Tragic Teardrop Star Reveals Hidden Supernova Doom

Artist’s impression of the HD265435 system at around 30 million years from now, with the smaller white dwarf distorting the hot subdwarf into a distinct ‘teardrop’ shape. Credit: University of Warwick/Mark Garlick

Astronomers have made the rare sighting of two stars spiraling to their doom by spotting the tell-tale signs of a teardrop-shaped star.

The tragic shape is caused by a massive nearby white dwarf distorting the star with its intense gravity, which will also be the catalyst for an eventual supernova that will consume both. Found by an international team of astronomers and astrophysicists led by the University of Warwick, it is one of only a very small number of star systems discovered that will one day see a white dwarf star reignite its core.

The team’s new research was published on July 12, 2021, in the journal Nature Astronomy.

With the help of W. M. Keck Observatory on Maunakea in Hawaiʻi, the astronomers were able to confirm that the two stars are in the early stages of a spiral that will likely end in a Type Ia supernova – a type that helps astronomers determine how fast the universe is expanding.

The couple – a binary star system called HD265435 – is located roughly 1,500 light-years away; it is comprised of a hot subdwarf star and a white dwarf star orbiting each other closely at a dizzying rate of around 100 minutes. White dwarfs are ‘dead’ stars that have burned all their fuel and collapsed in on themselves, making them small but extremely dense.

A type Ia supernova is generally thought to occur when a white dwarf star’s core reignites, leading to a thermonuclear explosion. There are two scenarios where this can happen. In the first, the white dwarf gains enough mass to reach 1.4 times the mass of our Sun, known as the Chandrasekhar limit. HD265435 fits in the second scenario, in which the total mass of a close stellar system of multiple stars is near or above this limit. Only a handful of other star systems have been discovered that will reach this threshold and result in a Type Ia supernova.

Lead author Ingrid Pelisoli from the University of Warwick Department of Physics explains: “We don’t know exactly how these supernovae explode, but we know it has to happen because we see it happening elsewhere in the universe.”

“One way is if the white dwarf accretes enough mass from the hot subdwarf, so as the two of them are orbiting each other and getting closer, matter will start to escape the hot subdwarf and fall onto the white dwarf. Another way is that because they are losing energy to gravitational wave emissions, they will get closer until they merge. Once the white dwarf gains enough mass from either method, it will go supernova,” she says.

Using data from NASA’s Transiting Exoplanet Survey Satellite, the team was able to observe the hot subdwarf. While they did not detect the white dwarf, the researchers observed the brightness of the hot subdwarf varied over time; this suggests a nearby massive object was distorting the star into a teardrop shape.

The astronomers then used Palomar Observatory and Keck Observatory’s Echellette Spectrograph and Imager (ESI) to measure the radial velocity and rotational velocity of the hot subdwarf star, which allowed them to confirm that the hidden white dwarf is as heavy as our Sun, but just slightly smaller than the Earth’s radius. Combined with the mass of the hot subdwarf, which is a little over 0.6 times the mass of our Sun, both stars have the mass needed to cause a Type Ia supernova.

“Keck’s ESI data was crucial in determining that the compact binary system exceeds the Chandrasekhar mass limit, which makes HD265435 one of the very few supernova Ia progenitor systems known,” says co-author Thomas Kupfer, assistant professor at Texas Tech University’s Department of Physics and Astronomy.

As the two stars are already close enough to begin spiraling closer together, the white dwarf will inevitably go supernova in around 70 million years. Theoretical models produced specifically for this study also predict that the hot subdwarf will contract to become a white dwarf star before merging with its companion.

Type Ia supernovae are important for cosmology as ‘standard candles.’ Their brightness is constant and of a specific type of light, which means astronomers can compare what luminosity they should be with what we observe on Earth, and from that work out how distant they are with a good degree of accuracy. By observing supernovae in distant galaxies, astronomers combine what they know of how fast this galaxy is moving with our distance from the supernova and calculate the expansion of the universe.

“The more we understand how supernovae work, the better we can calibrate our standard candles. This is very important at the moment because there’s a discrepancy between what we get from this kind of standard candle, and what we get through other methods,” says Pelisoli.

She adds, “The more we understand about how supernovae form, the better we can understand whether this discrepancy we are seeing is because of new physics that we’re unaware of and not taking into account, or simply because we’re underestimating the uncertainties in those distances.”

“There is another discrepancy between the estimated and observed galactic supernovae rate, and the number of progenitors we see. We can estimate how many supernovae are going to be in our galaxy through observing many galaxies, or through what we know from stellar evolution, and this number is consistent. But if we look for objects that can become supernovae, we don’t have enough. This discovery was very useful to put an estimate of what a hot subdwarf and white dwarf binaries can contribute. It still doesn’t seem to be a lot, none of the channels we observed seems to be enough,” Pelisoli says.

Read Impending Supernova Doom: Astronomers Rare Sighting of a Teardrop-Shaped Star for more on this research.

Reference: “A hot subdwarf–white dwarf super-Chandrasekhar candidate supernova Ia progenitor” by Ingrid Pelisoli, P. Neunteufel, S. Geier, T. Kupfer, U. Heber, A. Irrgang, D. Schneider, A. Bastian, J. van Roestel, V. Schaffenroth and B. N. Barlow, 12 July 2021, Nature Astronomy.
DOI: 10.1038/s41550-021-01413-0

This research was funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) and the Science and Technology Facilities Council, part of UK Research and Innovation.

About ESI

The Echellette Spectrograph and Imager (ESI) is a medium-resolution visible-light spectrograph that records spectra from 0.39 to 1.1 microns in each exposure. Built at UCO/Lick Observatory by a team led by Prof. Joe Miller, ESI also has a low-resolution mode and can image in a 2 x 8 arc min field of view. An upgrade provided an integral field unit that can provide spectra everywhere across a small, 5.7 x4.0 arc sec field. Astronomers have found a number of uses for ESI, from observing the cosmological effects of weak gravitational lensing to searching for the most metal-poor stars in our galaxy.

About W. M. Keck Observatory

The W. M. Keck Observatory telescopes are among the most scientifically productive on Earth. The two 10-meter optical/infrared telescopes atop Maunakea on the Island of Hawaiʻi feature a suite of advanced instruments including imagers, multi-object spectrographs, high-resolution spectrographs, integral-field spectrometers, and world-leading laser guide star adaptive optics systems. Some of the data presented herein were obtained at Keck Observatory, which is a private 501(c) 3 non-profit organization operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the Native Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain.

