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Early March Astronomy Bulletin
« on: March 07, 2021, 09:00 »
COMET STOPS NEAR JUPITER’S ASTEROIDS
NASA

After traveling toward the Sun, a young comet-like object orbiting among the giant planets has found a temporary parking place along the way. The object has settled near a family of captured ancient asteroids, called Trojans, that are orbiting the Sun alongside Jupiter. This is the first time a comet-like object has been spotted near the Trojan population. The unexpected visitor belongs to a class of icy bodies found in space between Jupiter and Neptune. Called "Centaurs," they become active for the first time when heated as they approach the Sun, and dynamically transition into becoming more comet-like. Visible-light snapshots by NASA's Hubble Space Telescope reveal that the object shows signs of comet activity, such as a tail, outgassing in the form of jets, and an enshrouding coma of dust and gas. Earlier observations by NASA's Spitzer Space Telescope gave clues to the composition of the comet-like object and the gasses driving its activity. Describing the Centaur's capture as a rare event, astronomers added say that the visitor had to have come into the orbit of Jupiter at just the right trajectory to have this kind of configuration that gives it the appearance of sharing its orbit with the planet. The research team's computer simulations show that the icy object, called P/2019 LD2 (LD2), probably swung close to Jupiter about two years ago.

The nomadic object was discovered in early June 2019 by the University of Hawaii's Asteroid Terrestrial-impact Last Alert System (ATLAS) telescopes located on the extinct volcanoes, one on Mauna Kea and one on Haleakala. Although LD2's location is surprising, this could be a common pull-off for some sunward-bound comets. "he unexpected guest probably will not stay among the asteroids for very long. Computer simulations show that it will have another close encounter with Jupiter in about another two years. The hefty planet will boot the comet from the system, and it will continue its journey to the inner solar system. The icy interloper is most likely one of the latest members of the so-called "bucket brigade" of comets to get kicked out of its frigid home in the Kuiper belt and into the giant planet region through interactions with another Kuiper belt object. Located beyond Neptune's orbit, the Kuiper belt is a haven of icy, leftover debris from our planets' construction 4.6 billion years ago, containing millions of objects, and occasionally these objects have near misses or collisions that drastically alter their orbits from the Kuiper belt inward into the giant planet region. The bucket brigade of icy relics endure a bumpy ride during their journey sunward. They bounce gravitationally from one outer planet to the next in a game of celestial pinball before reaching the inner solar system, warming up as they come closer to the Sun. The researchers say the objects spend as much or even more time around the giant planets, gravitationally pulling on them--about 5 million years--than they do crossing into the inner system where we live.


MILKY WAY MAY BE SWARMING WITH EARTH-LIKE PLANETS
University of Copenhagen

Astronomers have long been looking into the Universe in hopes of discovering alien civilisations. But for a planet to have life, liquid water must be present. The chances of finding that scenario have seemed impossible to calculate because it has been the assumption that planets like Earth get their water by chance if a large, ice asteroid hits the planet. Now, researchers have published an eye-opening study, indicating that water may be present during the very formation of a planet. According to the study's calculations, this is true for Earth, Venus and Mars. Using a computer model, astronomers have calculated how quickly planets are formed, and from which building blocks. The study indicates that it was millimetre-sized dust particles of ice and carbon -- which are known to orbit around all young stars in the Milky Way -- that 4.5 billion years ago accreted in the formation of what would later become Earth. Up to the point where Earth had grown to one percent of its current mass, our planet grew by capturing masses of pebbles filled with ice and carbon. Earth then grew faster and faster until, after five million years, it became as large as we know it today. Along the way, the temperature on the surface rose sharply, causing the ice in the pebbles to evaporate on the way down to the surface so that, today, only 0.1 percent of the planet is made up of water, even though 70 percent of Earth's surface is covered by water. The theory, called 'pebble accretion', is that planets are formed by pebbles that are clumping together, and that the planets then grow larger and larger. Water molecules H2O are found everywhere in our galaxy, and the theory therefore opens up the possibility that other planets may have been formed in the same way as Earth, Mars and Venus. If planets in our galaxy had the same building blocks and the same temperature conditions as Earth, there will also be good chances that they may have about the same amount of water and continents as our planet. If, on the other hand, it was random how much water was present on planets, the planets might look vastly different. Some planets would be too dry to develop life, while others would be completely covered by water.


