Extraterrestrial chemistry with earthbound possibilities

Who are we? Why are we here? As the Crosby, Stills, Nash & Young song suggests, we are stardust, the result of chemistry occurring throughout vast clouds of interstellar gas and dust. To better understand how that chemistry could create prebiotic molecules -- the seeds of life on Earth and possibly elsewhere -- researchers investigated the role of low-energy electrons created as cosmic radiation traverses through ice particles. Their findings may also inform medical and environmental applications on our home planet.

Undergraduate student Kennedy Barnes will present the team's results at the fall meeting of the American Chemical Society (ACS).

"The first detection of molecules in space was made by Wellesley College alum Annie Jump Cannon more than a hundred years ago," says Barnes, who, with fellow undergraduate Rong Wu, led this study at Wellesley, mentored by chemistry professor Christopher Arumainayagam and physics professor James Battat. Since Cannon's discovery, scientists have been interested in finding out how extraterrestrial molecules form. "Our goal is to explore the relative importance of low-energy electrons versus photons in instigating the chemical reactions responsible for the extraterrestrial synthesis of these prebiotic molecules," Barnes explains.

The few studies that previously probed this question suggested that both electrons and photons can catalyze the same reactions. Studies by Barnes and colleagues, however, hint that the prebiotic molecule yield from low-energy electrons and photons could be significantly different in space. "Our calculations suggest that the number of cosmic-ray-induced electrons within cosmic ice could be much greater than the number of photons striking the ice," Barnes explains. "Therefore, electrons likely play a more significant role than photons in the extraterrestrial synthesis of prebiotic molecules."

Aside from cosmic ice, her research into low-energy electrons and radiation chemistry also has potential applications on Earth. Barnes and colleagues recently studied the radiolysis of water, finding evidence of electron-stimulated release of hydrogen peroxide and hydroperoxyl radicals, which destroy stratospheric ozone and act as damaging reactive oxygen species in cells.

"A lot of our water radiolysis research findings could be used in medical applications and medical simulations," Barnes shares, offering the example of using high-energy radiation to treat cancer. "I once had a biochemistry professor say that humans are basically bags of water. So, other scientists are investigating how low-energy electrons produced in water affect our DNA molecules."

She also says the team's findings are applicable to environmental remediation efforts where wastewater is being treated with high-energy radiation, which produces large numbers of low-energy electrons that are assumed to be responsible for the destruction of hazardous chemicals.

Back to space chemistry, in attempting to better understand prebiotic molecule synthesis, the researchers didn't limit their efforts to mathematical modeling; they also tested their hypothesis by mimicking the conditions of space in the lab. They use an ultrahigh-vacuum chamber containing an ultrapure copper substrate that they can cool to ultralow temperatures, along with an electron gun that produces low-energy electrons and a laser-driven plasma lamp that produces low-energy photons. The scientists then bombard nanoscale ice films with electrons or photons to see what molecules are produced.

"Although we have previously focused on how this research is applicable to interstellar submicron ice particles, it is also relevant to cosmic ice on a much larger scale, like that of Jupiter's moon Europa, which has a 20-mile-thick ice shell," says Barnes.

Thus, she suggests their research will help astronomers understand data from space exploration missions such as NASA's James Webb Space Telescope as well as the Europa Clipper, initially expected to launch in October 2024. Barnes hopes that their findings will inspire other researchers to incorporate low-energy electrons into their astrochemistry models that simulate what happens within cosmic ices.

Barnes and colleagues are also varying the molecular composition of ice films and exploring atom addition reactions to see if low-energy electrons can produce other prebiotic chemistries. This work is being performed in collaboration with researchers at the Laboratory for the Study of Radiation and Matter in Astrophysics and Atmospheres in France.

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Falling stardust, wobbly jets explain blinking gamma ray bursts

A Northwestern University-led team of astrophysicists has developed the first-ever full 3D simulation of an entire evolution of a jet formed by a collapsing star, or a "collapsar."