Distorted Deja Vu: How the Universe Is Reflected Near Black Holes

A disk of glowing gas swirls into the black hole “Gargantua” from the movie Interstellar. Because space curves around the black hole, it is possible to look round its far side and see the part of the gas disk that would otherwise be hidden by the hole. Our understanding of this mechanism has now been increased by Danish master’s student at NBI, Albert Sneppen. Credit: Interstellar.wiki/CC BY-NC

In the vicinity of black holes, space is so warped that even light rays may curve around them several times. This phenomenon may enable us to see multiple versions of the same thing. While this has been known for decades, only now do we have an exact, mathematical expression, thanks to Albert Sneppen, student at the Niels Bohr Institute. The result, which even is more useful in realistic black holes, has just been published in the journal Scientific Reports.

You have probably heard of black holes — the marvelous lumps of gravity from which not even light can escape. You may also have heard that space itself and even time behave oddly near black holes; space is warped.

Light from the background galaxy circles a black hole an increasing number of times, the closer it passes the hole, and we therefore see the same galaxy in several directions. Credit: Peter Laursen

In the vicinity of a black hole, space curves so much that light rays are deflected, and very nearby light can be deflected so much that it travels several times around the black hole. Hence, when we observe a distant background galaxy (or some other celestial body), we may be lucky to see the same image of the galaxy multiple times, albeit more and more distorted.

Galaxies in multiple versions

The mechanism is shown on the figure below: A distant galaxy shines in all directions — some of its light comes close to the black hole and is lightly deflected; some light comes even closer and circumvolves the hole a single time before escaping down to us, and so on. Looking near the black hole, we see more and more versions of the same galaxy, the closer to the edge of the hole we are looking.

How much closer to the black hole do you have to look from one image to see the next image? The result has been known for over 40 years, and is some 500 times (for the math aficionados, it is more accurately the “exponential function of two pi,” written e2π).

The situation seen “face-on,” i.e. how we would actually observe it from Earth. The extra images of the galaxy become increasingly squeezed and distorted, the closer we look at the black hole. Credit: Peter Laursen

Calculating this is so complicated that, until recently, we had not yet developed a mathematical and physical intuition as to why it happens to be this exact factor. But using some clever, mathematical tricks, master’s student Albert Sneppen from the Cosmic Dawn Center — a basic research center under both the Niels Bohr Institute and DTU Space — has now succeeded in proving why.

“There is something fantastically beautiful in now understanding why the images repeat themselves in such an elegant way. On top of that, it provides new opportunities to test our understanding of gravity and black holes,” Albert Sneppen clarifies.

Proving something mathematically is not only satisfying in itself; indeed, it brings us closer to an understanding of this marvelous phenomenon. The factor “500” follows directly from how black holes and gravity work, so the repetitions of the images now become a way to examine and test gravity.

Spinning black holes

As a completely new feature, Sneppen’s method can also be generalized to apply not only to “trivial” black holes, but also to black holes that rotate. Which, in fact, they all do.

“It turns out that when the it rotates really fast, you no longer have to get closer to the black hole by a factor 500, but significantly less. In fact, each image is now only 50, or 5, or even down to just 2 times closer to the edge of the black hole,” explains Albert Sneppen.

Having to look 500 times closer to the black hole for each new image, means that the images are quickly “squeezed” into one annular image, as seen in the figure on the right. In practice, the many images will be difficult to observe. But when black holes rotate, there is more room for the “extra” images, so we can hope to confirm the theory observationally in a not-too-distant future. In this way, we can learn about not just black holes, but also the galaxies behind them:

The travel time of the light increases, the more times it has to go around the black hole, so the images become increasingly “delayed.” If, for example, a star explodes as a supernova in a background galaxy, one would be able to see this explosion again and again.

Reference: “Divergent reflections around the photon sphere of a black hole” by Albert Snepppen, 9 July 2021, Scientific Reports.
DOI: 10.1038/s41598-021-93595-w

Ariane 6 Rocket Targets New Missions With Astris Kick Stage

Astris added to the Ariane 6 upper stage. Credit: ArianeGroup

ESA will enhance the versatility of Europe’s Ariane 6 rocket with a kick stage called Astris in a €90 m development contract with prime contractor, ArianeGroup. This is part of ESA’s strategy to extend Ariane 6’s capabilities to serve a wider range of space transportation requirements.

Astris is planned to fly by mid 2024 as an optional add-on to Ariane 6’s upper stage and will interface directly with the payload. This will enable Ariane 6 to offer a range of new space transportation services by allowing complex orbital transfers.

Astris will simplify missions by taking over some of the required built-in propulsion capabilities of payloads to move themselves to their final position in orbit. This will reduce the burden on satellite manufacturers to factor this into their design.

The modular architecture of Astris makes it versatile, giving potential for even more capabilities. Structures will include a flight proven family of propellant tanks. This approach makes it possible to develop mission specific kits that offer a tailored solution to each customer.

Future space missions, especially for telecommunications applications and space exploration, could use Astris to reduce mission cost and risk. ESA’s Hera spacecraft, a planetary defense mission to the Didymos asteroid system, is set to be the first to benefit.

The Astris engine can be restarted several times. Credit: ArianeGroup

“ESA’s Astris kick stage is a major development to ensure that Ariane 6 can serve the widest possible range of present and future space transportation requirements. It is an important element to enable flexible in-space transportation services, such as space logistics, in-orbit servicing and specific exploration missions,” commented Daniel Neuenschwander, ESA Director of Space Transportation.

“From the beginning of the Ariane 6 program, the launcher was designed to be scalable and incorporate innovations throughout its operating cycle. This contract rewards the expertise and innovation capacity of our Bremen site in the field of launcher upper stages, while our teams near Munich are currently developing the new Berta engine. By pooling our skills, this project further strengthens Germany’s role in the new European launcher, Ariane 6,” added Pierre Godart, CEO at ArianeGroup in Germany.

Through Astris, Ariane 6 could enable deep space exploration for ridesharing payloads with destinations such as asteroids, the Moon and Mars. The Astris concept will make objects in the Solar System more accessible to a wider range of payloads.

Astris can take a payload to its final geostationary orbit. Credit: ArianeGroup

Closer to Earth, Astris will augment Ariane 6’s ability to deploy multiple payloads into separate low Earth orbits on a single launch.

Alternatively, Ariane 6 could place one payload in a transfer orbit then Astris would separate from the upper stage to take a second payload directly into its final position in geostationary orbit.

The Berta engine, a mid-size storable propellant propulsion system for Astris is in development and qualification at ArianeGroup in Ottobrunn, Germany, carried out within ESA’s Future Launchers Preparatory Programme (FLPP). This type of engine can be reliably reignited several times, making it particularly suitable for extended missions or for transport to different orbits.

