Topic: Early April Astronomy Bulletin

[Clive]

SCIENTISTS SOLVE SOLAR SECRET

University of Otago

The further we move away from a heat source, the cooler the air gets. Bizarrely, the same can't be said for the Sun, but scientists may have just explained a key part of why. The surface of the Sun starts at 6000 degree C, but over a short distance of only a few hundred kilometers, it suddenly heats up to more than a million degrees, becoming its atmosphere, or corona. This is so hot that the gas escapes the Sun's gravity as 'solar wind', and flies into space, smashing into Earth and other planets. Scientists know from measurements and theory that the sudden temperature jump is related to magnetic fields which thread out of the Sun's surface. But, exactly how these work to heat the gas is not well understood -- this is known as the Coronal Heating Problem. Astrophysicists have several different ideas about how the magnetic-field energy could be converted into heat to explain the heating, but most have difficulty explaining some aspect of observations. The popular theories are based on heating caused by turbulence, and heating caused by a type of magnetic wave called ion cyclotron waves. Both, however, have some problem -- turbulence struggles to explain why Hydrogen, Helium and Oxygen in the gas become as hot as they do, while electrons remain surprisingly cold; while the magnetic waves theory could explain this feature, there doesn't seem to be enough of the waves coming off the Sun's surface to heat up the gas. The group used six-dimensional supercomputer simulations of the coronal gas to show how these two theories are actually part of the same process, linked together by a bizarre effect called the 'helicity barrier'. If we imagine plasma heating as occurring a bit like water flowing down a hill, with electrons heated right at the bottom, then the helicity barrier acts like a dam, stopping the flow and diverting its energy into ion cyclotron waves. In this way, the helicity barrier links the two theories and resolves each of their individual problems.

For this latest study, the group stirred the magnetic field lines in simulations and found the turbulence created the waves, which then caused the heating. As this happens, the structures and eddies that form end up looking extremely similar to cutting-edge measurements from NASA's Parker Solar Probe spacecraft, which has recently become the first human-made object to actually fly into the corona. This gives astronomer confidence that they are accurately capturing key physics in the corona, which -- coupled with the theoretical findings about the heating mechanisms -- is a promising path to understanding the coronal heating problem. Understanding more about the Sun's atmosphere and the subsequent solar wind is important because of the profound impacts they have on Earth. Effects which result from solar wind's interaction with the Earth's magnetic field is called 'space weather', which causes everything from Aurora to satellite-destroying radiation and geomagnetic currents which damage the power grid. All of this is sourced, fundamentally, by the corona and its heating by magnetic fields, so as well as being interesting for our general understanding of the solar system, the solar-corona's dynamics can have profound impacts on Earth.

MERCURY HAS MAGNETIC STORMS

University of Alaska Fairbanks

A team of scientists has proved that Mercury, our solar system's smallest planet, has geomagnetic storms similar to those on Earth. Their finding, a first, answers the question of whether other planets, including those outside our solar system, can have geomagnetic storms regardless of the size of their magnetosphere or whether they have an Earth-like ionosphere. The team proves the planet has a ring current, a doughnut-shaped field of charged particles flowing laterally around the planet and excluding the poles and also the existence of geomagnetic storms triggered by the ring current. A geomagnetic storm is a major disturbance in a planet's magnetosphere caused by the transfer of energy from the solar wind. Such storms in Earth's magnetosphere produce the aurora and can disrupt radio communications. Confirmation about geomagnetic storms on Mercury results from research made possible by a fortuitous coincidence: a series of coronal mass ejections from the Sun on April 8-18, 2015, and the end of NASA's Messenger space probe, which launched in 2004 and crashed into the planet's surface on April 30, 2015, at the expected end of its mission. A coronal mass ejection, or CME, is an ejected cloud of the sun's plasma -- a gas made of charged particles. That cloud includes the plasma's embedded magnetic field. The coronal mass ejection of April 14 proved to be the key for scientists. It compressed Mercury's ring current on the sun-facing side and increased the current's energy.