Because these jets generate gamma ray bursts (GRBs) -- the most energetic and luminous events in the universe since the Big Bang -- the simulations have shed light on these peculiar, intense bursts of light. Their new findings include an explanation for the longstanding question of why GRBs are mysteriously punctuated by quiet moments -- blinking between powerful emissions and an eerily quiet stillness. The new simulation also shows that GRBs are even rarer than previously thought.

The new study will be published on June 29 in Astrophysical Journal Letters. It marks the first full 3D simulation of the entire evolution of a jet -- from its birth near the black hole to its emission after escaping from the collapsing star. The new model also is the highest-ever resolution simulation of a large-scale jet.

"These jets are the most powerful events in the universe," said Northwestern's Ore Gottlieb, who led the study. "Previous studies have tried to understand how they work, but those studies were limited by computational power and had to include many assumptions. We were able to model the entire evolution of the jet from the very beginning -- from its birth by a black hole -- without assuming anything about the jet's structure. We followed the jet from the black hole all the way to the emission site and found processes that have been overlooked in previous studies."

Gottlieb is a Rothschild Fellow in Northwestern's Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA). He coauthored the paper with CIERA member Sasha Tchekhovskoy, an assistant professor of physics and astronomy at Northwestern's Weinberg College of Arts and Sciences.

Weird wobbling

The most luminous phenomenon in the universe, GRBs emerge when the core of a massive star collapses under its own gravity to form a black hole. As gas falls into the rotating black hole, it energizes -- launching a jet into the collapsing star. The jet punches the star until finally escaping from it, accelerating at speeds close to the speed of light. After breaking free from the star, the jet generates a bright GRB.

"The jet generates a GRB when it reaches about 30 times the size of the star -- or a million times the size of the black hole," Gottlieb said. "In other words, if the black hole is the size of a beach ball, the jet needs to expand over the entire size of France before it can produce a GRB."

Due to the enormity of this scale, previous simulations have been unable to model the full evolution of the jet's birth and subsequent journey. Using assumptions, all previous studies found that the jet propagates along one axis and never deviates from that axis.

But Gottlieb's simulation showed something very different. As the star collapses into a black hole, material from that star falls onto the disk of magnetized gas that swirls around the black hole. The falling material causes the disk to tilt, which, in turn, tilts the jet. As the jet struggles to realign with its original trajectory, it wobbles inside the collapsar.

This wobbling provides a new explanation for why GRBs blink. During the quiet moments, the jet doesn't stop -- its emission beams away from Earth, so telescopes simply cannot observe it.

"Emission from GRBs is always irregular," Gottlieb said. "We see spikes in emission and then a quiescent time that lasts for a few seconds or more. The entire duration of a GRB is about one minute, so these quiescent times are a non-negligible fraction of the total duration. Previous models were not able to explain where these quiescent times were coming from. This wobbling naturally gives an explanation to that phenomenon. We observe the jet when its pointing at us. But when the jet wobbles to point away from us, we cannot see its emission. This is part of Einstein's theory of relativity."

Rare becomes rarer

These wobbly jets also provide new insights into the rate and nature of GRBs. Although previous studies estimated that about 1% of collapsars produce GRBs, Gottlieb believes that GRBs are actually much rarer.

If the jet were constrained to moving along one axis, then it would only cover a thin slice of the sky -- limiting the likelihood of observing it. But the wobbly nature of the jet means that astrophysicists can observe GRBs at different orientations, increasing the likelihood of spotting them. According to Gottlieb's calculations, GRBs are 10 times more observable than previously thought, which means that astrophysicists are missing 10 times fewer GRBs than previously thought.