Activities for the Astris kick stage are carried out within ESA’s Ariane 6 Competitiveness Improvement Programme. It anticipates future space transportation needs and works with industry to create solutions to ensure Europe remains competitive in the global market.

40-Year Mystery Solved: Source of Jupiter’s Strange X-Ray Flares Uncovered

The purple hues in this image show X-ray emissions from Jupiter’s auroras, detected by NASA’s Chandra Space Telescope in 2007. They are overlaid on an image of Jupiter taken by NASA’s Hubble Space Telescope. Jupiter is the only gas giant planet where scientists have detected X-ray auroras. Credit: (X-ray) NASA/CXC/SwRI/R.Gladstone et al.; (Optical) NASA/ESA/Hubble Heritage (AURA/STScI)

A puzzler about the gas giant’s intense northern and southern lights has been deciphered.

Planetary astronomers combined measurements taken by NASA’s Juno spacecraft orbiting Jupiter, with data from ESA’s (the European Space Agency’s) Earth-orbiting XMM-Newton mission, to solve a 40-year-old mystery about the origins of Jupiter’s unusual X-ray auroras. For the first time, they have seen the entire mechanism at work: The electrically charged atoms, or ions, responsible for the X-rays are “surfing” electromagnetic waves in Jupiter’s magnetic field down into the gas giant’s atmosphere.

A paper on the study was published on July 9, 2021, in the journal Science Advances.

Auroras have been detected on seven planets in our solar system. Some of these light shows are visible to the human eye; others generate wavelengths of light we can only see with specialized telescopes. Shorter wavelengths require more energy to produce. Jupiter has the most powerful auroras in the solar system and is the only one of t

Planetary astronomers have been fascinated with Jupiter’s X-ray auroral emission since its discovery four decades ago because it was not immediately clear how the energy required to produce it is generated. They knew these surprising Jovian northern and southern lights are triggered by ions crashing into Jupiter’s atmosphere. But until now scientists had no idea how the ions responsible for the X-ray light show are able to get to the atmosphere in the first place.

At Earth, auroras are usually visible only in a belt surrounding the magnetic poles, between 65 and 80 degrees latitude. Beyond 80 degrees, auroral emission disappears because the magnetic field lines leave Earth and connect to the magnetic field in the solar wind, which is the constant flux of electrically charged particles ejected by the Sun. These are called open field lines, and in the traditional picture, Jupiter’s and Saturn’s high-latitude polar regions are not expected to emit substantial auroras, either.

However, Jupiter’s X-ray auroras are different. They exist poleward of the main auroral belt and pulsate, and those at the north pole often differ from those at the south pole. These are typical features of a closed magnetic field, where the magnetic field line exits the planet at one pole and reconnects with the planet at the other. All planets with magnetic fields have both open and closed field components.

Scientists studying the phenomena turned to computer simulations and found that the pulsating X-ray auroras could be linked to closed magnetic fields that are generated inside Jupiter and then stretch out millions of miles into space before turning back. But how to prove the model was viable?

The study authors turned to data acquired by both Juno and XMM-Newton from July 16 to 17, 2017. During the two-day span, XMM-Newton observed Jupiter continuously for 26 hours and saw X-ray aurora pulsating every 27 minutes.

At the same time, Juno had been traveling between 62 and 68 Jupiter radii (about 2.8 to 3 million miles, or 4.4 to 4.8 million kilometers) above the planet’s pre-dawn area. This was exactly the region that the team’s simulations suggested was important for triggering the pulsations, so they searched the Juno data for any magnetic processes that were occurring at the same rate.

They found that fluctuations of Jupiter’s magnetic field caused the pulsating X-ray auroras. The outer boundary of the magnetic field is struck directly by the particles of the solar wind and compressed. These compressions heat ions that are trapped in Jupiter’s extensive magnetic field, which are millions of miles away from the planet’s atmosphere.

This triggers a phenomenon called electromagnetic ion cyclotron (EMIC) waves, in which the particles are directed along the field lines. Guided by the field, the ions ride the EMIC wave across millions of miles of space, eventually slamming into the planet’s atmosphere and triggering the X-ray auroras.

“What we see in the Juno data is this beautiful chain of events. We see the compression happen, we see the EMIC wave triggered, we see the ions, and then we see a pulse of ions traveling along the field line,” said William Dunn of the Mullard Space Science Laboratory, University College London, and a co-author of the paper. “Then, a few minutes later, XMM sees a burst of X-rays.”

Now that the missing piece of the process has been identified for the first time, it opens up a wealth of possibilities for where it could be studied next. For example, at Jupiter, the magnetic field is filled with sulfur and oxygen ions being emitted by the volcanoes on the moon Io. At Saturn, the moon Enceladus jets water into space, filling Saturn’s magnetic field with water group ions.

For more on this discovery, see Scientists Solve 40-Year Mystery Over Jupiter’s Spectacularly Powerful X-ray Aurora.

Reference: “Revealing the source of Jupiter’s x-ray auroral flares” by Zhonghua Yao, William R. Dunn, Emma E. Woodfield, George Clark, Barry H. Mauk, Robert W. Ebert, Denis Grodent, Bertrand Bonfond, Dongxiao Pan, I. Jonathan Rae, Binbin Ni, Ruilong Guo, Graziella Branduardi-Raymont, Affelia D. Wibisono, Pedro Rodriguez, Stavros Kotsiaros, Jan-Uwe Ness, Frederic Allegrini, William S. Kurth, G. Randall Gladstone, Ralph Kraft, Ali H. Sulaiman, Harry Manners, Ravindra T. Desai and Scott J. Bolton, 9 July 2021, Science Advances.
DOI: 10.1126/sciadv.abf0851

More About the Mission

JPL, a division of Caltech in Pasadena, California, manages the Juno mission for the principal investigator, Scott J. Bolton, of the Southwest Research Institute in San Antonio. Juno is part of NASA’s New Frontiers Program, which is managed at NASA’s Marshall Space Flight Center in Huntsville, Alabama, for the agency’s Science Mission Directorate in Washington. Lockheed Martin Space in Denver built and operates the spacecraft.

“What’s This Weirdo?” – A New Type of Supernova Illuminates an Old Mystery

Las Cumbres Observatory and Hubble Space Telescope color composite of the electron-capture supernova 2018zd (the large white dot on the right) and the host starburst galaxy NGC 2146 (toward the left). Credit: NASA/STScI/J. DePasquale; Las Cumbres Observatory

A worldwide team led by scientists at Las Cumbres Observatory has discovered the first convincing evidence for a new type of stellar explosion — an electron-capture supernova. While they have been theorized for 40 years, real-world examples have been elusive. They are thought to arise from the explosions of massive super-asymptotic giant branch (SAGB) stars, for which there has also been scant evidence. The discovery also sheds new light on the thousand-year mystery of the supernova from A.D. 1054 that was seen all over the world in the daytime, before eventually becoming the Crab Nebula.

