most recent discovery by Stovall and collaborators brings that number up to three, for a grand total of 16 binary-neutron-star systems (confirmed and unconfirmed) known to date.

Arecibo

The 305-m Arecibo Radio Telescope, built into the landscape at Arecibo, Puerto Rico. [NOAO/AURA/NSF/H. Schweiker/WIYN]

The newest binary in this collection, PSR J1946+2052, exhibits a pulsar with a 17-millisecond spin period that whips around its compact companion at a terrifying rate: the binary period is just 1.88 hours. Follow-up observations with the Jansky Very Large Array and other telescopes allowed the team to identify the binary’s location to high precision and establish additional parameters of the system.

PSR J1946+2052 is a system of extremes. The binary’s total mass is found to be ~2.5 solar masses, placing it among the lightest binary-neutron-star systems known. Its orbital period is the shortest we’ve observed, and the two neutron stars are on track to merge in less time than any other known neutron-star binaries: in just 46 million years. When the two stars reach the final stages of their merger, the effects of the pulsar’s rapid spin on the gravitational-wave signal will be the largest of any such system discovered to date.

More Tests of General Relativity

What can PSR J1946+2052 do for us? This extreme system will be especially useful as a gravitational laboratory. Continued observations of PSR J1946+2052 will pin down with unprecedented precision parameters like the Einstein delay and the rate of decay of the binary’s orbit due to the emission of gravitational waves, testing the predictions of general relativity to an order of magnitude higher precision than was possible before.

As we expect there to be thousands of systems like PSR J1946+2052 in our galaxy alone, better understanding this binary — and finding more like it — continue to be important steps toward interpreting compact-object merger observations in the future.

Citation

K. Stovall et al 2018 ApJL 854 L22. doi:10.3847/2041-8213/aaad06

Large Magellanic Cloud

For the first time, data from the Atacama Large Millimeter/submillimeter Array (ALMA) reveal the presence of methyl formate and dimethyl ether in a star-forming region outside our galaxy. This discovery has important implications for the formation and survival of complex organic compounds — important for the formation of life — in low-metallicity galaxies both young and old.

No Simple Picture of Complex Molecule Formation

ALMA

ALMA, pictured here with the Magellanic Clouds above, has observed organic molecules in our Milky Way Galaxy —
and beyond. [ESO/C. Malin]

Complex organic molecules (those with at least six atoms, one or more of which must be carbon) are the precursors to the building blocks of life. Knowing how and where complex organic molecules can form is a key part of understanding how life came to be on Earth — and how it might arise elsewhere in the universe. From exoplanet atmospheres to interstellar space, complex organic molecules are ubiquitous in the Milky Way.

In our galaxy, complex organic molecules are often found in the intense environments of hot cores — clumps of dense molecular gas surrounding the sites of star formation. However, it’s not yet fully understood how the complex organic molecules found in hot cores come to be. One possibility is that the compounds condense onto cold dust grains long before the young stars begin heating their natal shrouds. Alternatively, they might assemble themselves from the hot, dense gas surrounding the blazing protostars.

LMC Star-Forming Region

Composite infrared and optical image of the N 113 star-forming region in the LMC. The ALMA coverage is indicated by the gray line. Click to enlarge. [Sewiło et al. 2018]

Detecting Complexity, a Galaxy Away

Using ALMA, a team of researchers led by Marta Sewiło (NASA Goddard Space Flight Center) recently detected two complex organic molecules — methyl formate and dimethyl ether — for the first time in our neighboring galaxy, the Large Magellanic Cloud (LMC). Previous searches for organic molecules in the LMC detected small amounts of methanol, the parent molecule of the two newly-discovered compounds. By revealing the spectral signatures of dimethyl ether and methyl formate, Sewiło and collaborators further prove that organic chemistry is hard at work in hot cores in the LMC.

