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Leaving so soon? Unusual planetary nebula fades mere decades after it arrived

Stars are rather patient. They can live for billions of years, and they typically make slow transitions — sometimes over many millions of years — between the different stages of their lives.

So when a previously typical star’s behavior rapidly changes in a few decades, astronomers take note and get to work.

Such is the case with a star known as SAO 244567, which lies at the center of Hen 3-1357, commonly known as the Stingray Nebula. The Stingray Nebula is a planetary nebula — an expanse of material sloughed off from a star as it enters a new phase of old age and then heated by that same star into colorful displays that can last for up to a million years.

The tiny Stingray Nebula unexpectedly appeared in the 1980s and was first imaged by scientists in the 1990s using NASA’s Hubble Space Telescope. It is by far the youngest planetary nebula in our sky. A team of astronomers recently analyzed a more recent image of the nebula, taken in 2016 by Hubble, and found something unexpected: As they report in a paper accepted to the Astrophysical Journal, the Stingray Nebula has faded significantly and changed shape over the course of just 20 years.

If dimming continues at current rates, in 20 or 30 years the Stingray Nebula will be barely perceptible, and was likely already fading when Hubble obtained the first clear images of it in 1996, according to lead author Bruce Balick, an emeritus professor of astronomy at UW.

“This is an unprecedented departure from typical behavior for a planetary nebula,” said Balick. “Over time, we would expect it to imperceptibly brighten and expand, which could easily go unnoticed in a century or more. But here we’re seeing the Stingray nebula fade significantly in an incredibly compressed time frame of just 20 years. Moreover, its brightest inner structure has contracted — not expanded — as the nebula fades.”

Planetary nebulae form after most stars, including stars like our own sun, swell into red giants as they exhaust hydrogen fuel. At the end of the red giant phase, the star then expels large amounts of its outer material as it gradually — over the course of a million years — transforms into a small, compact white dwarf. The sloughed-off material expands outward for several thousand years while the star heats the material, which eventually becomes ionized and glows.

Balick and his co-authors, Martín Guerrero at the Institute of Astrophysics of Andalusia in Spain and Gerardo Ramos-Larios at the University of Guadalajara in Mexico, compared Hubble images of the Stingray Nebula taken in 1996 and 2016. Hen 3-1357 changed shape markedly over 20 years, losing the sharp, sloping edges that gave the Stingray Nebula its name. Its colors have faded overall and once-prominent blue expanses of gas near its center are largely gone.

“In a planetary nebula, the star is really the center of all the activity,” said Balick. “The material around it is directly responsive to the energy from its parent star.”

The team analyzed light spectra from Hen 3-1357 emitted by chemical elements in the nebula. Emission levels of hydrogen, nitrogen, sulfur and oxygen all dropped between 1996 and 2016, particularly oxygen, which dropped by a factor of 900. The resulting fade in color and the nebula’s change in shape are likely connected to the cooling of its parent star — from a peak of about 107,500 degrees Fahrenheit in 2002 to just under 90,000 degrees Fahrenheit in 2015 — which means it is giving off less ultraviolet ionizing radiation that heats the expelled gas and makes it glow.

“Like a doused forest fire, the smoke wanes more slowly than the flames that created it,” said Balick. “Even so, we were amazed when the Hubble images revealed how quickly the nebula was fading. It took a month of work to believe it.”

Astronomers have yet to understand why SAO 244567 made the Stingray Nebula light up and then fade almost as quickly. One theory, posited by a team led by Nicole Reindl at the University of Potsdam, is that the star underwent a brief burst of fresh helium fusion around its core, which stirred up its outer layers and caused its surface to both shrink and heat.

If so, then as its outer layers settle back down, the star may return to a more typical transition from red giant to white dwarf. Only future observations of the star and its nebula can confirm this.

“Unfortunately, the best tool to follow future changes in the Stingray Nebula, the Hubble Space Telescope, is near the end of its life as well,” said Balick. “We can hope, but the odds aren’t good for Hubble’s survival as its three remaining gyroscopes start to fail. It’s a good race to the finish.”

