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Late February Astronomy Bulletin
« on: February 21, 2021, 09:40 »
THE COMET THAT KILLED THE DINOSAURS
Harvard University

It was tens of miles wide and forever changed history when it crashed into Earth about 66 million years ago. The Chicxulub impactor, as it's known, left behind a crater off the coast of Mexico that spans 93 miles and goes 12 miles deep. Its devastating impact brought the reign of the dinosaurs to an abrupt and calamitous end by triggering their sudden mass extinction, along with the end of almost three-quarters of the plant and animal species then living on Earth. The enduring puzzle has always been where the asteroid or comet that set off the destruction originated, and how it came to strike the Earth. And now researchers believe they have the answer. Using statistical analysis and gravitational simulations, researchers show that a significant fraction of a type of comet originating from the Oort cloud, a sphere of debris at the edge of the solar system, was bumped off-course by Jupiter's gravitational field during its orbit and sent close to the Sun, whose tidal force broke apart pieces of the rock. That increases the rate of comets like Chicxulub because these fragments cross the Earth's orbit and hit the planet once every 250 to 730 million years or so. It's because of this that long-period comets, which take more than 200 years to orbit the Sun, are called Sun grazers. When you have these Sun grazers, it's not so much the melting that goes on, which is a pretty small fraction relative to the total mass, but the comet is so close to the Sun that the part that's closer to the Sun feels a stronger gravitational pull than the part that is farther from the Sun, causing a tidal force. You get what's called a tidal disruption event and so these large comets that come really close to the Sun break up into smaller comets. And basically, on their way out, there's a statistical chance that these smaller comets hit the Earth. The calculations from the theory increase the chances of long-period comets impacting Earth by a factor of about 10, and show that about 20 percent of long-period comets become Sun grazers. That finding falls in line with research from other astronomers. The team claim that their new rate of impact is consistent with the age of Chicxulub, providing a satisfactory explanation for its origin and other impactors like it.

The hypothesis predicts that other Chicxulub-size craters on Earth are more likely to correspond to an impactor with a primitive (carbonaceous chondrite) composition than expected from the conventional main-belt asteroids. This is important because a popular theory on the origin of Chicxulub claims the impactor is a fragment of a much larger asteroid that came from the main belt, which is an asteroid population between the orbit of Jupiter and Mars. Only about a tenth of all main-belt asteroids have a composition of carbonaceous chondrite, while it's assumed most long-period comets have it. Evidence found at the Chicxulub crater and other similar craters that suggests they had carbonaceous chondrite. This includes an object that hit about 2 billion years ago and left the Vredefort crater in South Africa, which is the largest confirmed crater in Earth's history, and the impactor that left the Zhamanshin crater in Kazakhstan, which is the largest confirmed crater within the last million years. The researchers say that composition evidence supports their model and that the years the objects hit support both their calculations on impact rates of Chicxulub-sized tidally disrupted comets and for smaller ones like the impactor that made the Zhamanshin crater. If produced the same way, they say those would strike Earth once every 250,000 to 730,000 years. The hypothesis can be tested by further studying these craters, others like them, and even ones on the surface of the Moon to determine the composition of the impactors. Space missions sampling comets can also help. Aside from composition of comets, the new Vera Rubin Observatory in Chile may be able to see the tidal disruption of long-period comets after it becomes operational next year.


SOLAR SYSTEM’S MOST DISTANT KNOWN OBJECT
NSF's NOIRLab

Astronomers have confirmed that a faint object discovered in 2018 and nicknamed "Farfarout" is indeed the most distant object yet found in our Solar System. The object has just received its designation from the International Astronomical Union. Farfarout was first spotted in January 2018 by the Subaru Telescope, located on Maunakea in Hawai'i. Its discoverers could tell it was very far away, but they weren't sure exactly how far. They needed more observations. At that time they did not know the object's orbit as they only had the Subaru discovery observations over 24 hours, but it takes years of observations to get an object's orbit around the Sun. All astronomers knew was that the object appeared to be very distant at the time of discovery. They spent the next few years tracking the object with the Gemini North telescope (also on Maunakea in Hawai'i) and the Carnegie Institution for Science's Magellan Telescopes in Chile to determine its orbit and have now confirmed that Farfarout currently lies 132 astronomical units (au) from the Sun, which is 132 times farther from the Sun than Earth is. (For comparison, Pluto is 39 au from the Sun, on average.) Farfarout is even more remote than the previous Solar System distance record-holder, which was discovered by the same team and nicknamed "Farout." Provisionally designated 2018 VG18, Farout is 124 au from the Sun.