"The idea is that we observe GRBs on the sky in a certain rate, and we want to learn about the true rate of GRBs in the universe," Gottlieb explained. "The observed and true rates are different because we can only see the GRBs that are pointing at us. That means we need to assume something about the angle that these jets cover on the sky, in order to infer the true rate of GRBs. That is, what fraction of GRBs we are missing. Wobbling increases the number of detectable GRBs, so the correction from the observed to true rate is smaller. If we miss fewer GRBs, then there are fewer GRBs overall in the sky."

If this is true, Gottlieb posits, then most of the jets either fail to be launched at all or never succeed in escaping from the collapsar to produce a GRB. Instead, they remain buried inside.

Mixed energy

The new simulations also revealed that some of the magnetic energy in the jets partially converts to thermal energy. This suggests that the jet has a hybrid composition of magnetic and thermal energies, which produce the GRB. In a major step forward in understanding the mechanisms that power GRBs, this is the first time researchers have inferred the jet composition of GRBs at the time of emission.

"Studying jets enables us to 'see' what happens deep inside the star as it collapses," Gottlieb said. "Otherwise, it's difficult to learn what happens in a collapsed star because light cannot escape from the stellar interior. But we can learn from the jet emission -- the history of the jet and the information that it carries from the systems that launch them."

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From stardust to pale blue dot: Carbon's interstellar journey to Earth

 We are made of stardust, the saying goes, and a pair of studies including University of Michigan research finds that may be more true than we previously thought.

The first study, led by U-M researcher Jie (Jackie) Li and published in Science Advances, finds that most of the carbon on Earth was likely delivered from the interstellar medium, the material that exists in space between stars in a galaxy. This likely happened well after the protoplanetary disk, the cloud of dust and gas that circled our young sun and contained the building blocks of the planets, formed and warmed up.

Carbon was also likely sequestered into solids within one million years of the sun's birth -- which means that carbon, the backbone of life on earth, survived an interstellar journey to our planet.

Previously, researchers thought carbon in the Earth came from molecules that were initially present in nebular gas, which then accreted into a rocky planet when the gases were cool enough for the molecules to precipitate. Li and her team, which includes U-M astronomer Edwin Bergin, Geoffrey Blake of the California Institute of Technology, Fred Ciesla of the University of Chicago and Marc Hirschmann of the University of Minnesota, point out in this study that the gas molecules that carry carbon wouldn't be available to build the Earth because once carbon vaporizes, it does not condense back into a solid.

"The condensation model has been widely used for decades. It assumes that during the formation of the sun, all of the planet's elements got vaporized, and as the disk cooled, some of these gases condensed and supplied chemical ingredients to solid bodies. But that doesn't work for carbon," said Li, a professor in the U-M Department of Earth and Environmental Sciences.

Much of carbon was delivered to the disk in the form of organic molecules. However, when carbon is vaporized, it produces much more volatile species that require very low temperatures to form solids. More importantly, carbon does not condense back again into an organic form. Because of this, Li and her team inferred most of Earth's carbon was likely inherited directly from the interstellar medium, avoiding vaporization entirely.

To better understand how Earth acquired its carbon, Li estimated the maximum amount of carbon Earth could contain. To do this, she compared how quickly a seismic wave travels through the core to the known sound velocities of the core. This told the researchers that carbon likely makes up less than half a percent of Earth's mass. Understanding the upper bounds of how much carbon the Earth might contain tells the researchers information about when the carbon might have been delivered here.

"We asked a different question: We asked how much carbon could you stuff in the Earth's core and still be consistent with all the constraints," Bergin said, professor and chair of the U-M Department of Astronomy. "There's uncertainty here. Let's embrace the uncertainty to ask what are the true upper bounds for how much carbon is very deep in the Earth, and that will tell us the true landscape we're within."

A planet's carbon must exist in the right proportion to support life as we know it. Too much carbon, and the Earth's atmosphere would be like Venus, trapping heat from the sun and maintaining a temperature of about 880 degrees Fahrenheit. Too little carbon, and Earth would resemble Mars: an inhospitable place unable to support water-based life, with temperatures around minus 60.