New analysis of data from Messenger, which had dropped closer to the planet, shows the presence of a ring current intensification that is essential for triggering magnetic storms. But this doesn't mean Mercury has auroral displays like those on Earth. On Earth, the storms produce aurora displays when solar wind particles interact with the particles of the atmosphere. On Mercury, however, solar wind particles don't encounter an atmosphere. Instead, they reach the surface unimpeded and may therefore be visible only through X-ray and gamma ray examination.The results show that magnetic storms are "potentially a common feature of magnetized planets. The results obtained from Messenger provide a further fascinating insight into Mercury's place in the evolution of the solar system following the discovery of its intrinsic planetary magnetic field.

SPEED OF SOUND ON MARS IS SLOWER THAN ON EARTH

NASA/JPL-Caltech

A team of scientists has revealed how fast sound travels through the extremely thin, mostly carbon dioxide atmosphere of Mars and how it might sound to human ears. Most of the sounds in the study were recorded using the microphone on Perseverance’s SuperCam, mounted on the head of the rover’s mast. The study also refers to sounds recorded by another microphone mounted on the chassis of the rover. This second microphone recently recorded the puffs and pings of the rover’s Gaseous Dust Removal Tool, or gDRT, which blows shavings off rocks that the rover has scraped in order to examine. The result of the recordings reveal a new understanding of strange characteristics of the Martian atmosphere, where the speed of sound is slower than on Earth – and varies with pitch (or frequency). On Earth, sounds typically travel at 343 metres per second. But on Mars, low-pitched sounds travel at about 240 metres per second, while higher-pitched sounds move at 250 metres per second. The variable sound speeds on the Red Planet are an effect of the thin, cold, carbon dioxide atmosphere. Prior to the mission, scientists expected Mars’ atmosphere would influence sound speed, but the phenomenon had never been observed until these recordings were made. Another effect of this thin atmosphere: Sounds carry only a short distance, and higher-pitched tones carry hardly at all. On Earth, sound might drop off after about 65 metres; on Mars, it falters at just 8 metres, with high-pitched sounds being lost completely at that distance. The recordings from SuperCam’s microphone also reveal previously unobserved pressure variations produced by turbulence in the Martian atmosphere as its energy changes at tiny scales. Martian wind gusts at very short timescales also were measured for the first time. One of the most striking features of the sound recordings is the silence that seems to prevail on Mars. That, too, is a consequence of Mars having such a thin atmosphere. That means, in the Martian autumn months to come, Mars might get noisier – and provide even more insights into its otherworldly air and weather.

'PRENATAL' PROTOPLANET DISCOVERED

National Institutes of Natural Sciences

An international research team has discovered a new planet so young that it has yet to emerge from the womb of matter where it is forming. This is the youngest protoplanet discovered to date. It's location and the surrounding patterns of matter suggest that an alternative method of planet formation may be at work. This discovery could help to explain the histories and features of extrasolar planets seen around other stars. In the standard model of planet formation, a large Jupiter-like gas planet starts as a rocky core in a protoplanetary disk around a young star. This core then accretes gas from the disk, growing into a giant planet. While this model works well for the planets in the Solar System, it has trouble explaining exoplanets which have been discovered around other stars at distances much larger than the orbit of Neptune, the outermost Solar System planet. Rocky cores aren't expected to form far away from the central star, so core accretion can't drive distant planet formation. One theory holds that outlying planets form close to the central star and move outwards. But new observations using an extreme adaptive optics system which allows the Subaru Telescope to directly image faint objects close to brighter stars show what appears to be a protoplanet in the process of forming directly at a distance of 93 au: over three times the distance between the Sun and Neptune.

Analysis of this object, named AB Aur b, shows that a simple model of starlight reflected from an anomaly in the disk can't reproduce the observations; but neither can a model of a naked planet. The best fit models indicate that AB Aur b is a protoplanet so young that it is still forming in a womb of matter in the protoplanetary disk. Nearby spiral structures in the disk match models where a planet forms directly from the gravitational collapse of the surrounding matter. This discovery has profound implications for explaining the many observed outlying exoplanets and the overall theoretical model of planet formation.