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Titanium atom that exists in two places at once in crystal to blame for unusual phenomenon

The crystalline solid BaTiS3 (barium titanium sulfide) is terrible at conducting heat, and it turns out that a wayward titanium atom that exists in two places at the same time is to blame.

The discovery, made by researchers from Caltech, USC, and the Department of Energy’s Oak Ridge National Laboratory (ORNL), was published on November 27 in the journal Nature Communications. It provides a fundamental atomic-level insight into an unusual thermal property that has been observed in several materials. The work is of particular interest to researchers who are exploring the potential use of crystalline solids with poor thermal conductivity in thermoelectric applications, in which heat is directly converted into electric energy and vice versa.

“We have found that quantum mechanical effects can play a huge role in setting the thermal transport properties of materials even under familiar conditions like room temperature,” says Austin Minnich, professor of mechanical engineering and applied physics at Caltech and co-corresponding author of the Nature Communications paper.

Crystals are usually good at conducting heat. By definition, their atomic structure is highly organized, which allows atomic vibrations — heat — to flow through them as a wave. Glasses, on the other hand, are terrible at conducting heat. Their internal structure is disordered and random, which means that vibrations instead hop from atom to atom as they pass through.

BaTiS3 belongs to a class of materials called Perovskite-related chalcogenides. Jayakanth Ravichandran, an assistant professor in USC Viterbi’s Mork Family Department of Chemical Engineering and Materials Science, and his team have been investigating them for their optical properties and recently started studying their thermoelectric applications.

“We had a hunch that BaTiS3 will have low thermal conductivity, but the value was unexpectedly low. Our study shows a new mechanism to achieve low thermal conductivity, so the next question is whether the electrons in the system flow seamlessly unlike heat to achieve good thermoelectric properties,” says Ravichandran.

The team discovered that BaTiS3, along with several other crystalline solids, possessed “glass-like” thermal conductivity. Not only is its thermal conductivity comparable to those of disordered glasses, it actually gets worse as temperature goes down, which is the opposite of most materials. In fact, its thermal conductivity at cryogenic temperatures is among the worst ever observed in any fully dense (nonporous) solid.

The team found that the titanium atom in each BaTiS3 crystal exists in what is known as a double-well potential — that is, there are two spatial locations in the atomic structure where the atom wants to be. The titanium atom existing in two places at the same time gives rise to what is known as a “two-level system.” In this case, the titanium atom has two states: a ground state and an excited state. Passing atomic vibrations are absorbed by the titanium atom, which goes from the ground to the excited state, then quickly decays back to ground state. The absorbed energy is emitted in the form of a vibration and in a random direction.

The overall effect of this absorption and emission of vibrations is that energy is scattered rather than cleanly transferred. An analogy would be shining a light through a frosted glass, with the titanium atoms as the frost; incoming waves deflect off of the titanium, and only a portion make their way through the material.

Two-level systems have long been known to exist, but this is the first direct observation of one that was sufficient to disrupt thermal conduction in a single crystal material over an extended temperature range, measured here between 50 and 500 Kelvin.

The researchers observed the effect by bombarding BaTiS3 crystals with neutrons in a process known as inelastic scattering using the Spallation Neutron Source at ORNL. When they pass through the crystals, the neutrons gain or lose energy. This indicates that energy is absorbed from a two-level system in some cases and imparted to them in others.

“It took real detective work to solve this mystery about the structure and dynamics of the titanium atoms. At first it seemed that the atoms were just positionally disordered, but the shallowness of the potential well meant that they couldn’t stay in their positions for very long,” says Michael Manley, senior researcher at ORNL and co-corresponding author of the Nature Communications paper. That’s when Raphael Hermann, researcher at ORNL, suggested doing quantum calculations for the double well. “That atoms can tunnel is well known, of course, but we did not expect to see it at such a high frequency with such a large atom in a crystal. But the quantum mechanics is clear: if the barrier between the wells is small enough, then such high-frequency tunneling is indeed possible and should result in strong phonon scattering and thus glass-like thermal conductivity,” Manley says.