Historically, there have been two main supernova types. One is a thermonuclear supernova — the explosion of a white dwarf star after it gains matter in a binary star system. These white dwarfs are the dense cores of ash that remain after a low-mass star (one up to about 8 times the mass of the sun) reaches the end of its life. Another main supernova type is an iron core-collapse supernova where a massive star — one more than about 10 times the mass of the sun — runs out of nuclear fuel and has its iron core collapse, creating a black hole or neutron star. The electron-capture supernovae are on the borderline between these two types of supernovae. The stars stop fusion when their cores are made of oxygen, neon, and magnesium; they aren’t massive enough to create iron.

While gravity is always trying to crush a star, what keeps most stars from collapsing is either ongoing fusion, or in cores where fusion has stopped, the fact that you can’t pack the atoms any tighter. In an electron capture supernova, some of the electrons in the oxygen — neon — magnesium core get smashed into their atomic nuclei, in a process called electron capture. This removal of electrons causes the core of the star to buckle under its own weight and collapse, resulting in an electron-capture supernova.

If the star had been slightly heavier, the core elements could have fused to create heavier elements, prolonging its life. So it is a kind of reverse-Goldilocks situation: the star isn’t light enough to escape its core collapsing, nor is it heavy enough to prolong its life and die later via different means.

That’s the theory that was formulated beginning 1980 by Ken’ichi Nomoto of the University of Tokyo, and others. Over the decades, theorists have formulated predictions of what to look for in an electron-capture supernova and their SAGB star progenitors. The stars should have a lot of mass, lose much of it before exploding, and this mass near the dying star should be of an unusual chemical composition. Then the electron-capture supernova should be weak, have little radioactive fallout, and have neutron-rich elements in the core.

Artist impressions of a super-asymptotic giant branch star (left) and its core (right) made up of oxygen (O), neon (Ne), and magnesium (Mg). A super-asymptotic giant branch star is the end state of stars in a mass range of around 8-10 solar masses, whose core is pressure supported by electrons (e-). When the core becomes dense enough, neon and magnesium start to eat up electrons (so called electron-capture reactions), reducing the core pressure and inducing a core-collapse supernova explosion. Credit: S. Wilkinson; Las Cumbres Observatory

The new study, published in Nature Astronomy, is led by Daichi Hiramatsu, a graduate student at the University of California, Santa Barbara (UCSB), and Las Cumbres Observatory (LCO). Hiramatsu is a core member of the Global Supernova Project, a worldwide team of scientists using dozens of telescopes around and above the globe. The team found that the supernova SN 2018zd had many unusual characteristics, some of which were seen for the first time in a supernova.

It helped that the supernova was relatively nearby — only 31 million light-years away — in the galaxy NGC 2146. This allowed the team to examine archival images taken prior to the explosion from the Hubble Space Telescope and to detect the likely progenitor star before it exploded. The observations were consistent with another recently identified SAGB star in the Milky Way, but inconsistent with models of red supergiants, the progenitors of normal iron core-collapse supernovae.

The study looked through all published data on supernovae, and found that while some had a few of the indicators predicted for electron-capture supernovae, only SN 2018zd had all six — an apparent SAGB progenitor, strong pre-supernova mass loss, an unusual stellar chemical composition, a weak explosion, little radioactivity, and a neutron-rich core.

“We started by asking ‘what’s this weirdo?’” Hiramatsu said. “Then we examined every aspect of SN 2018zd and realized that all of them can be explained in the electron-capture scenario.”

The new discoveries also illuminate some mysteries of the most famous supernova of the past. In A.D. 1054 a supernova happened in the Milky Way Galaxy, and according to Chinese and Japanese records, it was so bright that it could be seen in the daytime for 23 days, and at night for nearly two years. The resulting remnant, the Crab Nebula, has been studied in great detail. It was previously the best candidate for an electron-capture supernova, but this was uncertain partly because the explosion happened nearly a thousand years ago. The new result increases the confidence that the historic SN 1054 was an electron-capture supernova. It also explains why that supernova was relatively bright compared to the models: its luminosity was probably artificially enhanced by the supernova ejecta colliding with material cast off by the progenitor star as was seen in SN 2018zd.

Dr. Ken Nomoto at the Kavli IPMU of the University of Tokyo was excited that his theory had been confirmed, adding “I am very pleased that the electron-capture supernova was finally discovered, which my colleagues and I predicted to exist and have a connection to the Crab Nebula 40 years ago. I very much appreciate the great efforts involved in obtaining these observations. This is a wonderful case of the combination of observations and theory.”

Hiramatsu added, “It was such a ‘Eureka moment’ for all of us that we can contribute to closing the 40-year-old theoretical loop, and for me personally because my career in astronomy started when I looked at the stunning pictures of the Universe in the high school library, one of which was the iconic Crab Nebula taken by the Hubble Space Telescope.”

“The term Rosetta Stone is used too often as an analogy when we find a new astrophysical object,” said Dr. Andrew Howell, a staff scientist at Las Cumbres Observatory and adjunct faculty at UCSB, “but in this case I think it is fitting. This supernova is literally helping us decode thousand-year-old records from cultures all over the world. And it is helping us associate one thing we don’t fully understand, the Crab Nebula, with another thing we have incredible modern records of, this supernova. In the process it is teaching us about fundamental physics: how some neutron stars get made, how extreme stars live and die, and about how the elements we’re made of get created and scattered around the universe.” Dr. Howell is the leader of the Global Supernova Project, and the lead author’s PhD advisor.

For more on this research:

Reference: “The electron-capture origin of supernova 2018zd” by Daichi Hiramatsu, D. Andrew Howell, Schuyler D. Van Dyk, Jared A. Goldberg, Keiichi Maeda, Takashi J. Moriya, Nozomu Tominaga, Ken’ichi Nomoto, Griffin Hosseinzadeh, Iair Arcavi, Curtis McCully, Jamison Burke, K. Azalee Bostroem, Stefano Valenti, Yize Dong, Peter J. Brown, Jennifer E. Andrews, Christopher Bilinski, G. Grant Williams, Paul S. Smith, Nathan Smith, David J. Sand, Gagandeep S. Anand, Chengyuan Xu, Alexei V. Filippenko, Melina C. Bersten, Gastón Folatelli, Patrick L. Kelly, Toshihide Noguchi and Koichi Itagaki, 28 June 2021, Nature Astronomy.
DOI: 10.1038/s41550-021-01384-2

D.H., D.A.H., G.H., C.M., and J.B. were supported by the U.S. National Science Foundation (NSF) grants AST-1313484 and AST-1911225, as well as by the National Aeronautics and Space Administration (NASA) grant 80NSSC19kf1639.

