Webb locates dust reservoirs in two supernovae

Researchers using NASA's James Webb Space Telescope have made major strides in confirming the source of dust in early galaxies. Observations of two Type II supernovae, Supernova 2004et (SN 2004et) and Supernova 2017eaw (SN 2017eaw), have revealed large amounts of dust within the ejecta of each of these objects. The mass found by researchers supports the theory that supernovae played a key role in supplying dust to the early universe.

Dust is a building block for many things in our universe -- planets in particular. As dust from dying stars spreads through space, it carries essential elements to help give birth to the next generation of stars and their planets. Where that dust comes from has puzzled astronomers for decades. One significant source of cosmic dust could be supernovae -- after the dying star explodes, its leftover gas expands and cools to create dust.

"Direct evidence of this phenomenon has been slim up to this point, with our capabilities only allowing us to study the dust population in one relatively nearby supernova to date -- Supernova 1987A, 170,000 light-years away from Earth," said lead author Melissa Shahbandeh of Johns Hopkins University and the Space Telescope Science Institute in Baltimore, Maryland. "When the gas cools enough to form dust, that dust is only detectable at mid-infrared wavelengths provided you have enough sensitivity."

For supernovae more distant than SN 1987A like SN 2004et and SN 2017eaw, both in NGC 6946 about 22 million light-years away, that combination of wavelength coverage and exquisite sensitivity can only be obtained with Webb's MIRI (Mid-Infrared Instrument).

The Webb observations are the first breakthrough in the study of dust production from supernovae since the detection of newly formed dust in SN 1987A with the Atacama Large Millimeter/submillimeter Array (ALMA) telescope nearly a decade ago.

Another particularly intriguing result of their study isn't just the detection of dust, but the amount of dust detected at this early stage in the supernova's life. In SN 2004et, the researchers found more than 5,000 Earth masses of dust.

"When you look at the calculation of how much dust we're seeing in SN 2004et especially, it rivals the measurements in SN 1987A, and it's only a fraction of the age," added program lead Ori Fox of the Space Telescope Science Institute. "It's the highest dust mass detected in supernovae since SN 1987A."

Observations have shown astronomers that young, distant galaxies are full of dust, but these galaxies are not old enough for intermediate mass stars, like the Sun, to have supplied the dust as they age. More massive, short-lived stars could have died soon enough and in large enough numbers to create that much dust.

While astronomers have confirmed that supernovae produce dust, the question has lingered about how much of that dust can survive the internal shocks reverberating in the aftermath of the explosion. Seeing this amount of dust at this stage in the lifetimes of SN 2004et and SN 2017eaw suggests that dust can survive the shockwave -- evidence that supernovae really are important dust factories after all.

Researchers also note that the current estimations of the mass may be the tip of the iceberg. While Webb has allowed researchers to measure dust cooler than ever before, there may be undetected, colder dust radiating even farther into the electromagnetic spectrum that remains obscured by the outermost layers of dust.

The researchers emphasized that the new findings are also just a hint at newfound research capabilities into supernovae and their dust production using Webb, and what that can tell us about the stars from which they came.

"There's a growing excitement to understand what this dust also implies about the core of the star that exploded," Fox said. "After looking at these particular findings, I think our fellow researchers are going to be thinking of innovative ways to work with these dusty supernovae in the future."

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A star is born: Study reveals complex chemistry inside 'stellar nurseries'

An international team of researchers has uncovered what might be a critical step in the chemical evolution of molecules in cosmic "stellar nurseries." In these vast clouds of cold gas and dust in space, trillions of molecules swirl together over millions of years. The collapse of these interstellar clouds eventually gives rise to young stars and planets.

Like human bodies, stellar nurseries contain a lot of organic molecules, which are made up mostly of carbon and hydrogen atoms. The group's results, published Feb. 6 in the journal Nature Astronomy, reveal how certain large organic molecules may form inside these clouds. It's one tiny step in the eons-long chemical journey that carbon atoms undergo -- forming in the hearts of dying stars, then becoming part of planets, living organisms on Earth and perhaps beyond.

"In these cold molecular clouds, you're creating the first building blocks that will, in the end, form stars and planets," said Jordy Bouwman, research associate at the Laboratory for Atmospheric and Space Physics (LASP) and assistant professor in the Department of Chemistry at the University of Colorado Boulder.

For the new study, Bouwman and his colleagues took a deep dive into one stellar nursery in particular: the Taurus Molecular Cloud (TMC-1). This region sits in the constellation Taurus and is roughly 440 light years (more than 2 quadrillion miles) from Earth. This chemically complex environment is an example of what astronomers call an "accreting starless core." Its cloud has begun to collapse, but scientists haven't yet detected embryonic stars emerging inside it.