However, the orbit of Farfarout is quite elongated, taking it 175 au from the Sun at its farthest point and around 27 au at its closest, which is inside the orbit of Neptune. Because its orbit crosses Neptune's, Farfarout could provide insights into the history of the outer Solar System. Farfarout was likely thrown into the outer Solar System by getting too close to Neptune in the distant past and will likely interact with Neptune again in the future since their orbits still intersect. Farfarout is very faint. Based on its brightness and distance from the Sun, the team estimates it to be about 400 kilometres across, putting it at the low end of possibly being designated a dwarf planet by the International Astronomical Union (IAU). The IAU's Minor Planet Center in Massachusetts has announced that it has given Farfarout the provisional designation 2018 AG37. The Solar System's most distant known member will receive an official name after more observations are gathered and its orbit becomes even more refined in the coming years. Farfarout takes a millennium to go around the Sun once. Because of this, it moves very slowly across the sky, requiring several years of observations to precisely determine its trajectory. Discoverers are confident that even more distant objects remain to be discovered on the outskirts of the Solar System, and that its distance record might not stand for long.


ORIGINS OF ‘SUPER-EARTHS’ UNCOVERED
McGill University

Mini-Neptunes and super-Earths up to four times the size of our own are the most common exoplanets orbiting stars beyond our solar system. Until now, super-Earths were thought to be the rocky cores of mini-Neptunes whose gassy atmospheres were blown away. In a new study astronomers show that some of these exoplanets never had gaseous atmospheres to begin with, shedding new light on their mysterious origins. From observations, we know about 30 to 50 percent of host stars have one or the other, and the two populations appear in about equal proportion. But where did they come from? One theory is that most exoplanets are born as mini-Neptunes but some are stripped of their gas shells by radiation from host stars, leaving behind only a dense, rocky core. This theory predicts that our Galaxy has very few Earth-sized and smaller exoplanets known as Earths and mini-Earths. However, recent observations show this may not be the case. To find out more, the astronomers used a simulation to track the evolution of these mysterious exoplanets. The model used thermodynamic calculations based on how massive their rocky cores are, how far they are from their host stars, and how hot the surrounding gas is.

Contrary to previous theories, the study shows that some exoplanets can never build gaseous atmospheres to begin with. The findings suggest that not all super-Earths are remnants of mini-Neptunes. Rather, the exoplanets were formed by a single distribution of rocks, born in a spinning disk of gas and dust around host stars. Some of the rocks grew gas shells, while others emerged and remained rocky super-Earths. Planets are thought to form in a spinning disk of gas and dust around stars. Rocks larger than the moon have enough gravitational pull to attract surrounding gas to form a shell around its core. Over time this shell of gas cools down and shrinks, creating space for more surrounding gas to be pulled in, and causing the exoplanet to grow. Once the entire shell cools down to the same temperature as the surrounding nebular gas, the shell can no longer shrink and growth stops. For smaller cores, this shell is tiny, so they remain rocky exoplanets. The distinction between super-Earths and mini-Neptunes comes about from the ability of these rocks to grow and retain gas shells. The findings help explain the origin of the two populations of exoplanets, and perhaps their prevalence" says Lee. "Using the theory proposed in the study, we could eventually decipher how common rocky exoplanets like Earths and mini-Earths may be.


CRAB NEBULA REVEALS HONEYCOMBE CENTRE
RAS

A unique ‘heart-shape’, with wisps of gas filaments showing an intricate honeycomb-like arrangement, has been discovered at the centre of the iconic supernova remnant, the Crab Nebula. Astronomers have mapped the void in unprecedented detail, creating a realistic three-dimensional reconstruction. The Crab, formally known as Messier 1, exploded as a dramatic supernova in 1054 AD, and was observed over the subsequent months and years by ancient astronomers across the world. The resulting nebula - the remnant of this enormous explosion - has been studied by amateur and professional astronomers for centuries. However, despite this rich history of investigation, many questions remain about what type of star was originally there and how the original explosion took place. Astronomers hope to answer these questions using a new 3D reconstruction of the nebula. The team used the powerful SITELLE imaging spectrometer on the Canada-Hawaii-France Telescope (CFHT) in Mauna Kea, Hawaii, to compare the 3D shape of the Crab to two other supernova remnants. Remarkably, they found that all three remnants had ejecta arranged in large-scale rings, suggesting a history of turbulent mixing and radioactive plumes expanding from a collapsed iron core. The Crab is often understood as being the result of an electron-capture supernova triggered by the collapse of an oxygen-neon-magnesium core, but the observed honeycomb structure may not be consistent with this scenario. The new reconstruction was made possible by the ground-breaking technology used by SITELLE, which incorporates a Michelson interferometer design allowing scientists to obtain over 300,000 high-resolution spectra of every single point of the nebula. SITELLE was designed with objects like the Crab Nebula in mind; but its wide field of view and adaptability make it ideal to study nearby galaxies and even clusters of galaxies at large distances.