The conventional approach to creating crystalline solids with low thermal conductivity is to create a lot of defects in those solids, which is detrimental to other properties such as electrical conductivity. So, a method to design low-thermal-conductivity crystalline materials without any detriment to electrical and optical properties is highly desirable for thermoelectric applications. A small handful of crystalline solids exhibit the same poor thermal conductivity, so the team next plans to explore whether this phenomenon is to blame in those materials as well.

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Spamhaus Technology Launches Threat Intelligence API Beta

Spamhaus Technology Ltd has recently announced the release of its Intelligence API Beta. This is the first time Spamhaus has released its extensive threat intelligence via API, providing enriched data relating to IP addresses exhibiting compromised behavior.  

Available free of charge, developers can readily access enhanced data that catalogues IP addresses compromised by malware, worms, Trojan infections, devices controlled by botnets, and third party exploits, such as open proxies.

The API features live and historical data, including bot names, first seen dates, and valid until dates, providing security developers with the capacity to create additional applications to enhance network security. Using a combination of machine learning, manual investigations, and heuristics, Spamhaus’s researchers derive this information from data shared by the industry and beyond – including from hosting companies, ISPs, internet governing bodies such as ICANN, and from its own honeypots and spam traps.

“For years, the researchers at the Spamhaus Project have recorded a wealth of intelligence relating to IPs and domains. They’ve been working with big data long before it became the buzzword it is today,” explains Simon Forster, CEO at Spamhaus Technology. “It’s a pleasure to share this in a readily available API format with the wider internet community. We’re looking forward to seeing the security challenges this data can resolve in live environments.”

There are multiple applications for this data. Spamhaus Intelligence API can be integrated with current applications to provide increased visibility as to where issues have occurred, such as Splunk applications, for example. The use cases are numerous, and include improved incident response, online real-time risk assessment, and trend monitoring.

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Shuttering fossil fuel power plants may cost less than expected

Decarbonizing U.S. electricity production will require both construction of renewable energy sources and retirement of power plants now operated by fossil fuels. A generator-level model described in the December 4 issue of the journal Science suggests that most fossil fuel power plants could complete normal lifespans and still close by 2035 because so many facilities are nearing the end of their operational lives.

Meeting a 2035 deadline for decarbonizing U.S. electricity production, as proposed by the incoming U.S. presidential administration, would eliminate just 15% of the capacity-years left in plants powered by fossil fuels, says the article by Emily Grubert, a Georgia Institute of Technology researcher. Plant retirements are already underway, with 126 gigawatts of fossil generator capacity taken out of production between 2009 and 2018, including 33 gigawatts in 2017 and 2018 alone.

“Creating an electricity system that does not contribute to climate change is actually two processes — building carbon-free infrastructure like solar plants, and closing carbon-based infrastructure like coal plants,” said Grubert, an assistant professor in Georgia Tech’s School of Civil and Environmental Engineering. “My work shows that because a lot of U.S. fossil fuel plants are already pretty old, the target of decarbonization by 2035 would not require us to shut most of these plants down earlier than their typical lifespans.”

Of U.S. fossil fuel-fired generation capacity, 73% (630 out of 840 gigawatts) will reach the end of its typical lifespan by 2035; that percentage would reach 96% by 2050, she says in the Policy Forum article. About 13% of U.S. fossil fuel-fired generation capacity (110 GW) operating in 2018 had already exceeded its typical lifespan.

Because typical lifespans are averages, some generators operate for longer than expected. Allowing facilities to run until they retire is thus likely insufficient for a 2035 decarbonization deadline, the article notes. Closure deadlines that strand assets relative to reasonable lifespan expectations, however, could create financial liability for debts and other costs. The research found that a 2035 deadline for completely retiring fossil-based electricity generators would only strand about 15% (1700 gigawatt-years) of fossil fuel-fired capacity life, along with about 20% (380,000 job-years) of direct power plant and fuel extraction jobs that existed in 2018.