The team's findings hinge on a deceptively simple molecule called ortho-benzyne. Drawing on experiments on Earth and computer simulations, the researchers showed that this molecule can readily combine with others in space to form a wide range of larger organic molecules.

Small building blocks, in other words, become big building blocks.

And, Bouwman said, those reactions could be a sign that stellar nurseries are a lot more interesting than scientists give them credit for.

"We're only at the start of truly understanding how we go from these small building blocks to larger molecules," he said. "I think we'll find that this chemistry is so much more complex than we thought, even at the earliest stages of star formation."

Fateful observation

Bouwman is a cosmochemist, studying a field that blends chemistry and astronomy to understand the churning chemical reactions that happen deep in space.

On the surface, he said, cold molecular clouds might not seem like a hotbed of chemical activity. As their name suggests, these galactic primordial soups tend to be frigid, often hovering around -263 degrees Celsius (about -440 degrees Fahrenheit), just 10 degrees above absolute zero. Most reactions need at least a little bit of heat to get a kick-start.

But cold or not, complex chemistry seems to be happening in stellar nurseries. TMC-1, in particular, contains surprising concentrations of relatively large organic molecules with names like fulvenallene and 1- and 2-ethynylcyclopentadiene. Chemists call them "five-membered ring compounds" because they each contain a ring of carbon atoms shaped like a pentagon.

"Researchers kept detecting these molecules in TMC-1, but their origin was unclear," Bouwman said.

Now, he and his colleagues think they have an answer.

In 2021, researchers using the Yebes 40-metre Radiotelescope in Spain found an unexpected molecule hiding in the clouds of gas of TMC-1: ortho-benzyne. Bouwman explained that this small molecule, made up of a ring of six carbon atoms with four hydrogens, is one of the extroverts of the chemistry world. It easily interacts with a number of other molecules and doesn't require a lot of heat to do so.

"There's no barrier to reaction," Bouwman said. "That means that it has the potential to drive complex chemistry in cold environments."

Identifying the culprit

To find out what kind of complex chemistry was happening in TMC-1, Bouwman and his colleagues -- who hail from the United States, Germany, the Netherlands and Switzerland -- turned to a technique called "photoelectron photoion coincidence spectroscopy." The team used light generated by a giant facility called a synchrotron light source to identify the products of chemical reactions. They saw that ortho-benzyne and methyl radicals, another common constituent of molecular clouds, readily combine to form larger and more complex organic compounds.

"We knew we were onto something good," Bouwman said.

The team then drew on computer models to explore the role of ortho-benzyne in a stellar nursery spread out over several light years deep in space. The results were promising: The models generated clouds of gas containing roughly the same mix of organic molecules that astronomers had observed in TMC-1 using telescopes.

Ortho-benzyne, in other words, seems to be a prime candidate for driving the gas-phase organic chemistry that occurs within these stellar nurseries, Bouwman said.

He added that scientists still have a lot of work to do to fully understand all the reactions happening in TMC-1. He wants to examine, for example, how organic molecules in space also pick up nitrogen atoms -- key components of the DNA and amino acids of living organisms on Earth.

"Our findings may just change the view on what ingredients we have in the first place to form new stars and new planets," Bouwman said.

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Rare sighting of luminous jet spewed by supermassive black hole

What happens when a dying star flies too close to a supermassive black hole?

According to University of Maryland astronomer Igor Andreoni, several things happen: first, the star is violently ripped apart by the black hole's gravitational tidal forces -- similar to how the Moon pulls tides on Earth but with greater strength. Then, pieces of the star are captured into a swiftly spinning disk orbiting the black hole. Finally, the black hole consumes what remains of the doomed star in the disk. This is what astronomers call a tidal disruption event (TDE).

But in some extremely rare cases, the supermassive black hole launches "relativistic jets" -- beams of matter traveling close to the speed of light -- after destroying a star. Andreoni, who is a postdoctoral associate in the Department of Astronomy at UMD and NASA Goddard Space Flight Center, discovered one such case with his team in the Zwicky Transient Facility (ZTF) survey in February 2022. After the group publicly announced the sighting, the event was named "AT2022cmc." The team published its findings in the journal Nature on November 30, 2022.

"The last time scientists discovered one of these jets was well over a decade ago," said Michael Coughlin, an assistant professor of astronomy at the University of Minnesota Twin Cities and co-lead on the project. "From the data we have, we can estimate that relativistic jets are launched in only 1% of these destructive events, making AT2022cmc an extremely rare occurrence. In fact, the luminous flash from the event is among the brightest ever observed."