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CASSIOPEIA  A SUPERNOVA EJECTION NEBULA

Netherlands Research School for Astronomy

Cassiopeia A is the remnant of an exploded star in the Cassiopeia constellation, about 11,000 light years away from us. Light from the explosion should have reached Earth for the first time around 1670. However, there was too much gas and dust around the star for the explosion to be seen with the naked eye or with the then very basic telescopes. The Cassiopeia A explosion nebula is expanding at an average rate of 4,000 to 6,000 kilometres per second and has a temperature of about 30 million degrees Celsius. The expansion is most likely occurring in gas that was blown out by the star long before the explosion. Cassiopeia A is now about 16 light years across. The researchers analysed 19 years of data from the Chandra X-ray Observatory that orbits the Earth in a high elliptical orbit. The scientists observed that on the western side of Cassiopeia A, the inner regions of the explosion nebula are not expanding, but moving inwards. The researchers also took measurements of the acceleration or deceleration of the outer shock wave. This outer shock wave turned out to accelerate in the west instead of decelerating as was expected. The backward movement in the west can mean two things. Either there is a hole somewhere, a kind of vacuum, in the supernova material, causing the hot shell to suddenly move inwards locally. Or the nebula has collided with something. From the computer models, a collision appears most likely. They predict that after a collision, the shock first decreases in speed but then accelerates. Exactly as the team has measured.

MYSTERIOUS DEATH THROES OF CARBON STAR

National Radio Astronomy Observatory

Scientists studying V Hydrae (V Hya) have witnessed the star's mysterious death throes in unprecedented detail. Using the Atacama Large Millimeter/submillimeter Array (ALMA) and data from the Hubble Space Telescope (HST), the team discovered six slowly-expanding rings and two hourglass-shaped structures caused by the high-speed ejection of matter out into space. V Hya is a carbon-rich asymptotic giant branch (AGB) star located approximately 1,300 light-years from Earth in the constellation Hydra. More than 90-percent of stars with a mass equal to or greater than the Sun evolve into AGB stars as the fuel required to power nuclear processes is stripped away. Among these millions of stars, V Hya has been of particular interest to scientists due to its so-far unique behaviours and features, including extreme-scale plasma eruptions that happen roughly every 8.5 years and the presence of a nearly invisible companion star that contributes to V Hya's explosive behaviour. The six rings have expanded outward from V Hya over the course of roughly 2,100 years, adding matter to and driving the growth of a high-density flared and warped disk-like structure around the star. The team has dubbed this structure the DUDE, or Disk Undergoing Dynamical Expansion. V Hya is in the brief but critical transition phase that does not last very long, and it is difficult to find stars in this phase. In addition to a full set of expanding rings and a warped disk, V Hya's final act features two hourglass-shaped structures -- and an additional jet-like structure -- that are expanding at high speeds of more than half a million miles per hour (240 km/s). Large hourglass structures have been observed previously in planetary nebulae, including MyCn 18 -- also known as the Engraved Hourglass Nebula -- a young emission nebula located roughly 8,000 light-years from Earth in the southern constellation of Musca, and the more well-known Southern Crab Nebula, an emission nebula located roughly 7,000 light-years from Earth in the southern constellation Centaurus. Scientists first observed the presence of very fast outflows in 1981. Then, in 2022, they found a jet-like flow consisting of compact plasma blobs ejected at high speeds from V Hya. And now, the discovery of wide-angle outflows in V Hya connects the dots, revealing how all these structures can be created during the evolutionary phase that this extra-luminous red giant star is now in.