In a second study by the same group of authors, but led by Hirschmann of the University of Minnesota, the researchers looked at how carbon is processed when the small precursors of planets, known as planetesimals, retain carbon during their early formation. By examining the metallic cores of these bodies, now preserved as iron meteorites, they found that during this key step of planetary origin, much of the carbon must be lost as the planetesimals melt, form cores and lose gas. This upends previous thinking, Hirschmann says.

"Most models have the carbon and other life-essential materials such as water and nitrogen going from the nebula into primitive rocky bodies, and these are then delivered to growing planets such as Earth or Mars," said Hirschmann, professor of earth and environmental sciences. "But this skips a key step, in which the planetesimals lose much of their carbon before they accrete to the planets."

Hirschmann's study was recently published in Proceedings of the National Academy of Sciences.

"The planet needs carbon to regulate its climate and allow life to exist, but it's a very delicate thing," Bergin said. "You don't want to have too little, but you don't want to have too much."

Bergin says the two studies both describe two different aspects of carbon loss -- and suggest that carbon loss appears to be a central aspect in constructing the Earth as a habitable planet.

"Answering whether or not Earth-like planets exist elsewhere can only be achieved by working at the intersection of disciplines like astronomy and geochemistry," said Ciesla, a U. of C. professor of geophysical sciences. "While approaches and the specific questions that researchers work to answer differ across the fields, building a coherent story requires identifying topics of mutual interest and finding ways to bridge the intellectual gaps between them. Doing so is challenging, but the effort is both stimulating and rewarding."

Blake, a co-author on both studies and a Caltech professor of cosmochemistry and planetary science, and of chemistry, says this kind of interdisciplinary work is critical.

"Over the history of our galaxy alone, rocky planets like the Earth or a bit larger have been assembled hundreds of millions of times around stars like the Sun," he said. "Can we extend this work to examine carbon loss in planetary systems more broadly? Such research will take a diverse community of scholars."

Read more at Science Daily

New method to determine the origin of stardust in meteorites

 Scientists have made a key discovery thanks to stardust found in meteorites, shedding light on the origin of crucial chemical elements.

Meteorites are critical to understanding the beginning of our solar system and how it has evolved over time. However, some meteorites contain grains of stardust that predate the formation of our solar system and are now providing important information about how the elements in the universe formed.

In a study published by Physical Review Letters, researchers from the University of Surrey detail how they made a key discovery connected to the "pre-solar grains" found in primitive meteorites. This discovery has provided new insights into the nature of stellar explosions and the origin of the chemical elements. It has also provided a new method for astronomical research.

Dr Gavin Lotay, Nuclear Astrophysicist and Director of Learning and Teaching at the University of Surrey, said: "Tiny pre-solar grains, about one micron in size, are the residuals of stellar explosions that occurred in the distant past, long before our solar system existed. Stellar debris eventually became wedged into meteorites that, in turn, crashed into the Earth."

One of the most frequent stellar explosions to occur in our galaxy is called a nova, which involves a binary star system consisting of a main sequence star orbiting a white dwarf star -- an extremely dense star that can be the size of Earth but has the mass of our Sun. Matter from the main star is continually pulled away by the white dwarf because of its intense gravitational field. This deposited material initiates a thermonuclear explosion every 1,000 to 100,000 years and the white dwarf ejects the equivalent of the mass of more than thirty Earths into interstellar space. In contrast, a supernova involves a single collapsing star and, when it explodes, it ejects almost all of its mass.

As novae continually enrich our galaxy with chemical elements, they have been the subject of intense astronomical investigations for decades. Much has been learned from them about the origin of the heavier elements, for example. However, a number of key puzzles remain.

Dr Lotay continues: "A new way of studying these phenomena is by analysing the chemical and isotopic composition of the pre-solar grains in meteorites. Of particular importance to our research is a specific nuclear reaction that occurs in novae and supernovae -- proton capture on an isotope of chlorine -- which we can only indirectly study in the laboratory."