Mysterious Population of Rogue Planets Spotted Near the Center of Our Galaxy

Artist’s impression of a free-floating planet.

Tantalizing evidence has been uncovered for a mysterious population of “rogue” (or “free-floating”) planets, planets that may be alone in deep space, unbound to any host star. The results include four new discoveries that are consistent with planets of similar masses to Earth, published today (July 6, 2021) in Monthly Notices of the Royal Astronomical Society.

The study, led by Iain McDonald of the University of Manchester, UK, (now based at the Open University, UK) used data obtained in 2016 during the K2 mission phase of NASA’s Kepler Space Telescope. During this two-month campaign, Kepler monitored a crowded field of millions of stars near the center of our Galaxy every 30 minutes in order to find rare gravitational microlensing events.

The study team found 27 short-duration candidate microlensing signals that varied over timescales of between an hour and 10 days. Many of these had been previously seen in data obtained simultaneously from the ground. However, the four shortest events are new discoveries that are consistent with planets of similar masses to Earth.

These new events do not show an accompanying longer signal that might be expected from a host star, suggesting that these new events may be free-floating planets. Such planets may perhaps have originally formed around a host star before being ejected by the gravitational tug of other, heavier planets in the system.

Predicted by Albert Einstein 85 years ago as a consequence of his General Theory of Relativity, microlensing describes how the light from a background star can be temporarily magnified by the presence of other stars in the foreground. This produces a short burst in brightness that can last from hours to a few days. Roughly one out of every million stars in our Galaxy is visibly affected by microlensing at any given time, but only a few percent of these are expected to be caused by planets.

Kepler was not designed to find planets using microlensing, nor to study the extremely dense star fields of the inner Galaxy. This meant that new data reduction techniques had to be developed to look for signals within the Kepler dataset.

Iain notes: “These signals are extremely difficult to find. Our observations pointed an elderly, ailing telescope with blurred vision at one the most densely crowded parts of the sky, where there are already thousands of bright stars that vary in brightness, and thousands of asteroids that skim across our field. From that cacophony, we try to extract tiny, characteristic brightenings caused by planets, and we only have one chance to see a signal before it’s gone. It’s about as easy as looking for the single blink of a firefly in the middle of a motorway, using only a handheld phone.”

Co-author Eamonn Kerins of the University of Manchester also comments, “Kepler has achieved what it was never designed to do, in providing further tentative evidence for the existence of a population of Earth-mass, free-floating planets. Now it passes the baton on to other missions that will be designed to find such signals, signals so elusive that Einstein himself thought that they were unlikely ever to be observed. I am very excited that the upcoming ESA Euclid mission could also join this effort as an additional science activity to its main mission.”

Confirming the existence and nature of free-floating planets will be a major focus for upcoming missions such as the NASA Nancy Grace Roman Space Telescope, and possibly the ESA Euclid mission, both of which will be optimized to look for microlensing signals.

Reference: “Kepler K2 Campaign 9 – I. Candidate short-duration events from the first space-based survey for planetary microlensing” by I McDonald, E Kerins, R Poleski, M T Penny, D Specht, S Mao, P Fouqué, W Zhu and W Zang, 6 July 2021, Monthly Notices of the Royal Astronomical Society.
DOI: 10.1093/mnras/stab1377

Science Made Simple: What Are Atomic Nuclei?

In 1911, Ernest Rutherford discovered that at the core of every atom is a nucleus. Atomic nuclei consist of electrically positive protons and electrically neutral neutrons. These are held together by the strongest known fundamental force, called the strong force. The nucleus makes up much less than .01% of the volume of the atom, but typically contains more than 99.9% of the mass of the atom.

The chemical properties of a substance are determined by the negatively charged electrons enshrouding the nucleus. The number of electrons usually matches the number of protons in the nucleus. Some nuclei are unstable and may undergo radioactive decay, eventually arriving at a stable state through the emission of photons (gamma decay), emission or capture of electrons or positrons (beta decay), emission of helium nuclei (alpha decay), or a combination of these processes. Most nuclei are spherical or ellipsoidal, though some exotic shapes exist. Nuclei can vibrate and rotate when struck by other particles. Some are unstable and will break apart or change their relative number of protons and neutrons.

Nuclei Facts

• A typical grain of sand contains more than 10 million trillion nuclei. That’s 100 times more than the number of seconds since the beginning of the Universe.
• The nucleus accounts for more than 99.9994% of the total atomic mass, but occupies less than one ten-trillionth of the atomic volume.
• All nuclei have approximately the same density. If the Moon was smashed to the same density, it would fit inside Yankee Stadium.

DOE Office of Science: Contributions to Nuclei Research

The DOE Office of Nuclear Physics in the Office of Science supports research to understand all forms of nuclear matter. This research includes mechanisms that form heavy nuclei in cosmic neutron star mergers. It also includes unraveling previously unknown properties of nuclei in their natural state for important applications in medicine, commerce, and national defense. Another area of study is understanding precisely how nuclei are structured depending on the number of protons and neutrons inside them. Other research focuses on heating nuclei to the temperature of the early universe to understand how they condensed out of the quark-gluon soup that existed at the time.

Surprising Insights Into Star Formation From Unprecedented Survey of the “Nurseries” Where Stars Are Born

Study of nearby galaxies gives new insights into star formation.

Astronomers have taken a big step forward in understanding the dark and violent places where stars are born.

Over the past five years, an international team of researchers has conducted the first systematic survey of “stellar nurseries” across our part of the universe, charting the more than 100,000 of these nurseries across more than 90 nearby galaxies and providing new insights into the origins of stars.

“Every star in the sky, including our own sun, was born in one of these stellar nurseries,” said Adam Leroy, associate professor of astronomy at The Ohio State University and one of the leaders of the project.

“These nurseries are responsible for building galaxies and making planets, and they’re just an essential part in the story of how we got here. But this is really the first time we have gotten a complete view of these stellar nurseries across the whole nearby universe.”

The project is called PHANGS-ALMA, and the research was possible thanks to the ALMA telescope array high in the Andes mountains in Chile.

ALMA, the most powerful radio telescope in the world, is an international facility with heavy U.S. involvement led by the National Science Foundation and National Radio Astronomy Observatory.

The power of this facility allowed the team to survey the stellar nurseries across a diverse set of 90 galaxies, while previous studies had mostly focused only on an individual galaxy or a part of one galaxy.

“When optical telescopes take pictures, they capture the light from stars. When ALMA takes a picture, it sees the glow from the gas and dust that will form stars,” said Jiayi Sun, an Ohio State PhD student who is completing a dissertation based on the survey this month.