HUBBLE SPOTS FARTHEST STAR EVER SEEN

NASA/Goddard Space Flight Center

The Hubble Space Telescope has established an extraordinary new benchmark: detecting the light of a star that existed within the first billion years after the Universe's birth in the big bang -- the farthest individual star ever seen to date. The find is a huge leap further back in time from the previous single-star record holder; detected by Hubble in 2018. That star existed when the Universe was about 4 billion years old, or 30 percent of its current age, at a time that astronomers refer to as "redshift 1.5." Scientists use the word "redshift" because as the Universe expands, light from distant objects is stretched or "shifted" to longer, redder wavelengths as it travels toward us. The newly detected star is so far away that its light has taken 12.9 billion years to reach Earth, appearing to us as it did when the Universe was only 7 percent of its current age, at redshift 6.2. The smallest objects previously seen at such a great distance are clusters of stars, embedded inside early galaxies. Normally at these distances, entire galaxies look like small smudges, with the light from millions of stars blending together. The galaxy hosting this star has been magnified and distorted by gravitational lensing into a long crescent that has been named the Sunrise Arc. After studying the galaxy in detail, astronomers determined that one feature is an extremely magnified star that they called Earendel, which means "morning star" in Old English. The discovery holds promise for opening up an uncharted era of very early star formation. It will be a window into an era of the Universe that we are unfamiliar with, but that led to everything we do know.

The research team estimates that Earendel is at least 50 times the mass of our Sun and millions of times as bright, rivalling the most massive stars known. But even such a brilliant, very high-mass star would be impossible to see at such a great distance without the aid of natural magnification by a huge galaxy cluster, WHL0137-08, sitting between us and Earendel. The mass of the galaxy cluster warps the fabric of space, creating a powerful natural magnifying glass that distorts and greatly amplifies the light from distant objects behind it. Thanks to the rare alignment with the magnifying galaxy cluster, the star Earendel appears directly on, or extremely close to, a ripple in the fabric of space. This ripple, which is defined in optics as a "caustic," provides maximum magnification and brightening. The effect is analogous to the rippled surface of a swimming pool creating patterns of bright light on the bottom of the pool on a sunny day. The ripples on the surface act as lenses and focus sunlight to maximum brightness on the pool floor. This caustic causes the star Earendel to pop out from the general glow of its home galaxy. Its brightness is magnified a thousandfold or more. At this point, astronomers are not able to determine if Earendel is a binary star, though most massive stars have at least one smaller companion star. Astronomers expect that Earendel will remain highly magnified for years to come. It will be observed by NASA's James Webb Space Telescope. Webb's high sensitivity to infrared light is needed to learn more about Earendel, because its light is stretched (redshifted) to longer infrared wavelengths due to the Universe's expansion.

SCIENTISTS DISCOVER FARTHEST GALAXY EVER

RAS

Astronomers have discovered the most distant astronomical object ever: a galaxy 13.5 billion light years away. Named HD1, scientists have begun to speculate exactly what it is. Shining only ~300 million years after the Big Bang, it may be home to the oldest stars in the Universe, or a supermassive black hole. HD1 was discovered after more than 1,200 hours of observing time with the Subaru Telescope, VISTA Telescope, UK Infrared Telescope and Spitzer Space Telescope. The team then conducted follow-up observations using the Atacama Large Millimetre/submillimetre Array (ALMA) to confirm the distance, which is 100 million light years further than GN-z11, the current record-holder for the furthest galaxy.  The team proposes two ideas: HD1 may be forming stars at an astounding rate and is possibly even home to the Universe’s very first stars, known as Population III stars — which, until now, have never been observed. Alternatively, HD1 may contain a supermassive black hole about 100 million times the mass of our Sun. 

The very first population of stars that formed in the Universe were more massive, more luminous and hotter than modern stars. If we assume the stars produced in HD1 are these first, or Population III, stars, then its properties could be explained more easily. In fact, Population III stars are capable of producing more UV light than normal stars, which could clarify the extreme ultraviolet luminosity of HD1. Answering questions about the nature of a source so far away can be challenging. It’s like guessing the nationality of a ship from the flag it flies, while being faraway ashore, with the vessel in the middle of a gale and dense fog. One can maybe see some colours and shapes of the flag, but not in their entirety. It’s ultimately a long game of analysis and exclusion of implausible scenarios. Forming a few hundred million years after the Big Bang, a black hole in HD1 must have grown out of a massive seed at an unprecedented rate. Using the James Webb Space Telescope, the research team will soon once again observe HD1 to verify its distance from Earth. If current calculations are right, HD1 will be the most distant — and oldest — galaxy ever recorded. The same observations will allow the team to dig deeper into HD1’s identity and confirm if one of their theories is correct.