In conducting their experiment, the team, led by Dr Lotay and Surrey PhD student Adam Kennington (also a former Surrey undergraduate), pioneered a new research approach. It involves the use of the Gamma-Ray Energy Tracking In-beam Array (GRETINA) coupled to the Fragment Mass Analyzer at the Argonne Tandem Linac Accelerator System (ATLAS), USA. GRETINA is a state-of-the-art detection system able to trace the path of gamma rays (g-ray) emitted from nuclear reactions. It is one of only two such systems in the world that utilise this novel technology.

Using GRETINA, the team completed the first detailed g-ray spectroscopy study of an astronomically important nucleus, argon-34, and were able to calculate the expected abundance of sulfur isotopes produced in nova explosions.

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Meteorite contains the oldest material on Earth: 7-billion-year-old stardust

Illustration of meteor entering Earth's atmosphere

Stars have life cycles. They're born when bits of dust and gas floating through space find each other and collapse in on each other and heat up. They burn for millions to billions of years, and then they die. When they die, they pitch the particles that formed in their winds out into space, and those bits of stardust eventually form new stars, along with new planets and moons and meteorites. And in a meteorite that fell fifty years ago in Australia, scientists have now discovered stardust that formed 5 to 7 billion years ago -- the oldest solid material ever found on Earth.

"This is one of the most exciting studies I've worked on," says Philipp Heck, a curator at the Field Museum, associate professor at the University of Chicago, and lead author of a paper describing the findings in the Proceedings of the National Academy of Sciences. "These are the oldest solid materials ever found, and they tell us about how stars formed in our galaxy."

The materials Heck and his colleagues examined are called presolar grains-minerals formed before the Sun was born. "They're solid samples of stars, real stardust," says Heck. These bits of stardust became trapped in meteorites where they remained unchanged for billions of years, making them time capsules of the time before the solar system..

But presolar grains are hard to come by. They're rare, found only in about five percent of meteorites that have fallen to Earth, and they're tiny-a hundred of the biggest ones would fit on the period at the end of this sentence. But the Field Museum has the largest portion of the Murchison meteorite, a treasure trove of presolar grains that fell in Australia in 1969 and that the people of Murchison, Victoria, made available to science. Presolar grains for this study were isolated from the Murchison meteorite for this study about 30 years ago at the University of Chicago.

"It starts with crushing fragments of the meteorite down into a powder ," explains Jennika Greer, a graduate student at the Field Museum and the University of Chicago and co-author of the study. "Once all the pieces are segregated, it's a kind of paste, and it has a pungent characteristic-it smells like rotten peanut butter."

This "rotten-peanut-butter-meteorite paste" was then dissolved with acid, until only the presolar grains remained. "It's like burning down the haystack to find the needle," says Heck.

Once the presolar grains were isolated, the researchers figured out from what types of stars they came and how old they were. "We used exposure age data, which basically measures their exposure to cosmic rays, which are high-energy particles that fly through our galaxy and penetrate solid matter," explains Heck. "Some of these cosmic rays interact with the matter and form new elements. And the longer they get exposed, the more those elements form.

"I compare this with putting out a bucket in a rainstorm. Assuming the rainfall is constant, the amount of water that accumulates in the bucket tells you how long it was exposed," he adds. By measuring how many of these new cosmic-ray produced elements are present in a presolar grain, we can tell how long it was exposed to cosmic rays, which tells us how old it is.

The researchers learned that some of the presolar grains in their sample were the oldest ever discovered-based on how many cosmic rays they'd soaked up, most of the grains had to be 4.6 to 4.9 billion years old, and some grains were even older than 5.5 billion years. For context, our Sun is 4.6 billion years old, and Earth is 4.5 billion.

But the age of the presolar grains wasn't the end of the discovery. Since presolar grains are formed when a star dies, they can tell us about the history of stars. And 7 billion years ago, there was apparently a bumper crop of new stars forming-a sort of astral baby boom.

