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'Oumuamua

More than a year has passed since the discovery of 1I/2017 ’Oumuamua, a bizarre body that burst onto the scene and then disappeared into the distance as quickly as it had arrived. During ‘Oumuamua’s visit, astronomers gathered some 800+ observations from telescopes around the world, which together reveal a strange light curve that raises more questions than answers.

A few things are agreed upon. ‘Oumuamua’s orbit indicates it originated outside of our solar system, making it the first visiting interstellar body we’ve witnessed. Its shape appears to be highly elongated, suggesting it’s more cigar-shaped than spherical. And its light curve reveals a periodicity of roughly 8 hours, potentially indicating the speed at which this odd body rotates.

But many unsolved questions remain. What are ‘Oumuamua’s structure, composition, and shape? Where did it come from? How was it launched onto its journey to our solar system?

Solar Push for a Fluffy Body?

One of ‘Oumuamua’s biggest mysteries relates to the discovery late last year that this asteroid wasn’t moving just under the influence of gravity; instead, ‘Oumuamua was experiencing a mysterious additional acceleration away from the Sun.

light sail

Artist’s impression of an artificial light sail, a thin spacecraft that can be propelled by radiation pressure. [Josh Spradling / The Planetary Society]

What could cause ‘Oumuamua’s added boost? Some scientists have suggested that radiation pressure — the push from solar photons hitting the object — could speed it up enough to explain observations. But for this to work, the asteroid would need an enormous surface-area-to-mass ratio.

One study suggested this could be achieved if ‘Oumuamua took the form of a giant light sail less than a millimeter thick (naturally reinvigorating the “is it aliens?” debate). But a recent study by Amaya Moro-Martín (Space Telescope Science Institute) suggests there might be another way: ‘Oumuamua could have a more ordinary shape, but an exceedingly low density.

cosmic dust

Image through a microscope of a porous interplanetary dust particle. Could ‘Oumuamua be an extremely low-density aggregate of icy dust? [Donald E. Brownlee (U. of Washington) and Elmar Jessberger (Institute for Planetology, Germany)]

The least dense manmade solid, aerographene, has a density on the order of 10-4 g/cm3, or ~10% the density of air. Moro-Martín suggests that an object with a tenth of this density, ~10-5 g/cm3, could get enough of a boost from the Sun to explain our observations of ‘Oumuamua.

Given this low density value, is this scenario actually likely? It turns out that fluffy, porous materials occur naturally in space, in the form of aggregates of icy dust particles. If the icy-aggregate explanation for ‘Oumuamua is correct, then the asteroid could have formed in the outer reaches of a nearby protoplanetary disk — and this could open a new window onto the study of the building blocks of planets around young stars. 

comet 67P/Churyumov-Gerasimenko

Image of comet 67P/Churyumov-Gerasimenko outgassing as it is heated by the Sun. Could similar processes be occurring on ‘Oumuamua? [ESA/Rosetta/MPS for OSIRIS Team]

Added Nudge from Migrating Jets?

There’s a more mundane explanation for ‘Oumuamua’s anomalous acceleration than radiation pressure, however: outgassing, which occurs as volatiles heat beneath a body’s surface and evaporate. This process is commonly seen in the jetted tails of comets, but there are some problems with using it to explain ‘Oumuamua’s motion.

First, no outgassing was observed from ‘Oumuamua; in fact, Spitzer observations placed strict limits on the amount of carbon-based material that could be evaporating from it. Second, calculations show that traditional comet-like outgassing would create torques that would spin ‘Oumuamua up rapidly, causing it to fly apart.

A new study may have found a way around these problems, however. A publication led by Darryl Seligman (Yale University) suggests that ‘Oumuamua may have been accelerated by outgassing not from a fixed point, but from migrating jets that follow the warmth, tracking the side of the asteroid closest to the Sun. Instead of spinning out of control, ‘Oumuamua’s motion might then resemble a pendulum, gently rocking back and forth to produce the ~8-hr period seen in the light curve.

'Oumuamua light curve

Light curves (left) and periodograms (right) for actual ‘Oumuamua observations (top row) and synthetic observations for three of the authors’ outgassing models, using three different aspect ratios for the body (next three rows). The bottom row reflects a flat photometry. Click to enlarge. [Seligman et al. 2019]

What about the Spitzer constraints on the visible evaporation? Seligman and collaborators suggest that the outgassing was primarily in the form of water vapor rather than carbon-based material. This could occur if the body had an unusually carbon-poor composition compared to a typical comet.

Ready for the Next Visitor

Could one of these explanations solve the mystery of ‘Oumuamua’s odd acceleration? Or could the true answer be a combination of proposed scenarios? With ‘Oumuamua long gone, we can’t be sure until we spot another interstellar visitor like it — but you can bet we’ll be prepared next time!