“The new thing with PHANGS-ALMA is that we can use ALMA to take pictures of many galaxies, and these pictures are as sharp and detailed as those taken by optical telescopes. This just hasn’t been possible before.”

The survey has expanded the amount of data on stellar nurseries by more than tenfold, Leroy said. That has given astronomers a much more accurate perspective of what these nurseries are like across our corner of the universe.

Based on these measurements, they have found that stellar nurseries are surprisingly diverse across galaxies, live only a relatively short time in astronomical terms, and are not very efficient at making stars.

The diversity of these stellar nurseries came as something of a surprise.

“For a long time, conventional wisdom among astronomers was that all stellar nurseries looked more or less the same,” Sun said.

“But with this survey we can see that this is really not the case. While there are some similarities, the nature and appearance of these nurseries change within and among galaxies, just like cities or trees may vary in important ways as you go from place to place across the world.”

For example, nurseries in larger galaxies, and those in the center of galaxies, tend to be denser and more massive, and much more turbulent, he said. Star formation is much more violent in these clouds, findings suggest.

“So the properties of these nurseries and even their ability to make stars seem to depend on the galaxies they live in,” Sun said.

Results from the survey also showed that these stellar nurseries live for only 10 to 30 million years, which is a relatively short time in astronomical terms. And the team used the same measurements to gauge how efficiently these stellar nurseries turned their gas and dust into stars – and it turned out they weren’t that efficient.

“This survey is allowing us to build a much more complete picture of the life cycle of these regions, and we’re finding they are short-lived and inefficient,” Leroy said.

“It’s not random chance destroying these nurseries, but the new stars that they make. They are very ungrateful children.”

The radiation and heat that come out of these young stars begins to disperse and dissolve the clouds, eventually destroying them before they can convert most of their mass.

After more than five years of observations, the survey was recently completed and summarized by the PHANGS-ALMA team in two recent papers accepted to the Astrophysical Journal Supplement Series.

The publication of these two new papers marks a milestone, and the data collected by the project team is now publicly available. The researchers have already used PHANGS-ALMA to produce more than 20 scientific publications. Ten papers detailing the outcomes of the PHANGS survey were recently presented at the 238th meeting of the American Astronomical Society.

“We have an incredible dataset here that will continue to be useful,” Leroy said. “This is really a new view of galaxies and we expect to be learning from it for years to come.”

For more on this research, read Map of the Nearby Universe Created by Cosmic Cartographers Reveals the Diversity of Star-Forming Galaxies.

Reference: “PHANGS-ALMA: Arcsecond CO(2-1) Imaging of Nearby Star-Forming Galaxies” by Adam K. Leroy, Eva Schinnerer, Annie Hughes, Erik Rosolowsky, Jérôme Pety, Andreas Schruba, Antonio Usero, Guillermo A. Blanc, Mélanie Chevance, Eric Emsellem, Christopher M. Faesi, Cinthya N. Herrera, Daizhong Liu, Sharon E. Meidt, Miguel Querejeta, Toshiki Saito, Karin M. Sandstrom, Jiayi Sun, Thomas G. Williams, Gagandeep S. Anand, Ashley T. Barnes, Erica A. Behrens, Francesco Belfiore, Samantha M. Benincasa, Ivana Bešlić, Frank Bigiel, Alberto D. Bolatto, Jakob S. den Brok, Yixian Cao, Rupali Chandar, Jérémy Chastenet, I-Da Chiang, Enrico Congiu, Daniel A. Dale, Sinan Deger, Cosima Eibensteiner, Oleg V. Egorov, Axel García-Rodríguez, Simon C. O. Glover, Kathryn Grasha, Jonathan D. Henshaw, I-Ting Ho, Amanda A. Kepley, Jaeyeon Kim, Ralf S. Klessen, Kathryn Kreckel, Eric W. Koch, J. M. Diederik Kruijssen, Kirsten L. Larson, Janice C. Lee, Laura A. Lopez, Josh Machado, Ness Mayker, Rebecca McElroy, Eric J. Murphy, Eve C. Ostriker, Hsi-An Pan, Ismael Pessa, Johannes Puschnig, Alessandro Razza, Patricia Sánchez-Blázquez, Francesco Santoro, Amy Sardone, Fabian Scheuermann, Kazimierz Sliwa, Mattia C. Sormani, Sophia K. Stuber, David A. Thilker, Jordan A. Turner, Dyas Utomo, Elizabeth J. Watkins, Bradley Whitmore, Accepted, Astrophysical Journal Supplement.
arXiv: 2104.07739

Bursting the Hubble Bubble: Powerful Ground-Based Telescope Will See Further and Clearer Than Hubble Space Telescope

This computer model shows how MAVIS will look on the instrument platform of VLT Unit Telescope 4 (Yepun) at ESO’s Paranal Observatory. The boxes indicate the various submodules of the instrument. Credit: Macquarie University

Australian scientists will help construct one of the world’s most powerful ground-based telescopes that promises to see further and clearer than the Hubble Space Telescope and unlock mysteries of the early Universe.

The team will develop a new, world-first instrument that will produce images three times sharper than Hubble under the multimillion-dollar project.

The MAVIS instrument will be fitted to one of the eight-meter Unit Telescopes at the European Southern Observatory’s (ESO’s) Very Large Telescope in Chile, to remove blurring from telescope images caused by turbulence in Earth’s atmosphere. MAVIS will be built over seven years at a cost of $57 million.

The MAVIS consortium is led by The Australian National University (ANU), and involves Macquarie University, Italy’s National Institute for Astrophysics (INAF), and France’s Laboratoire d’Astrophysique (LAM).

MAVIS Principal Investigator Professor François Rigaut, from the ANU Research School of Astronomy and Astrophysics, said atmospheric turbulence is like the phenomenon of objects appearing blurry on the horizon during a hot day.

“MAVIS will remove this blurring and deliver images as sharp as if the telescope were in space, helping us to peer back into the early Universe by pushing the cosmic frontier of what is visible,” he said.

“The ability to deliver corrected optical images, over a wide field of view using one of the world’s largest telescope, is what makes MAVIS a first-of-its kind instrument, and means we will be able to observe very faint, distant objects.

“We will be able to use the new technology to explore how the first stars formed 13 billion years ago, as well as how weather changes on planets and moons in our Solar System.”

Associate Professor Richard McDermid, the MAVIS project scientist based at Macquarie University, said the project represents a significant milestone for Australia’s growing relationship with ESO, and the nation’s space research and work.

“MAVIS demonstrates that Australia can not only participate in the scientific life of the observatory, but can also be a core player in helping ESO maintain its leadership by developing unique and competitive instruments using Australian expertise,” he said.