"We have more young grains that we expected," says Heck. "Our hypothesis is that the majority of those grains, which are 4.9 to 4.6 billion years old, formed in an episode of enhanced star formation. There was a time before the start of the Solar System when more stars formed than normal."

This finding is ammo in a debate between scientists about whether or not new stars form at a steady rate, or if there are highs and lows in the number of new stars over time. "Some people think that the star formation rate of the galaxy is constant," says Heck. "But thanks to these grains, we now have direct evidence for a period of enhanced star formation in our galaxy seven billion years ago with samples from meteorites. This is one of the key findings of our study."

Heck notes that this isn't the only unexpected thing his team found. As almost a side note to the main research questions, in examining the way that the minerals in the grains interacted with cosmic rays, the researchers also learned that presolar grains often float through space stuck together in large clusters, "like granola," says Heck. "No one thought this was possible at that scale."

Heck and his colleagues look forward to all of these discoveries furthering our knowledge of our galaxy. "With this study, we have directly determined the lifetimes of stardust. We hope this will be picked up and studied so that people can use this as input for models of the whole galactic life cycle," he says.

Heck notes that there are lifetimes' worth of questions left to answer about presolar grains and the early Solar System. "I wish we had more people working on it to learn more about our home galaxy, the Milky Way," he says.

"Once learning about this, how do you want to study anything else?" says Greer. "It's awesome, it's the most interesting thing in the world."

"I always wanted to do astronomy with geological samples I can hold in my hand," says Heck. "It's so exciting to look at the history of our galaxy. Stardust is the oldest material to reach Earth, and from it, we can learn about our parent stars, the origin of the carbon in our bodies, the origin of the oxygen we breathe. With stardust, we can trace that material back to the time before the Sun."

"It's the next best thing to being able to take a sample directly from a star," says Greer.

Read more at Science Daily

Stardust in the Antarctic snow

Antarctica illustration

The rare isotope iron-60 is created in massive stellar explosions. Only a very small amount of this isotope reaches Earth from distant stars. Now, a research team with significant involvement from the Technical University of Munich (TUM) has discovered iron-60 in Antarctic snow for the first time. The scientists suggest that the iron isotope comes from the interstellar neighborhood.

The quantity of cosmic dust that trickles down to Earth each year ranges between several thousand and ten thousand tons. Most of the tiny particles come from asteroids or comets within our solar system. However, a small percentage comes from distant stars. There are no natural terrestrial sources for the iron-60 isotope contained therein; it originates exclusively as a result of supernova explosions or through the reactions of cosmic radiation with cosmic dust.

Antarctic Snow Travels around the World

The first evidence of the occurrence of iron-60 on Earth was discovered in deep-sea deposits by a TUM research team 20 years ago. Among the scientists on the team was Dr. Gunther Korschinek, who hypothesized that traces of stellar explosions could also be found in the pure, untouched Antarctic snow. In order to verify this assumption, Dr. Sepp Kipfstuhl from the Alfred Wegener Institute collected 500 kg of snow at the Kohnen Station, a container settlement in the Antarctic, and had it transported to Munich for analysis. There, a TUM team melted the snow and separated the meltwater from the solid components, which were processed at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) using various chemical methods, so that the iron needed for the subsequent analysis was present in the milligram range, and the samples could be returned to Munich.

Korschinek and Dominik Koll from the research area Nuclear, Particle and Astrophysics at TUM found five iron-60 atoms in the samples using the accelerator laboratory in Garching near Munich. "Our analyses allowed us to rule out cosmic radiation, nuclear weapons tests or reactor accidents as sources of the iron-60," states Koll. "As there are no natural sources for this radioactive isotope on Earth, we knew that the iron-60 must have come from a supernova."