Citation

“Could 1I/’Oumuamua be an Icy Fractal Aggregate?,” Amaya Moro-Martín 2019 ApJL 872 L32. doi:10.3847/2041-8213/ab05df
“On the Anomalous Acceleration of 1I/2017 U1 ‘Oumuamua,” Darryl Seligman et al 2019 ApJL 876 L26. doi:10.3847/2041-8213/ab0bb5

superluminous supernova

What causes the bizarre, extragalactic fast radio bursts we’ve detected over the last decade? A new study of an unusually bright supernova may have found the key to answering this question.

Clues from a Repeating Burst

FRB 121102 host

The host of FRB 121102 is placed in context in this Gemini image. [Gemini Observatory/AURA/NSF/NRC]

When a mysterious millisecond radio pulse of extragalactic origin — a fast radio burst (FRB) — was recently found to repeat, it gave astronomers a rare chance to hunt down this source’s host galaxy. FRB 121102 was isolated to a star-forming, low-metallicity dwarf galaxy located roughly 3 billion light-years away, and a dimmer, persistent radio source was discovered in the same region that produced the bursts.

This localization lent support to one theory for the origin of FRB 121102 (and possibly other FRBs): that the bursts are powered by a magnetized neutron star born decades ago in a superluminous supernova.

Magnetar Culprit?

Superluminous supernovae are a type of stellar explosion at least ten times more powerful than standard supernovae. These supernovae may shine extra bright due to the birth of a neutron star with extremely strong magnetic fields — a magnetar — that spins on millisecond timescales, emitting radiation and winds as its magnetic fields decay and further powering the explosion.

magnetar

Artist’s impression of a magnetar — an extremely magnetized neutron star — in a young star cluster. [ESO/L. Calçada]

In the superluminous-supernova explanation for FRBs, the magnetar born in the stellar explosion could, even a decade later, power brief radio bursts. In addition, it would generating a glowing nebula visible to us as a persistent radio source.

A team of scientists has now sought to test this picture by examining known superluminous supernovae and searching for signs of co-located persistent or bursting radio sources. In a recent publication led by Tarraneh Eftekhari (Harvard-Smithsonian Center for Astrophysics), they detail their first success.

Radio Source Found

Using the Very Large Array, Eftekhari and collaborators discovered a persistent radio source coincident with the superluminous supernova PTF10hgi, an explosion that went off 7.5 years ago, roughly 1.5 billion light-years away. This is the first time a radio source of any kind has ever been associated with a superluminous supernova, providing an important link between these explosions and other phenomena.

PTF10hgi

Left: Radio continuum map from VLA 6-GHz observations of PTF10hgi. Right: Near-ultraviolet image of the host galaxy of PTF10hgi from Hubble, with radio contours overlaid. The small red circle shows the optical position of PTF10hgi. [Eftekhari et al. 2019]

The authors test a variety of different potential origins for the radio emission seen, like star formation activity, an active galactic nucleus, and a supernova blast wave. Though none of these scenarios can yet be ruled out with certainty, Eftekhari and collaborators show that each of them is highly unlikely.

Instead, the radio emission is extremely reminiscent of the persistent radio source associated with FRB 121102. The authors show that all of the observations are consistent with a magnetar central engine powering a glowing nebula embedded in the supernova ejecta. And while a bursting radio source wasn’t found coincident with the supernova, the authors’ observations were only 40 minutes long; a longer observation time may yet discover a co-located FRB.

The authors detail future observations that should be able to rule out alternative origins for the radio emission and strengthen the case for a superluminous-supernova-born magnetar as a source of fast radio bursts. In the meantime, it’s exciting to watch as the pieces of this puzzle start to come together!

Citation

“A Radio Source Coincident with the Superluminous Supernova PTF10hgi: Evidence for a Central Engine and an Analog of the Repeating FRB 121102?,” T. Eftekhari et al 2019 ApJL 876 L10. doi:10.3847/2041-8213/ab18a5

brown dwarf clouds

Nearby, cool dwarfs offer us the unique opportunity to learn more about the atmospheres of planetary-mass bodies. In a new study, Hubble observations shed some light on the clouds of one such object.

Looking for Variability

Ross 458C

A false-color image of the Ross 458 system. The bright source in the top left is the binary star system; the planetary-mass brown dwarf companion Ross 458C is circled on the bottom right. [UKIDSS]

As the list of known planet-like bodies beyond our solar system grows, we continue to find new ways to study and understand these objects. One area of increasing interest and capability is the study of atmospheres.

When we directly image planet-like bodies using high-resolution telescopes, we can look for variability in the light from the body over time. Regular, periodic variability can indicate the presence of clouds in that object’s atmosphere — and observations of the variability can reveal information about the cloud structure and composition.