This discovery is momentous because dwarf galaxies like the LMC tend to have a lower abundance of the heavy elements that make up complex organic molecules — most importantly, oxygen, carbon, and nitrogen. Beyond lacking the raw materials necessary to create complex molecules, the gas of low-metallicity galaxies does a poorer job preventing the penetration of high-energy photons. The impinging photons warm dust grains, resulting in a lower probability of forming and maintaining complex organic molecules. Despite this, organic molecules appear to be able to develop and persist — which has exciting implications for organic chemistry in low-metallicity environments.

Methyl formate detection

ALMA observation of emission by methyl formate in a hot core in the LMC.
[Adapted from Sewiło et al. 2018]

A Lens into the Past

In the early universe, before the budding galaxies have had time to upcycle their abundant hydrogen into heavier elements, organic chemistry is thought to proceed slowly or not at all. The discovery of complex organic molecules in a nearby low-metallicity galaxy upends this theory and propels us toward a better understanding of the organic chemistry in the early universe.

Citation

Marta Sewiło et al 2018 ApJL 853 L19. doi:10.3847/2041-8213/aaa079

coronal jet

coronal jet vs. CME

Could coronal mass ejections (bottom panel) be driven by the same mechanism as the much smaller coronal jets (top panel)? [NASA]

What launches small jets and enormous coronal mass ejections (CMEs) from the Sun’s surface? New simulations explore how changing magnetic fields can drive these powerful eruptions.

Different Sizes, Same Jets?

Coronal jets are frequent, short-lived eruptions of plasma that are launched from low in the Sun’s atmosphere and travel outward through the corona. These ejections occur frequently across the Sun’s surface, lasting for ~10 minutes at a time and reaching lengths of ~50,000 km — a few times the Earth’s diameter, but still tiny compared to their enormous cousins, CMEs.

Despite the difference in size scales, a team of scientists led by Peter Wyper of Durham University has proposed that both coronal jets and CMEs are launched by the same mechanism, a process known as “magnetic breakout”. In a recent publication, Wyper and collaborators show the results of a series of 3D magnetohydrodynamic simulations of coronal jets to see what we would expect to observe from magnetic-breakout-driven eruptions.

Breaking Free

In the magnetic breakout model, magnetic field lines above filaments break and reconnect, removing confinement and allowing the filament to erupt from the Sun’s surface. Wyper and collaborators simulate this process by modeling a small bipolar structure on the Sun’s surface, embedded in a background magnetic field. They then observe how the magnetic fields rearrange themselves over time.

evolution of breakout jets

Schematic of the evolutionary sequence that produces breakout jets. The growing, twisted flux rope is shown by the yellow field lines. [Wyper et al. 2017]

The coronal jets that form in these simulations have four main stages:

  1. Filament channel formation
    Free energy is stored in a mini-filament or flux rope embedded within a background larger-scale magnetic field.
  2. Breakout
    Slow reconnection of the magnetic field lines above the growing flux rope eventually leads to a critical point where the flux rope can rapidly reconnect with the external field, breaking out.
  3. Eruptive jet
    As the filament escapes, it rapidly unwinds its twist and accelerates surrounding plasma along with it, causing the sudden eruption of an energetic, helical jet.
  4. Relaxation
    After several minutes, the jet propagates away, the reconnection subsides, and the fields relax into a new equilibrium similar to the starting point.

A Match to Observations

Wyper and collaborators run three sets of simulations with differing inclinations of the background magnetic fields, and they show that magnetic breakout in all three cases can lead to the production of broad, twisting jets with features consistent with what we’ve observed on the Sun.

In particular, the differing background field inclinations lead to a diversity of reconnection outflows preceding the jets. For highly inclined fields, for instance, a fast, dense outflow is driven with an inverted Y shape; for vertical fields, the outflows form much weaker spires.