Stardust Comes from the Interstellar Neighborhood

The research team was able to make a relatively precise determination as to when the iron-60 has been deposited on Earth: The snow layer that was analyzed was not older than 20 years. Moreover, the iron isotope that was discovered did not seem to come from particularly distant stellar explosions, as the iron-60 dust would have dissipated too much throughout the universe if this had been the case. Based on the half-life of iron-60, any atoms originating from the formation of Earth would have completely decayed by now. Koll therefore assumes that the iron-60 in the Antarctic snow originates from the interstellar neighborhood, for example from an accumulation of gas clouds in which our solar system is currently located.

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The rise and fall of Ziggy star formation and the rich dust from ancient stars

Based on the observations with ALMA and HST, researchers assume that this galaxy contains stellar clusters with a mix of old and young stars. The clouds of gas and dust are illuminated by stellar light.

Researchers have detected a radio signal from abundant interstellar dust in MACS0416_Y1, a galaxy 13.2 billion light-years away in the constellation Eridanus. Standard models can't explain this much dust in a galaxy this young, forcing us to rethink the history of star formation. Researchers now think MACS0416_Y1 experienced staggered star formation with two intense starburst periods 300 million and 600 million years after the Big Bang with a quiet phase in between.

Stars are the main players in the Universe, but they are supported by the unseen backstage stagehands: star dust and gas. Cosmic clouds of dust and gas are the sites of star formation and masterful storytellers of the cosmic history.

"Dust and relatively heavy elements such as oxygen are disseminated by the deaths of stars," said Yoichi Tamura, an associate professor at Nagoya University and the lead author of the research paper, "Therefore, a detection of dust at some point in time indicates that a number of stars have already formed and died well before that point."

Using ALMA (Atacama Large Millimeter/submillimeter Array), Tamura and his team observed the distant galaxy MACS0416_Y1. Because of the finite speed of light, the radio waves we observe from this galaxy today had to travel for 13.2 billion years to reach us. In other words they provide an image of what the galaxy looked like 13.2 billion years ago, which is only 600 million years after the Big Bang.

The astronomers detected a weak but telltale signal of radio emissions from dust particles in MACS0416_Y1 (Note 1). The Hubble Space Telescope, the Spitzer Space Telescope, and the European Southern Observatory's Very Large Telescope have observed the light from stars in the galaxy; and from its color they estimate the stellar age to be 4 million years.

"It ain't easy," said Tamura half-lost in a moonage daydream. "The dust is too abundant to have been formed in 4 million years. It is surprising, but we need to hang onto ourselves. Older stars might be hiding in the galaxy, or they may have died out and disappeared already."

"There have been several ideas proposed to overcome this 'dust budget crisis'," said Ken Mawatari, a researcher at the University of Tokyo. "However, no one is conclusive. We made a new model which doesn't need any extreme assumptions diverging far from our knowledge of the life of stars in today's Universe. The model well explains both the color of the galaxy and the amount of dust." In this model, the first burst of star formation started at 300 million years and lasted 100 million years. After that, the star formation activity went quiet for a time, and then restarted at 600 million years. The researchers think ALMA observed this galaxy at the beginning of its second generation of star formation.

Read more at Science Daily

Using stardust grains, scientists build new model for nova eruptions

What do tiny specks of silicon carbide stardust, found in meteorites and older than the solar system, have in common with pairs of aging stars prone to eruptions?

A collaboration between two Arizona State University scientists -- cosmochemist Maitrayee Bose and astrophysicist Sumner Starrfield, both of ASU's School of Earth and Space Exploration -- has uncovered the connection and pinpointed the kind of stellar outburst that produced the stardust grains.

Their study has just been published in The Astrophysical Journal.

The microscopic grains of silicon carbide -- a thousand times smaller than the average width of a human hair -- were part of the construction materials that built the Sun and planetary system. Born in nova outbursts, which are repeated cataclysmic eruptions by certain types of white dwarf stars, the silicon carbide grains are found today embedded in primitive meteorites.