In 2018, fossil fuel facilities operated in 1,248 of 3,141 counties, directly employing about 157,000 people at generators and fuel-extraction facilities. Plant closure deadlines can improve outcomes for workers and host communities by providing additional certainty, for example, by enabling specific advance planning for things like remediation, retraining for displaced workers, and revenue replacements.

“Closing large industrial facilities like power plants can be really disruptive for the people that work there and live in the surrounding communities,” Grubert said. “We don’t want to repeat the damage we saw with the collapse of the steel industry in the 70s and 80s, where people lost jobs, pensions, and stability without warning. We already know where the plants are, and who might be affected: using the 2035 decarbonization deadline to guide explicit, community grounded planning for what to do next can help, even without a lot of financial support.”

Planning ahead will also help avoid creating new capital investment where that may not be needed long-term. “We shouldn’t build new fossil fuel power plants that would still be young in 2035, and we need to have explicit plans for closures both to ensure the system keeps working and to limit disruption for host communities,” she said.

Underlying policies governing the retirement of fossil fuel-powered facilities is the concept of a “just transition” that ensures material well-being and distributional justice for individuals and communities affected by a transition from fossil to non-fossil electricity systems. Determining which assets are “stranded,” or required to close earlier than expected absent policy, is vital for managing compensation for remaining debt and/or lost revenue, Grubert said in the article.

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Physicists capture the sound of a perfect fluid

For some, the sound of a “perfect flow” might be the gentle lapping of a forest brook or perhaps the tinkling of water poured from a pitcher. For physicists, a perfect flow is more specific, referring to a fluid that flows with the smallest amount of friction, or viscosity, allowed by the laws of quantum mechanics. Such perfectly fluid behavior is rare in nature, but it is thought to occur in the cores of neutron stars and in the soupy plasma of the early universe.

Now MIT physicists have created a perfect fluid in the laboratory, and listened to how sound waves travel through it. The recording is a product of a glissando of sound waves that the team sent through a carefully controlled gas of elementary particles known as fermions. The pitches that can be heard are the particular frequencies at which the gas resonates like a plucked string.

The researchers analyzed thousands of sound waves traveling through this gas, to measure its “sound diffusion,” or how quickly sound dissipates in the gas, which is related directly to a material’s viscosity, or internal friction.

Surprisingly, they found that the fluid’s sound diffusion was so low as to be described by a “quantum” amount of friction, given by a constant of nature known as Planck’s constant, and the mass of the individual fermions in the fluid.

This fundamental value confirmed that the strongly interacting fermion gas behaves as a perfect fluid, and is universal in nature. The results, published today in the journal Science, demonstrate the first time that scientists have been able to measure sound diffusion in a perfect fluid.

Scientists can now use the fluid as a model of other, more complicated perfect flows, to estimate the viscosity of the plasma in the early universe, as well as the quantum friction within neutron stars — properties that would otherwise be impossible to calculate. Scientists might even be able to approximately predict the sounds they make.

“It’s quite difficult to listen to a neutron star,” says Martin Zwierlein, the Thomas A. Franck Professor of Physics at MIT. “But now you could mimic it in a lab using atoms, shake that atomic soup and listen to it, and know how a neutron star would sound.”

While a neutron star and the team’s gas differ widely in terms of their size and the speed at which sound travels through, from some rough calculations Zwierlein estimates that the star’s resonant frequencies would be similar to those of the gas, and even audible — “if you could get your ear close without being ripped apart by gravity,” he adds.

Zwierlein’s co-authors are lead author Parth Patel, Zhenjie Yan, Biswaroop Mukherjee, Richard Fletcher, and Julian Struck of the MIT-Harvard Center for Ultracold Atoms.

Tap, listen, learn

To create a perfect fluid in the lab, Zwierlein’s team generated a gas of strongly interacting fermions — elementary particles, such as electrons, protons, and neutrons, that are considered the building blocks of all matter. A fermion is defined by its half-integer spin, a property that prevents one fermion from assuming the same spin as another nearby fermion. This exclusive nature is what enables the diversity of atomic structures found in the periodic table of elements.