All of these behaviors have been observed, and Wyper and collaborators’ model ties them together into a unified picture of coronal jet launching that has also been proposed to describe CMEs on a larger scale. If this picture is correct, then it may be possible that the complexities of these different solar eruptions can all be boiled down to one underlying process.

coronal jet eruption

Eruption sequence for an inclined background field at a) 16 minutes, b) 24 minutes, and c) 31 minutes. [Wyper et al. 2018]

Citation

P. F. Wyper et al 2018 ApJ 852 98. doi:10.3847/1538-4357/aa9ffc

giant exoplanet

As part of a major survey of evolved stars, scientists have discovered the most eccentric planet known to orbit a giant. What can we learn from this unusual object before it’s eventually consumed by its host?

Planetary Diversity

planetary system diversity

An example of the diversity of just a few of the planetary systems discovered by the Kepler mission. [NASA]

In the early stages of exoplanet science, it was easy to assume that all systems around other stars would be similar to our own solar system: rocky worlds close in, gas giants further out — and all with co-planar, low-eccentricity orbits.

As we observed the first exoplanets and learned about their properties, however, it quickly became apparent that most other systems don’t resemble our own. The more exoplanets we observe, the more we become aware of the diversity of planetary systems — with planet compositions, masses, and orbits unlike any in the solar system.

Orbit of HD 76920b

Orbit of HD 76920b, oriented properly and overlaid with the solar system inner planets’ orbits to scale. A comet and asteroid from our solar system are shown as having comparably eccentric orbits. [Wittenmyer et al. 2017]

Relative Sizes Matter

Some systems are easier to study than others. Since exoplanet detection and characterization techniques rely on looking for the imprint of planets on stellar signals, systems consisting of a small star and large planet are favored. For this reason, exoplanets orbiting solar-like or dwarf stars are especially well studied — but we don’t have nearly as much information about planets orbiting massive, hot stars.

To combat this lack of data, several teams have begun surveys particularly targeting evolved, massive stars. One of these is known as the Pan-Pacific Planet Search, a survey that uses the 3.9m Anglo-Australian Telescope in Australia to study the spectra of metal-rich subgiants in the southern hemisphere. Fresh among the discoveries from this survey is a planet orbiting HD 76920, reported on in a recent publication led by Robert Wittenmyer (University of Southern Queensland and University of New South Wales, Australia).

An Extreme Orbit

eccentricity of HD 76920b

Orbital eccentricity vs. planet’s periastron distance for the 116 confirmed planets orbiting giant stars. HD 76920b, the most eccentric of them, is shown with the red dot. [Wittenmyer et al. 2017]

Wittenmyer and collaborators conducted follow-up spectroscopy with two additional telescopes to confirm the properties of HD 76920. The team reports that HD 76920b — a giant planet of perhaps 4 Jupiter masses, with a period of 415 days and an eccentricity of e = 0.86 — is the most eccentric planet ever discovered orbiting a giant star.

How did HD 76920b achieve its extreme orbit? The go-to explanation for such an orbit is gravitational influence from a distant, massive stellar companion — and yet the authors find no evidence in their observations for a second star in the system. Instead, the team suggests that HD 76020b arrived on its current orbit via planet–planet scattering interactions earlier in the system’s lifetime.

star-planet interaction

Artist’s impression of a planet being engulfed by its host star. [NASA/ESA/G. Bacon]

Toasty Future

Lastly, Wittenmyer and collaborators use modeling to explore HD 76020b’s future. This planet’s orbit is already so extreme that it nearly skims the surface of its host, dipping to within 4 stellar radii of the star’s surface at its closest approach. The authors show that the planet will be engulfed by its host on a timescale of ~100 million years due to a combination of the star’s expanding radius and tidal interactions.

Gathering more observations of this extreme planet — and hunting for others like it — will help us to continue to learn about the formation and evolution of the diverse planetary systems our universe houses.

Citation

Robert A. Wittenmyer et al 2017 AJ 154 274. doi:10.3847/1538-3881/aa9894

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