An Atlas of Clouds

To this end, a large Hubble Space Telescope Treasury program was begun in 2015: Cloud Atlas. The program’s aim is to use high-precision, time-resolved Hubble photometry and spectroscopy to explore the atmospheres and clouds of exoplanets, planetary-mass brown dwarfs, and more massive brown dwarfs — bodies with typical temperatures between 800 K and 1,700 K.

There are two major steps to the Cloud Atlas project:

  1. Construct a broad sample of observed planets and brown dwarfs showing variability. This will ultimately allow us to compare the cloud maps between different objects and determine how cloud properties depend on things like a body’s temperature and size.
  2. Conduct detailed studies of the most interesting objects in this sample, using longer follow-up observations that cover complete rotations of the bodies.

In a recent publication led by Elena Manjavacas (W.M. Keck Observatory; The University of Arizona), the Cloud Atlas team describes their detailed observations of the planetary-mass brown dwarf Ross 458C as part of the latter half of this project.

Ross 458C variability

Observations of Ross 458C’s light curve over time in three different bands (white, top; narrow J-band, center; narrow H-band, bottom) reveal a distinct periodic variation fit by a sine function. [Adapted from Manjavacas et al. 2019]

Clouds on a Planetary-Mass Companion

Ross 458C is a T8 brown dwarf that orbits a binary system less than 40 light-years away. This cool object lies near the blurry line between brown dwarf and planet, representing the coldest dwarf yet to undergo detailed cloud studies.

Manjavacas and collaborators’ long observations of Ross 458C over seven consecutive Hubble orbits — during which 77 spectra were captured — revealed variability in the dwarf’s spectrum at a level of 2.62 ± 0.2% over the wavelength range of 1.1–1.64 µm. This modulation suggests the dwarf’s rotation period is somewhere around 6.75 hours.

By exploring how the dwarf’s variability changes across different wavelengths, the authors hope to be able to understand what its clouds are made up of and how the cloud structure is distributed in the atmosphere. Current best estimates from modeling suggest that these may be heterogeneous sulfide clouds (very different from Earth’s clouds of water!) — which could also explain the reddish color of Ross 458C.

The discovery of Ross 458C’s rotational modulations is important in and of itself: it suggests that clouds are typical even in the atmospheres of the coolest dwarfs. We can look forward to plenty of follow-up work further exploring the atmosphere of a planetary-mass companion with this convenient laboratory.

Citation

“Cloud Atlas: Rotational Spectral Modulations and Potential Sulfide Clouds in the Planetary-mass, Late T-type Companion Ross 458C,” Elena Manjavacas et al 2019 ApJL 875 L15. doi:10.3847/2041-8213/ab13b9

Atacama Large Millimeter/submillimeter Array

Interferometric arrays collect massive amounts of information, leaving astronomers with a happy problem: too much data! How can we handle mountains of data in an efficient way?

Murchison Widefield Array

One of many tiles comprising the Murchison Widefield Array (MWA). Radio interferometric arrays like MWA generate vast amounts of data. [Dr. John Goldsmith/Celestial Visions]

Too Much of a Good Thing?

Astronomers have come a long way from the early days of manually cataloging stars and sketching sunspots by hand. Even though today’s data sets are larger and more complex, many astronomers still manually calibrate and process their data.

This hands-on data processing won’t always be feasible, though; interferometry — the process of linking together tens to thousands of telescopes or antennae to produce images with ever-finer angular resolution — generates far more data than humans could hope to handle manually. Just one minute’s worth of data from the Murchison Widefield Array (MWA), a radio interferometer made up of 4,096 antennae, yields roughly 10,000 images!

With the number of interferometers increasing, we’ll need to be smart about how we process all that data to minimize computing hours while maximizing the quality of the output. Among the many detectors requiring novel data-processing techniques is the planned Square Kilometer Array (SKA), which will comprise a million antennae and 2,000 radio telescopes. How can we get a handle on all this data without getting too hands-on?

Mondal et al. 2019 Fig. 2

An illustration of how increasing numbers of detectors are included in the model of the target for self-calibration. The first step includes only the blue detectors near the center, and subsequent steps add the red, teal, black, and yellow detectors to increase the complexity of the model. [Mondal et al. 2019]

Dealing with Data Pileup

To tackle this problem, a team led by Surajit Mondal (Tata Institute of Fundamental Research, India) developed an automated processing pipeline for interferometric data — the Automated Imaging Routine for Compact Arrays for the Radio Sun (AIRCARS). They focused on processing solar radio images, which need to capture a huge dynamic range — from extremely bright active regions to faint, wispy filaments.

One of the challenges in radio interferometry is removing the effects of instrumental artifacts and the plasma in Earth’s atmosphere. Most radio interferometry data are corrected with a self-calibration process that treats the instrumental artifacts and the brightness of the target as free parameters and iteratively minimizes the difference between the observations and a model of the target.