“If electrons were not fermions, but happy to be in the same state, hydrogen, helium, and all atoms, and we ourselves, would look the same, like some terrible, boring soup,” Zwierlein says.

Fermions naturally prefer to keep apart from each other. But when they are made to strongly interact, they can behave as a perfect fluid, with very low viscosity. To create such a perfect fluid, the researchers first used a system of lasers to trap a gas of lithium-6 atoms, which are considered fermions.

The researchers precisely configured the lasers to form an optical box around the fermion gas. The lasers were tuned such that whenever the fermions hit the edges of the box they bounced back into the gas. Also, the interactions between fermions were controlled to be as strong as allowed by quantum mechanics, so that inside the box, fermions had to collide with each other at every encounter. This made the fermions turn into a perfect fluid.

“We had to make a fluid with uniform density, and only then could we tap on one side, listen to the other side, and learn from it,” Zwierlein says. “It was actually quite diffult to get to this place where we could use sound in this seemingly natural way.”

“Flow in a perfect way”

The team then sent sound waves through one side of the optical box by simply varying the brightness of one of the walls, to generate sound-like vibrations through the fluid at particular frequencies. They recorded thousands of snapshots of the fluid as each sound wave rippled through.

“All these snapshots together give us a sonogram, and it’s a bit like what’s done when taking an ultrasound at the doctor’s office,” Zwierlein says.

In the end, they were able to watch the fluid’s density ripple in response to each type of sound wave. They then looked for the sound frequencies that generated a resonance, or an amplified sound in the fluid, similar to singing at a wine glass and finding the frequency at which it shatters.

“The quality of the resonances tells me about the fluid’s viscosity, or sound diffusivity,” Zwierlein explains. “If a fluid has low viscosity, it can build up a very strong sound wave and be very loud, if hit at just the right frequency. If it’s a very viscous fluid, then it doesn’t have any good resonances.”

From their data, the researchers observed clear resonances through the fluid, particularly at low frequencies. From the distribution of these resonances, they calculated the fluid’s sound diffusion. This value, they found, could also be calculated very simply via Planck’s constant and the mass of the average fermion in the gas.

This told the researchers that the gas was a perfect fluid, and fundamental in nature: Its sound diffusion, and therefore its viscosity, was at the lowest possible limit set by quantum mechanics.

Zwierlein says in addition to using the results to estimate quantum friction in more exotic matter, such as neutron stars, the results can be helpful in understanding how certain materials might be made to exhibit perfect, superconducting flow.

“This work connects directly to resistance in materials,” Zwierlein says. “Having figured out what’s the lowest resistance you could have from a gas tells us what can happen with electrons in materials, and how one might make materials where electrons could flow in a perfect way. That’s exciting.”

This research was supported, in part, by the National Science Foundation and the NSF Center for Ultracold Atoms, the Air Force Office of Scientific Research, the Office of Naval Research, and the David and Lucile Packard Foundation.

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Scientists peer into the 3D structure of the Milky Way

Scientists from Cardiff University have helped produce a brand-new, three-dimensional survey of our galaxy, allowing them to peer into the inner structure and observe its star-forming processes in unprecedented detail.

The large-scale survey, called SEDIGISM (Structure, Excitation and Dynamics of the Inner Galactic Interstellar Medium), has revealed a wide range of structures within the Milky Way, from individual star-forming clumps to giant molecular clouds and complexes, that will allow astronomers to start pushing the boundaries of what we know about the structure of our galaxy.

SEDIGISM has been unveiled today through the publication of three separate papers in the Monthly Notices of the Royal Astronomical Society, authored by an international team of over 50 astronomers.

“With the publication of this unprecedentedly detailed map of cold clouds in our Milky Way, a huge observational effort comes to fruition,” says Frederic Schuller from the Max Planck Institute for Radio Astronomy (MPIfR), lead author of one of the three publications, presenting the data release.

Dr Ana Duarte Cabral, a Royal Society University Research Fellow from Cardiff University’s School of Physics and Astronomy, was lead author on one of the papers and has provided a catalogue of over 10,000 clouds of molecular gas in our Milky Way.