Before AT2022cmc, the only two previously known jetted TDEs were discovered through gamma-ray space missions, which detect the highest-energy forms of radiation produced by these jets. As the last such discovery was made in 2012, new methods were required to find more events of this nature. To help address that need, Andreoni and his team implemented a novel, "big picture" tactic to find AT2022cmc: ground-based optical surveys, or general maps of the sky without specific observational targets. Using ZTF, a wide-field sky survey taken by the Samuel Oschin Telescope in California, the team was able to identify and uniquely study the otherwise dormant-looking black hole.

"We developed an open-source data pipeline to store and mine important information from the ZTF survey and alert us about atypical events in real time," Andreoni explained. "The rapid analysis of ZTF data, the equivalent to a million pages of information every night, allowed us to quickly identify the TDE with relativistic jets and make follow-up observations that revealed an exceptionally high luminosity across the electromagnetic spectrum, from the X-rays to the millimeter and radio."

Follow up observations with many observatories confirmed that AT2022cmc was fading rapidly and the ESO Very Large Telescope revealed that AT2022cmc was at cosmological distance, 8.5 billion light years away.

Hubble Space Telescope optical/infrared images and radio observations from the Very Large Array pinpointed the location of AT2022cmc with extreme precision. The researchers believe that AT2022cmc was at the center of a galaxy that is not yet visible because the light from AT2022cmc outshone it, but future space observations with Hubble or James Webb Space Telescopes may unveil the galaxy when the transient eventually disappears.

It is still a mystery why some TDEs launch jets while others do not seem to. From their observations, Andreoni and his team concluded that the black holes in AT2022cmc and other similarly jetted TDEs are likely spinning rapidly so as to power the extremely luminous jets. This suggests that a rapid black hole spin may be one necessary ingredient for jet launching -- an idea that brings researchers closer to understanding the physics of supermassive black holes at the center of galaxies billions of light years away.

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Dying stars' cocoons might explain fast blue optical transients

Ever since they were discovered in 2018, fast blue optical transients (FBOTs) have utterly surprised and completely confounded both observational and theoretical astrophysicists.

So hot that they glow blue, these mysterious objects are the brightest known optical phenomenon in the universe. But with only a few discovered so far, FBOTs' origins have remained elusive.

Now a Northwestern University astrophysics team presents a bold new explanation for the origin of these curious anomalies. Using a new model, the astrophysicists believe FBOTs could result from the actively cooling cocoons that surround jets launched by dying stars. It marks the first astrophysics model that is fully consistent with all observations related to FBOTs.

The research was published April 11 in the Monthly Notices of the Royal Astronomical Society.

As a massive star collapses, it can launch outflows of debris at rates near the speed of light. These outflows, or jets, collide into collapsing layers of the dying star to form a "cocoon" around the jet. The new model shows that as the jet pushes the cocoon outward -- away from the core of the collapsing star -- it cools, releasing heat as an observed FBOT emission.

"A jet starts deep inside of a star and then drills its way out to escape," said Northwestern's Ore Gottlieb, who led the study. "As the jet moves through the star, it forms an extended structure, known as the cocoon. The cocoon envelopes the jet, and it continues to do so even after the jet escapes the star, this cocoon escapes with the jet. When we calculated how much energy the cocoon has, it turned out to be as powerful as an FBOT."

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 Tchekovskoy, an assistant professor of physics and astronomy in Northwestern's Weinberg College of Arts and Sciences.

The hydrogen problem

FBOTs (pronounced F-bot) are a type of cosmic explosion initially detected in the optical wavelength. As their name implies, transients fade almost as quickly as they appear. FBOTs reach peak brightness within a matter of days and then quickly fade -- much faster than standard supernovae rise and decay.

After discovering FBOTs just eight years ago, astrophysicists wondered if the mysterious events were related to another transient class: gamma ray bursts (GRBs). The strongest and brightest explosions across all wavelengths, GRBs also are associated with dying stars. When a massive star exhausts its fuel and collapses into a black hole, it launches jets to produce a powerful gamma ray emission.

"The reason why we think GRBs and FBOTs might be related is because both are very fast -- moving at close to the speed of light -- and both are asymmetrically shaped, breaking the spherical shape of the star," Gottlieb said. "But there was a problem. Stars that produce GRBs lack hydrogen. We don't see any signs of hydrogen in GRBs, whereas in FBOTs, we see hydrogen everywhere. So, it could not be the same phenomenon."