AIRCARS works especially well when applied to a compact array — one with many detectors clustered in the center and fewer near the outskirts. This configuration allows the pipeline to start with relatively little information about the target from just a few central detectors and gradually build a complex model of the target to be used in its self-calibration routine.

Mondal et al. 2019 Fig. 5

An example of the improvement of the dynamic range of MWA images through the self-calibration process. The number of iterations increases from top to bottom and left to right. The dashed circle indicates the location of the Sun’s disk. Click to enlarge. [Mondal et al. 2019]

AIRCARS in Our Future

In their tests on MWA data, the authors find that AIRCARS is capable of capturing a dynamic range up to 100,000:1 — a huge improvement over previous processing methods.

Mondal and collaborators note that AIRCARS can be configured to attain the maximum possible dynamic range without constraints on computing time, or to accept user-imposed time limits to rapidly process large amounts of data, depending on the user’s computational requirements.

Because the pipeline needs no human supervision, astronomers can take a step back from processing the vast amount of incoming data and focus instead on the exciting science we can do with interferometry.

Citation

“Unsupervised Generation of High Dynamic Range Solar Images: A Novel Algorithm for Self-calibration of Interferometry Data,” Surajit Mondal et al 2019 ApJ 875 97. doi:10.3847/1538-4357/ab0a01

buckyball

From a jumble of confusing clues in Hubble observations of interstellar space, scientists have picked out evidence of a celebrity molecule: ionized Buckminsterfullerene, or buckyballs.

Sorting Out Diffuse Signals

What makes up the tenuous gas and dust that pervades our galaxy, filling the space between stars? What kinds of complex molecules can form naturally in our universe, outside of the potentially contrived conditions of Earth-side laboratories? Where might these molecules form, and how are they distributed throughout space?

interstellar sightlines

Hubble spectra of seven heavily-reddened interstellar sightlines (top seven black lines) and four unreddened standard stars (bottom four lines). The red line at the top indicates a laboratory spectrum for C60+. Positions of the four absorption features associated with C60+ are marked with vertical dashed lines. Click to enlarge. [Cordiner et al. 2019]

These are among the many open questions regarding the chemistry of our universe. One particular, longstanding puzzle for astronomers is the cause of what’s known as “diffuse interstellar bands”: hundreds of broad absorption features that appear in optical to near-infrared spectra of reddened stars.

These features are not caused by the stars themselves, so they must be due to absorption of light by the diffuse interstellar medium (ISM) between us and the stars. But the jumble of hundreds of features — and the unknown conditions under which they are produced — has made it incredibly challenging to identify the individual molecules present in the diffuse ISM.

A new study led by Martin Cordiner (NASA Goddard SFC; Catholic University of America) presents observations from the Hubble Space Telescope — thus avoiding the additional complication of absorption features from the Earth’s atmosphere — that explore these diffuse interstellar bands further. Hubble’s sightlines toward 11 stars provide confirmation of one special molecule within this jumble: Buckminsterfullerene.

buckyball

Model of the structure of a buckyball. [Mstroek]

A Celebrity Molecule

The C60+ ion, formally known as Buckminsterfullerene and informally known as a “buckyball”, is an enormous molecule consisting of 60 carbon atoms arranged in a soccer-ball shape. Previously, the largest known molecules definitively detected in the diffuse interstellar medium contained no more than three atoms heavier than hydrogen — so the detection of buckyballs represents a dramatic increase in the known size limit!

Cordiner and collaborators use a novel scanning technique to obtain ultra-high signal-to-noise spectra of seven stars that are significantly reddened by obscuring ISM and four stars that are not. They then search for absorption signals at four wavelengths — 9348, 9365, 9428, and 9577 Å — predicted by laboratory experiments to be associated with C60+.

mean spectra

Mean spectra for the observed sightlines for reddened (black, top) and unreddened (gray, bottom) stars, around four predicted absorption features for C60+. The laboratory comparison spectra for C60+ are overlaid as red lines. [Cordiner et al. 2019]

The authors find obtain reliable detections of the three strongest of these absorption lines in the spectra toward the seven reddened stars, and find no sign of this absorption in the four unobscured stars. The 9348 Å absorption was not detected, but as this is predicted to be a very weak feature, this result is not surprising. The relative strengths of the three detected lines also fit with laboratory predictions.

The consistency of Cordiner and collaborators’ results with prediction provides the strongest confirmation yet of the presence of buckyballs in the diffuse ISM. This detection may help us to characterize other components of the diffuse ISM and better understand the conditions under which complex molecules exist in the extreme, low-density environment of interstellar space.

Citation

“Confirming Interstellar C60+ Using the Hubble Space Telescope,” M. A. Cordiner et al 2019 ApJL 875 L28. doi:10.3847/2041-8213/ab14e5

binary neutron star

When two compact objects — neutron stars or black holes — merge, will they emit light? A recent study looks at a neglected factor that could affect the answer: electric charge.