The Milky Way, named after its hazy appearance from Earth, is a spiral galaxy with an estimated diameter between 170,000 and 200,000 light-years which contains between 100-400 billion stars.

The Milky Way consists of a core region that is surrounded by a warped disk of gas and dust that provides the raw materials from which new stars are formed.

For Dr Duarte Cabral, the new catalogue of gas clouds will allow scientists to probe exactly how the spiral structure of our own Milky Way affects the life cycle of clouds, their properties, and ultimately the star formation that goes on within them.

“What is most exciting about this survey is that it can really help pin down the global galactic structure of the Milky Way, providing an astounding 3D view of the inner galaxy,” she said.

“With this survey we really have the ability to start pushing the boundaries of what we know about the global effects of the galactic structures and dynamics, in the distribution of molecular gas and star formation, because of the improved sensitivity, resolution, and the 3D view.”

The catalogue of molecular gas clouds was created by measuring the rare isotope of the carbon monoxide molecule, 13CO, using the extremely sensitive 12-metre Atacama Pathfinder Experiment telescope on the Chajnantor plateau in Chile.

This allowed the team to produce more precise estimates of the mass of the gas clouds and discern information about their velocity, therefore providing a truly three-dimensional picture of the galaxy.

Dr Duarte Cabral and colleagues are already beginning to tease out information from the vast amount of data at their disposal.

“The survey revealed that only a small proportion, roughly 10%, of these clouds have dense gas with ongoing star formation,” said James Urquhart from the University of Kent, the lead author of the third publication.

Similarly, the results from the work led by Dr Duarte Cabral suggest that the structure of the Milky Way is not that well defined and that the spiral arms are not that clear.

They have also shown that the properties of clouds do not seem to be dependent on whether a cloud is located in a spiral arm or an inter-arm region, where they expected very different physics to be playing a role.

“Our results are already showing us that the Milky Way may not be a strong grand design type of spiral galaxy as we thought, but perhaps more flocculent in nature,” Dr Duarte Cabral continued.

“This survey can be used by anyone that wants to study the kinematics or physical properties of individual molecular clouds or even make statistical studies of larger samples of clouds, and so in itself has a huge legacy value for the star formation community.”

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Astronomers to release most accurate data ever for nearly two billion stars

On 3 December an international team of astronomers will announce the most detailed ever catalogue of the stars in a huge swathe of our Milky Way galaxy. The measurements of stellar positions, movement, brightness and colours are in the third early data release from the European Space Agency’s Gaia space observatory and will be publicly available. Initial findings include the first optical measurement of the acceleration of the Solar system. The data set, and early scientific discoveries, will be presented at a special briefing hosted by the Royal Astronomical Society.

Launched in 2013, Gaia operates in an orbit around the so-called Lagrange 2 (L2) point, located 1.5 million kilometres behind the Earth in the direction away from the Sun. At L2 the gravitational forces between the Earth and Sun are balanced, so the spacecraft stays in a stable position, allowing long-term essentially unobstructed views of the sky.

The primary objective of Gaia is measure stellar distances using the parallax method. In this case astronomers use the observatory to continuously scan the sky, measuring the apparent change in the positions of stars over time, resulting from the Earth’s movement around the Sun.

Knowing that tiny shift in the positions of stars allows their distances to be calculated. On Earth this is made more difficult by the blurring of the Earth’s atmosphere, but in space the measurements are only limited by the optics of the telescope.

Two previous releases included the positions of 1.6 billion stars. This release brings the total to just under 2 billion stars, whose positions are significantly more accurate than in the earlier data. Gaia also tracks the changing brightness and positions of the stars over time across the line of sight (their so-called proper motion), and by splitting their light into spectra, measures how fast they are moving towards or away from the Sun and assesses their chemical composition.

The new data include exceptionally accurate measurements of the 300,000 stars within the closest 326 light years to the Sun. The researchers use these data to predict how the star background will change in the next 1.6 million years. They also confirm that the Solar system is accelerating in its orbit around the Galaxy.