Professor Matthew Colless, Director of the ANU Research School of Astronomy and Astrophysics, said the coming decade represents a very exciting time for astronomy.

“ESO and Australia entered a 10-year strategic partnership in 2017, a partnership that the Australian astronomy community has embraced with enthusiasm,” he said.

“In return for building MAVIS, the consortium will get guaranteed observing time with the instrument, as well as a financial contribution from ESO for its hardware.

“From space, with the likes of the James Webb Space Telescope, and with ground-based facilities such as ESO’s Extremely Large Telescope, astronomers will explore the Universe in more depth than ever.

“By delivering the sharpest view possible using visible light, MAVIS will be a unique and powerful complement to these future large facilities, which target infrared wavelengths.”

Lower Exposure to UVB Light From the Sun May Increase Colorectal Cancer Risk

Inadequate exposure to UVB light from the sun may be associated with an increased risk of colorectal cancer, particularly in older age groups, according to a study using data on 186 countries, published in the open access journal BMC Public Health.

Researchers at the University of California San Diego, USA investigated possible associations between global levels of UVB light in 2017 and rates of colorectal cancer for different countries and age groups in 2018.

The authors found that lower UVB exposure was significantly correlated with higher rates of colorectal cancer across all age groups from 0 to over 75 years in people living in the 186 countries included in the study. The association between lower UVB and risk of colorectal cancer remained significant for those aged above 45 after other factors, such as skin pigmentation, life expectancy and smoking were taken into consideration. Data on these factors were available for 148 countries.

The authors suggest that lower UVB exposure may reduce levels of vitamin D. Vitamin D deficiency has previously been associated with an increased risk of colorectal cancer. Future research could look directly at the potential benefits on colorectal cancer of correcting vitamin D deficiencies, especially in older age groups, according to the authors.

Raphael Cuomo, co-author of the study said: “Differences in UVB light accounted for a large amount of the variation we saw in colorectal cancer rates, especially for people over age 45. Although this is still preliminary evidence, it may be that older individuals, in particular, may reduce their risk of colorectal cancer by correcting deficiencies in vitamin D.”

The authors used UVB estimates obtained by the NASA EOS Aura spacecraft in April 2017 and data on colorectal cancer rates in 2018 for 186 countries from the Global Cancer (GLOBOCAN) database. They also collected data for 148 countries on skin pigmentation, life expectancy, smoking, stratospheric ozone (a naturally-occurring gas that filters the sun’s radiation) and other factors which may influence health and UVB exposure from previous literature and databases. Countries with lower UVB included Norway, Denmark and Canada, while countries with higher UVB included United Arab Emirates, Sudan, Nigeria, and India.

The authors caution that other factors may affect UVB exposure and vitamin D levels, such as vitamin D supplements, clothing and air pollution, which were not included in the study. They also caution that the observational nature of the study does not allow for conclusions about cause and effect and more work is needed to understand the relationship between UVB and vitamin D with colorectal cancer in more detail.

Reference: “Could age increase the strength of inverse association between ultraviolet B exposure and colorectal cancer?” 5 July 2021, BMC Public Health.
DOI: 10.1186/s12889-021-11089-w

Axions Could Be the Fossil of the Universe Astrophysicists Have Been Waiting For

Finding the hypothetical particle axion could mean finding out for the first time what happened in the Universe a second after the Big Bang, suggests a new study published in Physical Review D.

How far back into the Universe’s past can we look today? In the electromagnetic spectrum, observations of the Cosmic Microwave Background — commonly referred to as the CMB — allow us to see back almost 14 billion years to when the Universe cooled sufficiently for protons and electrons to combine and form neutral hydrogen. The CMB has taught us an inordinate amount about the evolution of the cosmos, but photons in the CMB were released 400,000 years after the Big Bang making it extremely challenging to learn about the history of the universe prior to this epoch.

To open a new window, a trio of theoretical researchers, including Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU) Principal Investigator, University of California, Berkeley, MacAdams Professor of Physics and Lawrence Berkeley National Laboratory senior faculty scientist Hitoshi Murayama, Lawrence Berkeley National Laboratory physics researcher and University of California, Berkeley, postdoctoral fellow Jeff Dror (now at University of California, Santa Cruz), and UC Berkeley Miller Research Fellow Nicholas Rodd, looked beyond photons, and into the realm of hypothetical particles known as axions, which may have been emitted in the first second of the Universe’s history.

In their paper, they suggest the possibility of searching for an axion analog of the CMB, a so-called Cosmic axion Background or CaB.

While hypothetical, there are many reasons to suspect that the axion could exist in our Universe.

For one, axions are a generic prediction of string theory, one of today’s best hopes for a theory of quantum gravity. The existence of an axion could further help resolve the long standing puzzle of why we have yet to measure an electric dipole moment for the neutron, an issue more formally known as the “Strong CP Problem.” More recently, the axion has become a promising candidate for dark matter, and as a consequence researchers are rapidly searching for axion dark-matter.

In their paper, the researchers point out that as experimentalists develop more sensitive instruments to search for dark matter, they may stumble upon another sign of axions in the form of the CaB. But because the CaB shares similar properties with dark-matter axions, there is a risk the experiments would throw the CaB signal out as noise.

The variation in the form of the CaB as a function of energy and density can be seen for four different scenarios for its production. Finding any one of these would help answer the fundamental questions listed. Credit: Dror et al.

Finding the CaB at one of these instruments would be a double discovery. Not only would it confirm the existence of the axion, but researchers worldwide would immediately have a new fossil from the early Universe. Depending on how the CaB was produced, researchers could learn about various different aspects of the Universe’s evolution never possible before (Figure).

“What we have proposed is that, by changing the way current experiments analyze data, we may be able to search for left-over axions from the early universe. Then, we might be able to learn about the origin of dark matter, phase transition or inflation at the beginning of the Universe. There are already experimental groups who have shown interest in our proposal, and I hope we can find out something new about the early Universe that wasn’t known before,” says Murayama.

“The evolution of the universe can produce axions with a characteristic energy distribution. By detecting the energy density of the universe currently made up of axions, experiments such as MADMAX, HAYSTAC, ADMX, and DMRadio could give us answers to some of the most important puzzles in cosmology, such as, ‘How hot did our universe get?’, ‘What is nature of dark matter?’, ‘Did our universe undergo a period of rapid expansion known as inflation?’, and ‘Was there ever a cosmic phase transition?’,” says Dror.

The new study provides reason to be excited about the axion dark-matter program. Even if dark matter is not made of axions, these instruments may provide an image of the Universe when it was less than a second old.