Dark or Light?

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]

Most theories agree that a compact binary containing a neutron star can emit light when it merges. This is because these systems contain lots of neutron-rich matter that can then radiate in the final stages of merger, in the form of gamma-ray bursts, kilonovae, and afterglows.

But what about compact binaries containing two black holes? Or so-called “plunging” black-hole–neutron-star mergers in which the neutron star plunges directly into the black hole before it can be disrupted? Are these mergers all doomed to darkness?

Possible Charge

Not according to Bing Zhang, a scientist at University of Nevada Las Vegas. Recently, Zhang proposed that black holes might carry electric charge in a surrounding magnetosphere. As charged black holes spiral around and around each other during a merger, they could generate electromagnetic radiation: a characteristic signal that rises sharply just before merger.

Now Zhang is back with a generalized model for the merger of charged compact objects, which also explores possible signatures from electrically charged neutron stars. In a new study, he works out the details and reports on where we might be able to detect these signals.

Searching for a Signal

All compact binaries containing a neutron star should emit radiation from electric charge, since neutron stars are definitely charged — they’re essentially spinning magnets. But for most systems containing a neutron star, Zhang demonstrates, the radiation associated with the object’s charge will be non-detectable, since it’s so much dimmer than other electromagnetic signatures from merger (like a gamma-ray burst).

Crab pulsar

The Crab pulsar is a highly magnetized, spinning neutron star that powers the Crab nebula seen in this composite image. [X-ray: NASA/CXC/SAO/F.Seward; Optical: NASA/ESA/ASU/J.Hester & A.Loll; Infrared: NASA/JPL-Caltech/Univ. Minn./R.Gehrz]

There’s hope, though, in the scenario of a plunging neutron-star–black-hole merger. If the neutron star is less than 20% the size of the black hole, it can be consumed whole, preventing any of the typical electromagnetic signatures from occurring. In this case, the radiation from the charged, inspiralling neutron star is the only electromagnetic signal present.

If the neutron star in such a system has a magnetic field similar to that of the Crab pulsar — possible in young star clusters — the charge signal can reach detectable levels, according to Zhang’s calculations. In fact, it’s possible that we could observe such a signal as a fast radio burst, the mysterious millisecond radio bursts that we’ve seen originating from beyond our galaxy.

Looking Ahead

Many unknowns are still present in this picture. How is the electric radiation converted into observable emission? How commonly do we expect plunging neutron-star–black-hole mergers to occur as described? Will we be able to link radiation from charged mergers to a gravitational-wave chirp?

One thing is for certain: if we can, indeed, observe the light from charge in a compact-binary merger, this would provide an exciting new opportunity to further probe these distant, exotic systems.

Citation

“Charged Compact Binary Coalescence Signal and Electromagnetic Counterpart of Plunging Black Hole–Neutron Star Mergers,” Bing Zhang 2019 ApJL 873 L9. doi:10.3847/2041-8213/ab0ae8

solar filaments

Images of the Sun’s chromosphere often reveal dark threads cutting across the Sun’s face. New research has now explored how these solar filaments are built from magnetic fields and plasma.

Two-Faced Structures

solar prominence

A solar eruptive prominence as seen in extreme UV light on March 30, 2010, with Earth superimposed for a sense of scale. [NASA/SDO]

Solar filaments may look like deep cracks in the Sun’s façade, but in reality, they are enormous arcs of hot plasma that extend above the Sun’s surface. Because this plasma is slightly cooler than the solar surface below, they appear dark against the hotter background.

Unfamiliar with filaments? You’ve likely seen plenty of them in images — but from a different angle! Filaments are the same structures as solar prominences, the loops of plasma we can see suspended above the Sun’s limbs. When prominences appear on the side of the Sun facing us, they take the form of filaments from our point of view.

Shaped by Fields

Filaments are often associated with various forms of solar activity. They last for days, frequently hanging above active regions of the Sun; filament channels are often the origin of eruptions from the Sun’s surface. To better understand our active and energetic Sun, understanding the structures of filaments is an important step.

Unfortunately, this is challenging! We know that filament structure is largely due to the magnetic fields — whose forces suspend the filaments against the downward pull of gravity — but we don’t have the ability to directly measure the magnetic field in the Sun’s atmosphere. A team of scientists at the University of Science and Technology of China has instead taken an indirect approach: they explored filaments by looking at the motion of plasma along them.

filament motions

Top: time-distance map characterizing the oscillations at one position on the filament spine. Bottom: a Doppler map, averaged over time, that shows the rotation around the spine of the filament. Blue indicates motion toward the observer, red away. [Adapted from Awasthi et al. 2019]

A Double Decker?