This acceleration is gentle, and is what would be expected from a system in a circular orbit. Over a year the Sun accelerates towards the centre of the Galaxy by 7 mm per second, compared with its speed along its orbit of about 230 kilometres a second.

Gaia data additionally deconstruct the two largest companion galaxies to the Milky Way, the Small and Large Magellanic Clouds, allowing researchers to see their different stellar populations. A dramatic visualisation shows these subsets, and the bridge of stars between the two systems.

Dr Floor van Leeuwen of the Institute of Astronomy at the University of Cambridge, and UK Gaia DPAC Project Manager, comments: “Gaia is measuring the distances of hundreds of millions of objects that are many thousands of light years away, at an accuracy equivalent to measuring the thickness of hair at a distance of more than 2000 kilometres. These data are one of the backbones of astrophysics, allowing us to forensically analyse our stellar neighbourhood, and tackle crucial questions about the origin and future of our Galaxy.”

Gaia will continue gathering data until at least 2022, with a possible mission extension until 2025. The final data releases are expected to yield stellar positions 1.9 times as accurate as those released so far, and proper motions more than 7 times more accurate, in a catalogue of more than 2 billion objects.

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Voyager spacecraft detect new type of solar electron burst

More than 40 years since they launched, the Voyager spacecraft are still making discoveries.

In a new study, a team of physicists led by the University of Iowa report the first detection of bursts of cosmic ray electrons accelerated by shock waves originating from major eruptions on the sun. The detection, made by instruments onboard both the Voyager 1 and Voyager 2 spacecraft, occurred as the Voyagers continue their journey outward through interstellar space, thus making them the first craft to record this unique physics in the realm between stars.

These newly detected electron bursts are like an advanced guard accelerated along magnetic field lines in the interstellar medium; the electrons travel at nearly the speed of light, some 670 times faster than the shock waves that initially propelled them. The bursts were followed by plasma wave oscillations caused by lower-energy electrons arriving at the Voyagers’ instruments days later — and finally, in some cases, the shock wave itself as long as a month after that.

The shock waves emanated from coronal mass ejections, expulsions of hot gas and energy that move outward from the sun at about one million miles per hour. Even at those speeds, it takes more than a year for the shock waves to reach the Voyager spacecraft, which have traveled further from the sun (more than 14 billion miles and counting) than any human-made object.

“What we see here specifically is a certain mechanism whereby when the shock wave first contacts the interstellar magnetic field lines passing through the spacecraft, it reflects and accelerates some of the cosmic ray electrons,” says Don Gurnett, professor emeritus in physics and astronomy at Iowa and the study’s corresponding author. “We have identified through the cosmic ray instruments these are electrons that were reflected and accelerated by interstellar shocks propagating outward from energetic solar events at the sun. That is a new mechanism.”

The discovery could help physicists better understand the dynamics underpinning shock waves and cosmic radiation that come from flare stars (which can vary in brightness briefly due to violent activity on their surface) and exploding stars. The physics of such phenomena would be important to consider when sending astronauts on extended lunar or Martian excursions, for instance, during which they would be exposed to concentrations of cosmic rays far exceeding what we experience on Earth.

The physicists believe these electrons in the interstellar medium are reflected off of a strengthened magnetic field at the edge of the shock wave and subsequently accelerated by the motion of the shock wave. The reflected electrons then spiral along interstellar magnetic field lines, gaining speed as the distance between them and the shock increases.

In a 2014 paper in the journal Astrophysical Letters, physicists J.R. Jokipii and Jozsef Kota described theoretically how ions reflected from shock waves could be accelerated along interstellar magnetic field lines. The current study looks at bursts of electrons detected by the Voyager spacecraft that are thought to be accelerated by a similar process.

“The idea that shock waves accelerate particles is not new,” Gurnett says. “It all has to do with how it works, the mechanism. And the fact we detected it in a new realm, the interstellar medium, which is much different than in the solar wind where similar processes have been observed. No one has seen it with an interstellar shock wave, in a whole new pristine medium.”

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