"Silicon carbide is one of the most resistant bits found in meteorites," Bose said. "Unlike other elements, these stardust grains have survived unchanged from before the solar system was born."

Violent birth

A star becomes a nova -- a "new star" -- when it suddenly brightens by many magnitudes. Novae occur in pairs of stars where one star is a hot, compact remnant called a white dwarf. The other is a cool giant star so large its extended outer atmosphere feeds gas onto the white dwarf. When enough gas collects on the white dwarf, a thermonuclear eruption ensues, and the star becomes a nova.

Although powerful, the eruption doesn't destroy the white dwarf or its companion, so novae can erupt over and over, repeatedly throwing into space gas and dust grains made in the explosion. From there the dust grains merge with clouds of interstellar gas to become the ingredients of new star systems.

The Sun and solar system were born about 4.6 billion years ago from just such an interstellar cloud, seeded with dust grains from earlier stellar eruptions by many different kinds of stars. Almost all the original grains were consumed in making the Sun and planets, yet a tiny fraction remained. Today these bits of stardust, or presolar grains, can be identified in primitive solar system materials such as chondritic meteorites.

"The key that unlocked this for us was the isotopic composition of the stardust grains," Bose said. Isotopes are varieties of chemical elements that have extra neutrons in their nuclei. "Isotopic analysis lets us trace the raw materials that came together to form the solar system."

She added, "Each silicon carbide grain carries a signature of the isotopic composition of its parent star. This provides a probe of that star's nucleosynthesis -- how it made elements."

Bose collected published data on thousands of grains, and found that nearly all the grains grouped naturally into three main categories, each attributable to one kind of star or another.

But there were about 30 grains that couldn't be traced back to a particular stellar origin. In the original analyses, these grains were flagged as possibly originating in nova explosions.

But did they?

Making stardust

As a theoretical astrophysicist, Starrfield uses computer calculations and simulations to study various kinds of stellar explosions. These include novae, recurrent novae, X-ray bursts, and supernovae.

Working with other astrophysicists, he was developing a computer model to explain the ejected materials seen in the spectrum of a nova discovered in 2015. Then he attended a colloquium talk given by Bose before she had joined the faculty.

"I would not have pursued this if I hadn't heard Maitrayee's talk and then had our follow-up discussion," he said. That drew him deeper into the details of nova eruptions in general and what presolar grains could say about these explosions that threw them into space.

A problem soon arose. "After talking with her," Starrfield said, "I discovered our initial way of solving the problem was not agreeing with either the astronomical observations or her results.

"So I had to figure out a way to get around this."

He turned to multidimensional studies of classical nova explosions, and put together a wholly new way of doing the model calculations.

There are two major composition classes of nova, Starrfield said. "One is the oxygen-neon class which I've been working on for 20 years. The other is the carbon-oxygen class which I had not devoted as much attention to." The class designations for novae come from the elements seen in their spectra.

"The carbon-oxygen kind produce a lot of dust as part of the explosion itself," Starrfield said. "The idea is that the nova explosion reaches down into the white dwarf's carbon-oxygen core, bringing up all these enhanced and enriched elements into a region with high temperatures."

That, he said, can drive a much bigger explosion, adding, "It's really messy. It shoots out dust in tendrils, sheets, jets, blobs, and clumps."

Starrfield's calculations made predictions of 35 isotopes, including those of carbon, nitrogen, silicon, sulfur, and aluminum, that would be created by the carbon-oxygen nova outbursts.

It turned out that getting the right proportion of white dwarf core material and accreted material from the companion star was absolutely necessary for the simulations to work. Bose and Starrfield then compared the predictions with the published compositions of the silicon carbide grains.

This led them to a somewhat surprising conclusion. Said Bose, "We found that only five of the roughly 30 grains could have come from novae."

While this may seem a disappointing result, the scientists were actually pleased. Bose said, "Now we have to explain the compositions of the grains that didn't come from nova outbursts. This means there's a completely new stellar source or sources to be discovered."

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