Scientists Arun Awasthi, Rui Liu, and Yuming Wang examined observations of a filament that appeared near active region AR 12685 in October 2017, captured with the 1-m New Vacuum Solar Telescope in China. The team used these high-resolution images to explore bulk motions of plasma in the filament.

Awasthi and collaborators found that the filament displayed two different types of motion: rotation around its central spine, and longitudinal oscillations along its spine. The longitudinal oscillations in the eastern segment of the filament were distinct from those in the west, suggesting that the magnetic field lines underneath these two segments have different lengths and curvatures.

On the whole, the motions observed in the filament indicate that magnetic structure for filaments is complicated. The authors argue that more than one model is likely at work; they propose a “double-decker” picture for the filament in which a flux rope (a twisted bundle of magnetic field lines) sits on top of a sheared arcade (a series of distorted loops).

"double-decker" magnetic field

Proposed “double-decker” configuration of the magnetic field hosting the filament, consisting of a flux rope (red) atop a sheared arcade (blue). Left panel shows the cross section viewed from the west; right panel shows the structure viewed from above. [Adapted from Awasthi et al. 2019]

Awasthi and collaborators conclude with specific predictions of indicators we can look for in future filament observations to test this model. If correct, this view of filament structure brings us a little closer to understanding the complex magnetic fields that control solar activity.

Bonus

Check out the video below showing the motions in the filament in different wavelengths. Top panel: GOES light curve and an HMI magnetogram. Bottom panels: AIA 171 Å image, GONG Hα image, and NVST Hα image.

Citation

“Double-decker Filament Configuration Revealed by Mass Motions,” Arun Kumar Awasthi et al 2019 ApJ 872 109. doi:10.3847/1538-4357/aafdad

W49B

There’s plenty to learn from the skeletons left behind after supernova explosions tear through their surroundings. An X-ray view from space has revealed new details about a particularly extreme supernova remnant.

Unexpected Plasmas

overionized plasma in W49B

NuSTAR observations showing the spatial distribution of flux that indicates where overionized plasma resides in supernova remnant W49B. The overionized plasma is more highly concentrated on the western side of the remnant. [Yamaguchi et al. 2018]

When some stars explode as powerful supernovae at the end of their lifetimes, they expel material into their surroundings, enriching the galaxy with heavy elements. As this matter is flung outwards at high speeds, it slams into the interstellar medium, generating shocks that heat the gas and ionize it.

We can study the young remnants of supernovae — the structures of gas and dust left behind shortly after these explosions — to learn more about how supernovae interact with the interstellar medium. One type of source is particularly intriguing: very young, hot supernova remnants that are “overionized”.

Overionized plasmas send us mixed signals: their level of ionization is higher than what we expect from the temperature we measure from their electrons. This is most likely an indication that the plasma has recently started cooling very rapidly.

But what might cause this sudden cooling of the remnant? To learn more, we need detailed observations of a young, hot remnant. Luckily, we’ve got an ideal target — and an ideal instrument.

NuSTAR

Artist’s illustration of the NuSTAR spacecraft. [NASA]

Setting Sights

Supernova remnant W49B is one of the first sources in which we discovered signs of an overionized plasma. It’s the youngest (just ~1,000 years old!), hottest, and most highly ionized among all such objects exhibiting this trait. But W49B’s hot plasma is challenging to observe, and we haven’t yet managed to constrain its detailed high-energy properties.

A powerful telescope is up to the task, however: the Nuclear Spectroscopic Telescope Array (NuSTAR). This versatile space observatory is ideally suited for exploring the spectroscopic details of the X-rays emitted from the hot plasma in W49B.

Sudden Expansion

NuSTAR spectral analysis

Top panel: Diagram of the 1’ x 1’ box regions NuSTAR resolved for spectral analysis (overlaid on a color image of W49B from the Wide Field Infrared Camera at Palomar Observatory). Bottom panel: Plot of the electron temperature (bottom) vs. the density (right) measured for each of the labeled regions. [Adapted from Yamaguchi et al. 2018]

Led by Hiroya Yamaguchi (Institute of Space and Astronautical Science, JAXA, Japan), a team of scientists used NuSTAR to capture detailed images and spectroscopy of the W49B remnant.

Yamaguchi and collaborators first confirmed that the overionized plasma is most highly concentrated on the western side of the remnant. They then show that lower electron temperatures — i.e., signs of rapid cooling — are found in the same regions that also have lower density. They measure a gradient from lowest electron temperature and density in the west, to highest in the east.

Taken together with previous observations that reveal that W49B’s surroundings also have lower density on the western side, these results provide strong evidence that the remnant is cooling via adiabatic expansion. In this picture, the supernova blast wave punched through dense circumstellar matter in early stages of the explosion, expanding slowly. Now it’s suddenly breaking out into the lower-density interstellar medium — on the west side first, because it’s not exactly symmetric — leading to sudden expansion and cooling.