MOST DETAILED MODEL OF EARLY UNIVERSE TO DATE

Massachusetts Institute of Technology

It all started around 13.8 billion years ago with a big, cosmological "bang" that brought the Universe suddenly and spectacularly into existence. Shortly after, the infant Universe cooled dramatically and went completely dark. Then, within a couple hundred million years after the Big Bang, the Universe woke up, as gravity gathered matter into the first stars and galaxies. Light from these first stars turned the surrounding gas into a hot, ionized plasma -- a crucial transformation known as cosmic reionization that propelled the Universe into the complex structure that we see today. Now, scientists can get a detailed view of how the Universe may have unfolded during this pivotal period with a new simulation, known as Thesan, developed by scientists at MIT, Harvard University, and the Max Planck Institute for Astrophysics. Named after the Etruscan goddess of the dawn, Thesan is designed to simulate the "cosmic dawn," and specifically cosmic reionization, a period which has been challenging to reconstruct, as it involves immensely complicated, chaotic interactions, including those between gravity, gas, and radiation. The Thesan simulation resolves these interactions with the highest detail and over the largest volume of any previous simulation. It does so by combining a realistic model of galaxy formation with a new algorithm that tracks how light interacts with gas, along with a model for cosmic dust. With Thesan, the researchers can simulate a cubic volume of the Universe spanning 300 million light years across. They run the simulation forward in time to track the first appearance and evolution of hundreds of thousands of galaxies within this space, beginning around 400,000 years after the Big Bang, and through the first billion years.

So far, the simulations align with what few observations astronomers have of the early Universe. As more observations are made of this period, for instance with the newly launched James Webb Space Telescope, Thesan may help to place such observations in cosmic context. For now, the simulations are starting to shed light on certain processes, such as how far light can travel in the early Universe, and which galaxies were responsible for reionization. In the earliest stages of cosmic reionization, the Universe was a dark and homogenous space. For physicists, the cosmic evolution during these early "dark ages" is relatively simple to calculate. To fully simulate cosmic reionization, the team sought to include as many major ingredients of the early Universe as possible. They started off with a successful model of galaxy formation that their groups previously developed, called Illustris-TNG, which has been shown to accurately simulate the properties and populations of evolving galaxies. They then developed a new code to incorporate how the light from galaxies and stars interact with and reionize the surrounding gas -- an extremely complex process that other simulations have not been able to accurately reproduce at large scale. Finally, the team included a preliminary model of cosmic dust -- another feature that is unique to such simulations of the early universe. This early model aims to describe how tiny grains of material influence the formation of galaxies in the early, sparse Universe. With the simulation's ingredients in place, the team set its initial conditions for around 400,000 years after the Big Bang, based on precision measurements of relic light from the Big Bang. They then evolved these conditions forward in time to simulate a patch of the Universe, using the SuperMUC-NG machine -- one of the largest supercomputers in the world -- which simultaneously harnessed 60,000 computing cores to carry out Thesan's calculations over an equivalent of 30 million CPU hours (an effort that would have taken 3,500 years to run on a single desktop). The simulations have produced the most detailed view of cosmic reionization, across the largest volume of space, of any existing simulation. While some simulations model across large distances, they do so at relatively low resolution, while other, more detailed simulations do not span large volumes. Early analyses of the simulations suggest that towards the end of cosmic reionization, the distance light was able to travel increased more dramatically than scientists had previously assumed. The researchers also see hints of the type of galaxies responsible for driving reionization. A galaxy's mass appears to influence reionization, though the team says more observations, taken by James Webb and other observatories, will help to pin down these predominant galaxies.