IDENTITY OF GAMMA-RAY SOURCE REVEALED
University of Manchester

An international research team has shown that a rapidly rotating neutron star is at the core of a celestial object now known as PSR J2039-5617. The collaboration used novel data analysis methods and the enormous computing power of the citizen science project Einstein@Home to track down the neutron star's faint gamma-ray pulsations in data from NASA's Fermi Space Telescope. Their results show that the pulsar is in orbit with a stellar companion about a sixth of the mass of our Sun. The pulsar is slowly but surely evaporating this star. The team also found that the companion's orbit varies slightly and unpredictably over time. Using their search method, they expect to find more such systems with Einstein@Home in the future. Searching for the so-called 'Spider' pulsar systems -- rapidly spinning neutron stars whose high-energy outflows are destroying their binary companion star, required 10 years of precise data. The pulsars have been given arachnid names of 'Black widows' or 'Redbacks', after species of spider where the females have been seen to kill the smaller males after mating. New research details how researchers found a neutron star rotating 377 times a second in an exotic binary system using data from NASA's Fermi Space Telescope. The astronomer's findings were uniquely boosted by the Einstein@Home project, a network of thousands of civilian volunteers lending their home computing power to the efforts of the Fermi Telescope's work. The group's search required combing very finely through the data in order not to miss any possible signals. The computing power required is enormous. The search would have taken 500 years to complete on a single computer core. By using a part of the Einstein@Home resources it was done in 2 months.

The celestial object has been known since 2014 as a source of X-rays, gamma rays, and light. All evidence obtained so far pointed at a rapidly rotating neutron star in orbit with a light-weight star being at the heart of the source. But clear proof was missing. The first step to solving this riddle were new observations of the stellar companion with optical telescopes. They provided precise knowledge about the binary system without which a gamma-ray pulsar search (even with Einstein@Home's huge computing power) would be unfeasible. The system's brightness varies during an orbital period depending on which side of the neutron star's companion is facing the Earth. Additionally, the companion is distorted by the pulsar's gravitational pull causing the apparent size of the star to vary over the orbit. These observations allowed the team to get the most precise measurement possible of the binary star's 5.5-hour orbital period, as well as other properties of the system. With this information and the precise sky position from Gaia data, the team used the aggregated computing power of the distributed volunteer computing project Einstein@Home for a new search of about 10 years of archival observations of NASA's Fermi Gamma-ray Space Telescope. Improving on earlier methods they had developed for this purpose, they enlisted the help of tens of thousands of volunteers to search Fermi data for periodic pulsations in the gamma-ray photons registered by the Large Area Telescope onboard the space telescope. The volunteers donated idle compute cycles on their computers' CPUs and GPUs to Einstein@Home.


EXPLOSION REMAINS FOUND AT MILKY WAY CENTRE
NASA/Marshall Space Flight Center

Astronomers may have found our galaxy's first example of an unusual kind of stellar explosion. This discovery, made with NASA's Chandra X-ray Observatory, adds to the understanding of how some stars shatter and seed the Universe with elements critical for life on Earth. This intriguing object, located near the centre of the Milky Way, is a supernova remnant called Sagittarius A East, or Sgr A East for short. Based on Chandra data, astronomers previously classified the object as the remains of a massive star that exploded as a supernova, one of many kinds of exploded stars that scientists have catalogued. Using longer Chandra observations, a team of astronomers has now instead concluded that the object is left over from a different type of supernova. It is the explosion of a white dwarf, a shrunken stellar ember from a fuel-depleted star like our Sun. When a white dwarf pulls too much material from a companion star or merges with another white dwarf, the white dwarf is destroyed, accompanied by a stunning flash of light. Astronomers use these "Type Ia supernovae" because most of them mete out almost the same amount of light every time no matter where they are located. This allows scientists to use them to accurately measure distances across space and study the expansion of the Universe. Data from Chandra have revealed that Sgr A East, however, did not come from an ordinary Type Ia. Instead, it appears that it belongs to a special group of supernovae that produce different relative amounts of elements than traditional Type Ias do, and less powerful explosions. This subset is referred to as "Type Iax," a potentially important member of the supernova family.