Does this explanation apply to overionized plasmas in the skeletons of other, similar supernovae? We’ll need more observations to be sure — but NuSTAR has proven itself up to the task!

Citation

“Evidence for Rapid Adiabatic Cooling as an Origin of the Recombining Plasma in the Supernova Remnant W49B Revealed by NuSTAR Observations,” Hiroya Yamaguchi et al 2018 ApJL 868 L35. doi:10.3847/2041-8213/aaf055

Artist's impression of a planetesimal

When interstellar asteroid ‘Oumuamua sped through our solar system in 2017, it revealed that small bodies might make a habit of visiting other planetary systems. Could such objects occasionally be responsible for jump-starting planet formation?

Planet Formation in the Works

1I/2017 U1 ('Oumuamua)

An artist’s impression of interstellar asteroid 1I/2017 U1 (‘Oumuamua). [European Southern Observatory / M. Kornmesser]

One of the leading models of planet formation is core accretion, in which dust grains come together to form pebbles, planetesimals, and planets. While the core accretion model explains many features that we observe in protoplanetary disks, it’s not yet clear exactly how dust grains clump together to form larger objects.

It’s also expected to take up to tens of millions of years to form a planet, which clashes with observations of planets forming in as little as a few hundred thousand years in disks that dissipate after just a few million years.

Many models of planet formation show that once planetesimals in a disk reach a certain size, they rapidly accrete gas to form planets. What if core accretion could get a head start from long-lost planetesimals ejected from other stellar systems?

Pfalzner & Bannister 2019 Fig. 1

A qualitative illustration of the timeline and mechanisms of interstellar object production. The times here are typical for a solar-mass star. Higher-mass stars would begin the final stage of interstellar object production earlier. Click to enlarge. [Pfalzner & Bannister 2019]

One System’s Trash, Another System’s Treasure?

To estimate how the population of interstellar objects affects the formation of new planets, Susanne Pfalzner (Jülich Supercomputing Center and Max Planck Institute for Radio Astronomy, Germany) and Michele Bannister (Queen’s University Belfast, UK) calculated the number of interstellar objects that survive the journey from their nascent star systems and become incorporated into new systems.

They began with an estimate of ‘Oumuamua-sized objects based on 30 years of observations from the Pan-STARRS1/Catalina Sky Survey: 1015 objects in a cubic parsec (about 35 cubic light-years).

The authors estimated that only a few percent of these objects can be easily captured by molecular clouds, and even fewer make it into the surroundings of a young star, which consumes all but 1–10% of the interstellar objects capable of acting as seeds for planet formation.

Plentiful Planetesimals

Pfalzner & Bannister 2019 Fig. 2

Density of 100-meter interstellar objects over the course of the star and planet formation process. [Pfalzner & Bannister 2019]

Of the initial 1015 interstellar objects found in a single cubic parsec, the authors estimate that more than 10 million are able to be captured by molecular clouds, survive collisions, withstand heating, and become incorporated into a protoplanetary disk to participate in accretion. Most are about 100 meters in size, but as many as 105 are kilometer sized and a thousand are 100 km or larger.

It’s not yet clear what role the abundant 100-meter objects play in planet formation, but simulations have shown that planetesimals larger than ~200 km rapidly accrete gas from their surroundings, forming both terrestrial and gas-giant planets in a blazingly fast 1,000 years.

Since the number of interstellar objects increases over time, the number of potential seeds for planet formation increases as well, meaning that planet formation is likely faster now than it was billions of years ago. There’s still plenty of work to be done, but one thing is clear: we can’t ignore the importance of interstellar objects in the planet formation process.

Citation

“A Hypothesis for the Rapid Formation of Planets,” S. Pfalzner & M. T. Bannister 2019 ApJL 874 L34. doi:10.3847/2041-8213/ab0fa0

EHT observations of M87

Astronomers have used a telescope that spans the globe to capture the first detailed images of a black hole: the nearby supermassive black hole in the Messier 87 galaxy. The first results from the Event Horizon Telescope (EHT) are detailed in six articles that make up a new Focus Issue in The Astrophysical Journal Letters.

Hubble observation of M87

Hubble image of the elliptical galaxy M87, dominated by the visible jet extending to the northwest of the supermassive black hole at the galaxy’s center (click to enlarge). [NASA/ESA/Hubble Heritage Team (STScI/AURA)/P. Cote (Herzberg Institute of Astrophysics)/E. Baltz (Stanford University)]

Why M87?

Obtaining an up-close view of a black hole is a goal that has long remained out of reach. When astronomers first developed a plan to image a supermassive black hole’s event horizon — the close-in boundary from which not even light can escape — two sources were selected as targets: the black hole at the center of our galaxy, Sgr A*; and the gargantuan, jet-producing black hole in the neighboring galaxy M87.