This study was accepted as an “Editors’ Suggestion” in the journal Physical Review D.

Reference: “Cosmic axion background” by Jeff A. Dror, Hitoshi Murayama and Nicholas L. Rodd, 7 June 2021, Physical Review D.
DOI: 10.1103/PhysRevD.103.115004

New, Third Type of Supernova Discovered: An Electron-Capture Supernova

An international team of astronomers has observed the first example of a new type of supernova. The discovery, confirming a prediction made four decades ago, could lead to new insights into the life and death of stars. The work was published on June 28, 2021, in Nature Astronomy.

“One of the main questions in astronomy is to compare how stars evolve and how they die,” said Stefano Valenti, professor of physics and astronomy at the University of California, Davis, and a member of the team that discovered and described supernova 2018zd. “There are many links still missing, so this is very exciting.”

There are two known types of supernova. A core-collapse supernova occurs when a massive star, more than 10 times the mass of our sun, runs out of fuel and its core collapses into a black hole or neutron star. A thermonuclear supernova occurs when a white dwarf star — the remains of a star up to eight times the mass of the sun — explodes.

In 1980, Ken’ichi Nomoto of the University of Tokyo predicted a third type called an electron capture supernova.

What keeps most stars from collapsing under their own gravity is the energy produced in their central core. In an electron capture supernova, as the core runs out of fuel, gravity forces electrons in the core into their atomic nuclei, causing the star to collapse in on itself.

Supernova 2018zd, marked with a white circle on the outskirts of galaxy NGC2146, is the first example of a new, third type of supernova predicted 40 years ago. Composite image with data from the Hubble Space Telescope, Las Cumbres Observatory and other sources. Credit: Joseph Depasquale, STScI

Evidence from late spectrum

Supernova 2018zd was detected in March 2018, about three hours after the explosion. Archival images from the Hubble Space Telescope and Spitzer Space Telescope showed a faint object that was likely the star before explosion. The supernova is relatively close to Earth, at a distance of about 31 million light years in galaxy NGC2146.

The team, led by Daichi Hiramatsu, graduate student at UC Santa Barbara and Las Cumbres Observatory, collected data on the supernova over the next two years. Astronomers from UC Davis, including Valenti and graduate students Azalee Bostroem and Yize Dong, contributed a spectral analysis of the supernova two years after the explosion, one of the lines of evidence demonstrating that 2018zd was an electron capture supernova.

“We had a really exquisite, really complete dataset following its rise and fade,” Bostroem said. That included very late data collected with the 10-meter telescope at the W.M. Keck Observatory in Hawaii, Dong added.

Theory predicts that electron capture supernovae should show an unusual stellar chemical spectrum years later.

“The Keck spectra we observed clearly confirm that SN 2018zd is our best candidate to be an electron capture supernova,” Valenti said.

The late spectrum data were not the only piece of the puzzle. The team looked through all published data on supernovae, and found that while some had a few of the indicators predicted for electron capture supernovae, only SN 2018zd had all six: an apparent progenitor star of the Super-Asymptotic Giant Branch (SAGB) type; strong pre-supernova mass loss; an unusual stellar chemical spectrum; a weak explosion; little radioactivity; and a neutron-rich core.

“We started by asking ‘what’s this weirdo?’ Then we examined every aspect of SN 2018zd and realized that all of them can be explained in the electron-capture scenario,” Hiramatsu said.

This composite image of the Crab Nebula was assembled by combining data from five telescopes spanning nearly the entire breadth of the electromagnetic spectrum. Credit: NASA, ESA, NRAO/AUI/NSF and G. Dubner (University of Buenos Aires)

Explaining the Crab Nebula

The new discoveries also illuminate some mysteries of the most famous supernova of the past. In A.D. 1054 a supernova occurred in the Milky Way. According to Chinese records it was so bright that it could be seen in the daytime for 23 days, and at night for nearly two years. The resulting remnant — the Crab Nebula — has been studied in great detail. It was previously the best candidate for an electron capture supernova, but this was uncertain partly because the explosion happened nearly a thousand years ago. The new result increases the confidence that the event that formed the Crab Nebula was an electron capture supernova.

“I am very pleased that the electron capture supernova was finally discovered, which my colleagues and I predicted to exist and have a connection to the Crab Nebula 40 years ago. This is a wonderful case of the combination of observations and theory,” said Nomoto, who is also an author on the current paper.

Read Discovery of a New Type of Stellar Explosion – An Electron-Capture Supernova – Illuminates a Medieval Mystery for more on this research.

Reference: “The electron-capture origin of supernova 2018zd” by Daichi Hiramatsu, D. Andrew Howell, Schuyler D. Van Dyk, Jared A. Goldberg, Keiichi Maeda, Takashi J. Moriya, Nozomu Tominaga, Ken’ichi Nomoto, Griffin Hosseinzadeh, Iair Arcavi, Curtis McCully, Jamison Burke, K. Azalee Bostroem, Stefano Valenti, Yize Dong, Peter J. Brown, Jennifer E. Andrews, Christopher Bilinski, G. Grant Williams, Paul S. Smith, Nathan Smith, David J. Sand, Gagandeep S. Anand, Chengyuan Xu, Alexei V. Filippenko, Melina C. Bersten, Gastón Folatelli, Patrick L. Kelly, Toshihide Noguchi and Koichi Itagaki, 28 June 2021, Nature Astronomy.
DOI: 10.1038/s41550-021-01384-2

The research is part of the Global Supernova Project, led by Professor Andrew Howell at UCSB and Las Cumbres Observatory. Additional co-authors are: Curtis McCully and Jamison Burke, Las Cumbres Observatory and UCSB; Jared Goldberg and Chengyuan Xu, UCSB; Schuyler Van Dyk and Gagandeep Anand, California Institute of Technology; Keiichi Maeda, Kyoto University; Takashi Moriya, National Astronomical Observatory of Japan; Nozomu Tominaga, Konan University, Kobe, Japan; Griffin Hosseinzadeh, Center for Astrophysics, Harvard & Smithsonian; Iair Arcavi, Tel Aviv University, Israel; Peter Brown, Texas A&M University; Jennifer Andrews, Christopher Bilinski, G. Grant Williams, Paul Smith, Nathan Smith and David Sand, Steward Observatory, University of Arizona; Alexei Filippenko, UC Berkeley; Melina Bersten and Gastón Folatelli, Instituto de Astrofísica de La Plata and Universidad Nacional de La Plata, Argentina; Patrick Kelly, University of Minnesota; Toshi- hide Noguchi, Noguchi Astronomical Observatory and Koichi Itagaki, Itagaki Astronomical Observatory, Japan. The work was partly supported by grants from the National Science Foundation and NASA.