Using their new model, Gottlieb and his coauthors think they might have found an answer to this problem. Hydrogen-rich stars tend to house hydrogen in their outermost layer -- a layer too thick for a jet to penetrate.

"Basically, the star would be too massive for the jet to pierce through," Gottlieb said. "So the jet will never make it out of the star, and that's why it fails to produce a GRB. However, in these stars, the dying jet transfers all its energy to the cocoon, which is the only component to escape the star. The cocoon will emit FBOT emissions, which will include hydrogen. This is another area where our model is fully consistent with all FBOT observations."

Putting the picture together

Although FBOTs glow bright in optical wavelengths, they also emit radio waves and X-rays. Gottlieb's model explains these too.

When the cocoon interacts with the dense gas surrounding the star, this interaction heats up stellar material to release a radio emission. And when the cocoon expands far enough away from the black hole (formed from the collapsed star), X-rays can leak out from the black hole. The X-rays join radio and optical light to form a full picture of the FBOT event.

While Gottlieb is encouraged by his team's findings, he says more observations and models are needed before we can definitively understand FBOTs' mysterious origins.

"This is a new class of transients, and we know so little about them," Gottlieb said. "We need to detect more of them earlier in their evolution before we can fully understand these explosions. But our model is able to draw a line among supernovae, GRBs and FBOTs, which I think is very elegant."

"This study paves the way for more advanced simulations of FBOTs," Tchekovskoy said. "This next-generation model will allow us to directly connect the physics of the central black hole to the observables, enabling us to reveal otherwise hidden physics of the FBOT central engine."

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Even dying stars can still give birth to planets

Planets are usually not much older than the stars around which they revolve. Take the Sun: it was born 4.6 billion years ago, and not long after that, Earth came into the world. But KU Leuven astronomers have discovered that a completely different scenario is also possible. Even if they are near death, some types of stars can possibly still form planets. If this is confirmed, theories on planet formation will need to be adjusted.

Planets such as Earth, and all other planets in our solar system, were formed not long after the Sun. Our Sun started to burn 4.6 billion years ago, and in the next million years, the matter around it clumped into protoplanets. The birth of the planets in that protoplanetary disc, a gigantic pancake made of dust and gas, so to speak, with the Sun in the middle, explains why they all orbit in the same plane.

But such discs of dust and gas needn't necessarily only surround newborn stars. They can also develop independently from star formation, for example around binary stars of which one is dying (binary stars are two stars that orbit each other, also called a binary system). When the end approaches for a medium-sized star (like the Sun), it catapults the outer part of its atmosphere into space, after which it slowly dies out as a so-called white dwarf. However, in the case of binary stars, the gravitational pull of the second star causes the matter ejected by the dying star to form a flat, rotating disc. Moreover, this disc strongly resembles the protoplanetary discs that astronomers observe around young stars elsewhere in the Milky Way.

This we already knew. However, what is new is that the discs surrounding so-called evolved binary stars not uncommonly show signs that could point to planet formation, as discovered by an international team of astronomers led by KU Leuven researchers. What's more, their observations show that this is the case for one in ten of these binary stars. "In ten per cent of the evolved binary stars with discs we studied, we see a large cavity (a void/opening, ed.) in the disc," says KU Leuven astronomer Jacques Kluska, first author of the article in the journal Astronomy & Astrophysics in which the discovery is described. "This is an indication that something is floating around there that has collected all matter in the area of the cavity."

Second-generation planets

The clean-up of the matter could be the work of a planet. That planet might not have formed at the very beginning of one of the binary stars' life, but at the very end. The astronomers moreover found further strong indications for the presence of such planets. "In the evolved binary stars with a large cavity in the disc, we saw that heavy elements such as iron were very scarce on the surface of the dying star," says Kluska. "This observation leads one to suspect that dust particles rich in these elements were trapped by a planet." By the way, the Leuven astronomer doesn't rule out the possibility that in this way, several planets can be formed around these binary stars.

The discovery was made when the astronomers were drawing up an inventory of evolved binary stars in our Milky Way. They did that based on existing, publicly available observations. Kluska and his colleagues counted 85 of such binary star pairs. In ten pairs, the researchers came across a disc with a large cavity on the infrared images.

Current theories put to the test

If new observations confirm the existence of planets around evolved binary stars, and if it turns out the planets were only formed after one of the stars had reached the end of its life, the theories on planet formation will need to be adjusted. "The confirmation or refutation of this extraordinary way of planet formation will be an unprecedented test for the current theories," according to Professor Hans Van Winckel, head of the KU Leuven Institute of Astronomy.

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