Sgr A*, while the nearest supermassive black hole, suffers from a host of complicating factors for imaging. This monster’s rapid variability and its position in the galactic plane — where it’s blurred by the interstellar medium — provide additional challenges that must be overcome to accurately capture it.

M87 may be further away, but its black hole is a thousand times larger than Sgr A*, giving it longer and more manageable variability timescales. We’re also peering through much less of the interstellar medium when pointed at M87.

These factors contribute to making the neighboring behemoth in M87 comparatively accessible for imaging — and, consequently, it’s the first source for which the EHT is presenting images.

A Planet-Scale Telescope

So how do we zoom in on this distant object? We need a telescope unlike any other.

EHT 2017 campaign

The eight stations of the EHT 2017 campaign over six geographic locations. [EHT Collaboration et al 2019]

The EHT is an extensive virtual telescope created by combining simultaneous observations from radio arrays and dishes all around the planet. For the images of M87 released today, the observations were made by eight ground-based telescopes in Arizona, Hawai’i, Mexico, Chile, Spain, and the South Pole.

The EHT works by performing very-long-baseline interferometry; by combining different telescopes around the world, the EHT can function like a telescope with an effective size that’s the same as its longest baseline — the distance between component telescopes. In this way, the EHT is able to achieve unprecedented resolution: it can theoretically resolve down to 25 millionths of an arcsecond at its observing wavelength of 1.3 mm.

It’s been more than a decade since the EHT’s bold imaging plans were first begun. Scientists have patiently waited as existing facilities were upgraded and new facilities have been built — and in April 2017, conditions were finally right to obtain the first good look at M87’s event horizon.

Images of a Shadow

M87’s black hole was observed on four days in April 2017. Weather was uniformly good — planet-wide! — during those observations, allowing EHT scientists to combine the data from the eight telescopes and reconstruct images of the black hole.

EHT M87 observations

EHT observations of M87 taken over 4 days reveal a bright, asymmetric ring; north is up and east is left. The images remain consistent over the observations, providing evidence of a stable source. [EHT Collaboration et al 2019]

What they saw was spot-on with predictions: a ring of light spanning ~38–44 µas, with the southern part of the ring appearing brighter than the rest. 

While a naked black hole would simply appear dark in an image, an active black hole like M87 is surrounded by dust and gas that forms an accreting disk, as well as funnels at the base of its powerful jets. Images of such an encased black hole were predicted to reveal a dark region — the black hole’s “shadow” — surrounded by a ring of emission produced by the distorted paths of light from the surrounding material. The EHT observations of M87 beautifully confirm this picture!

Why is the southern part of the ring brighter? As fast-moving material rotates around the black hole, it speeds toward us on one side and away from us on the other. On the side of the ring where matter moves toward us, a relativistic effect beams light in our direction, causing this region to appear brighter.

From the asymmetry of the ring, EHT scientists determine that matter on the south side of M87’s black hole is moving toward us. Combined with previous observations of M87’s jet, which show it’s inclined at an angle of 17° relative to our line of sight, this tells us that M87’s black hole likely spins clockwise from our point of view, with its spin axis pointed away from us at an angle.

GRMHD models

Three of the authors’ example models of spinning black holes (top row) and their simulated observations (bottom row). All three models produce similar simulated observations that well-match the real observations, indicating a single good fit doesn’t imply that one model is preferred over the others. [EHT Collaboration et al 2019]

Models and Measurements

The EHT’s images of M87’s black hole were compared to an extensive library of synthetic observations obtained from simulations of this source under varying models and conditions. The simulated observations show remarkable consistency with the actual observations, confirming a picture of a spinning supermassive black hole as the source of the emission. The models also suggest the most likely way that M87’s jets are launched: via energy extraction directly from the black hole’s spin.

Previous estimates of the mass for M87’s black hole ranged from ~3–7 billion solar masses, depending on the method used to measure it. From the EHT’s images of M87, the authors were able to estimate a mass for the black hole of 6.5 ± 0.7 billion solar masses, providing an independent check of past measurements.

Future Plans

What’s next for the EHT? These images of M87 are just the start! The team still plans to perform polarimetric analysis of their images, which will probe the magnetic field and help determine the rate of accretion onto M87’s black hole. Future observations will test the stability, shape, and depth of its shadow more accurately. And higher-resolution images will be possible with the addition of new telescopes to the EHT and a possible push to shorter-wavelength observations.

Continued improvements to EHT’s observational technology and analysis techniques should also bring our own supermassive black hole, Sgr A*, into reach. Welcome to a new world of black-hole exploration!

For more information, check out the full ApJL Focus Issue here:
Focus on the First Event Horizon Telescope Results

Citation

“First M87 Event Horizon Telescope Results. I. The Shadow of the Supermassive Black Hole,” EHT Collaboration et al 2019 ApJL 875 L1. doi:10.3847/2041-8213/ab0ec7

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