The explosions of white dwarfs is one of the most important sources in the Universe of elements like iron, nickel, and chromium. The only place that scientists know these elements can be created is inside the nuclear furnace of stars or when they explode. Astronomers are still debating the cause of Type Iax supernova explosions, but the leading theory is that they involve thermonuclear reactions that travel much more slowly through the star than in Type Ia supernovae. This relatively slow walk of the blast leads to weaker explosions and, hence, different amounts of elements produced in the explosion. It is also possible that part of the white dwarf is left behind. Sgr A East is located very close to Sagittarius A*, the supermassive black hole in the centre of our Milky Way galaxy, and likely intersects with the disk of material surrounding the black hole. The team was able to use Chandra observations targeting the supermassive black hole and the region around it for a total of about 35 days to study Sgr A East and find the unusual pattern of elements in the X-ray data. The Chandra results agree with computer models predicting a white dwarf that has undergone slow-moving nuclear reactions, making it a strong candidate for a Type Iax supernova remnant. In other galaxies, scientists observe that Type Iax supernovae occur at a rate that is about one third that of Type Ia supernovae. In the Milky Way, there have been three confirmed Type Ia supernova remnants and two candidates that are younger than 2,000 years, corresponding to an age when remnants are still relatively bright before fading later. If Sgr A East is younger than 2,000 years and resulted from a Type Iax supernova, this study suggests that our galaxy is in alignment with respect to the relative numbers of Type Iax supernovae seen in other galaxies. Along with the suggestion that Sgr A East is the remnant from the collapse of a massive star, previous studies have also pointed out that a normal Type Ia supernova had not been ruled out. The latest study conducted with this deep Chandra data argue against both the massive star and the normal Type Ia interpretations.


CONCENTRATION OF SMALL BLACK HOLES
ESA/Hubble Information Centre

Globular clusters are extremely dense stellar systems, in which stars are packed closely together. They are also typically very old -- the globular cluster that is the focus of this study, NGC 6397, is almost as old as the Universe itself. It resides 7800 light-years away, making it one of the closest globular clusters to Earth. Because of its very dense nucleus, it is known as a core-collapsed cluster. When astronomers set out to study the core of NGC 6397, they expected to find evidence for an "intermediate-mass" black hole (IMBH). These are smaller than the supermassive black holes that lie at the cores of large galaxies, but larger than stellar-mass black holes formed by the collapse of massive stars. IMBH are the long-sought "missing link" in black hole evolution and their mere existence is hotly debated, although a few candidates have been found. To look for the IMBH, the team analysed the positions and velocities of the cluster's stars. They did this using previous estimates of the stars' proper motions from Hubble images of the cluster spanning several years, in addition to proper motions provided by ESA's Gaia space observatory, which precisely measures the positions, distances and motions of stars. Knowing the distance to the cluster allowed the astronomers to translate the proper motions of these stars into velocities. Analysis indicated that the orbits of the stars are close to random throughout the globular cluster, rather than systematically circular or very elongated. The team found very strong evidence for invisible mass in the dense central regions of the cluster, but was surprised to find that this extra mass is not point-like but extended to a few percent of the size of the cluster

This invisible component could only be made up of the remnants (white dwarfs, neutron stars, and black holes) of massive stars whose inner regions collapsed under their own gravity once their nuclear fuel was exhausted. The stars progressively sank to the cluster's centre after gravitational interactions with nearby less massive stars, leading to the small extent of the invisible mass concentration. Using the theory of stellar evolution, the scientists concluded that the bulk of the unseen concentration is made of stellar-mass black holes, rather than white dwarfs or neutron stars that are too faint to observe. Two recent studies had also proposed that stellar remnants and in particular, stellar-mass black holes, could populate the inner regions of globular clusters. The study is the first finding to provide both the mass and the extent of what appears to be a collection of mostly black holes in a core-collapsed globular cluster. The astronomers also note that this discovery raises the question of whether mergers of these tightly packed black holes in core-collapsed globular clusters may be an important source of gravitational waves recently detected by the Laser Interferometer Gravitational-Wave Observatory (LIGO) experiment.


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