MANY BINARY STARS IN LOCAL AREA
University of California - Berkeley

The latest star data from the Gaia space observatory has for the first time allowed astronomers to generate a massive 3D atlas of widely separated binary stars within about 3,000 light years of Earth -- 1.3 million of them. The atlas should be a boon for those who study binary stars -- which make up at least half of all sunlike stars -- and white dwarfs, exoplanets and stellar evolution, in general. Before Gaia, the last compilation of nearby binary stars, assembled using data from the now-defunct Hipparcos satellite, included about 200 likely pairs. This is a much bigger census that included 17,000 white dwarfs alone. White dwarfs are the end stages of most stars; the Sun will likely end up as a compact white dwarf in 5 billion years. The atlas includes 1,400 systems that consist of two white dwarfs and 16,000 binaries that consist of a white dwarf and another type of star. The vast majority of the 2.6 million individual stars are still in the prime of life, however. Astronomers refer to them as main sequence stars, because they cluster along a line when plotted on a graph showing temperature versus brightness. The team plans to focus in the future on the white dwarf binaries, because white dwarfs can be assigned an age more precisely than is possible with regular stars. Main sequence stars like the Sun can look the same for billions, or even tens of billions, of years, while white dwarfs change -- for one thing, they cool down at a well-defined rate. And since binary pairs are birthed at the same time, the age of the white dwarf tells astronomers the age of its main-sequence twin, or of any planets around the stars.

For a white dwarf, in general, it is easy to tell how old it is -- not just how old since it became a white dwarf, but what its total age is. You can also measure their masses, because white dwarfs have a well-understood mass-radius relation. As an example, the team recently used the Gaia data to estimate the age of a Jupiter-sized gas giant discovered by the TESS satellite around a white dwarf-K dwarf pair. That exoplanet, TOI-1259Ab, turned out to be about 4 billion years old, based on the age of the white dwarf. In this catalogue, there are something like 15 systems like this: star plus planet plus white dwarf, and there are another few hundred that are star plus planet plus another star. Those are also potentially interesting because, in some cases, the other star will do something dynamically to the planet. Until Gaia was launched by the European Space Agency in 2013 to precisely measure the distances and motions of millions of nearby stars, the only way to find binaries was to look for stars close together in the sky. This can be tricky, because stars that look very close from Earth could be hundreds to thousands of light-years from one another, merely sitting along the same line of site. Ruling out a chance alignment requires lots of observing time to confirm that the two candidates are actually at the same distance and moving together. Because of Earth's motion around the Sun, nearby stars appear to change position in the sky, and that parallax can be used to calculate how far away they are. The star's motion across the sky, known as proper motion, helps determine its velocity. Gaia conducts this tedious astrometry continuously for all nearby stars in the sky, 24/7, from its orbit at the Earth-Sun Lagrange point. The space telescope's survey is most useful for stars within about 3,000 light years of Earth, however, because beyond that, the parallax is usually too small to measure.


FIRST BLACK HOLE DETECTED IS MORE MASSIVE THAN PREVIOUSLY THOUGHT
International Centre for Radio Astronomy Research

New observations of the first black hole ever detected have led astronomers to question what they know about the Universe's most mysterious objects. The research shows the system known as Cygnus X-1 contains the most massive stellar-mass black hole ever detected without the use of gravitational waves. Cygnus X-1 is one of the closest black holes to Earth. It was discovered in 1964 when a pair of Geiger counters were carried on board a sub-orbital rocket launched from New Mexico. The object was the focus of a famous scientific wager between physicists Stephen Hawking and Kip Thorne, with Hawking betting in 1974 that it was not a black hole. Hawking conceded the bet in 1990. In this latest work, an international team of astronomers used the Very Long Baseline Array -- a continent-sized radio telescope made up of 10 dishes spread across the United States -- together with a clever technique to measure distances in space. Over six days the team observed a full orbit of the black hole and used observations taken of the same system with the same telescope array in 2011. This method and the new measurements show the system is further away than previously thought, with a black hole that's significantly more massive. The black hole in the Cygnus X-1 system began life as a star approximately 60 times the mass of the Sun and collapsed tens of thousands of years ago. Incredibly, it's orbiting its companion star -- a supergiant -- every five and a half days at just one-fifth of the distance between the Earth and the Sun. These new observations tell us the black hole is more than 20 times the mass of our Sun -- a 50 per cent increase over previous estimates. Using the updated measurements for the black hole's mass and its distance away from Earth, the team was able to confirm that Cygnus X-1 is spinning incredibly quickly -- very close to the speed of light and faster than any other black hole found to date.


NEUTRON STAR IN SUPERNOVA 1987A
NASA

What remains of the star that exploded just outside our galaxy in 1987?   Debris has obscured scientists’ view, but two of NASA’s X-ray telescopes have revealed new clues. Since astronomers captured the bright explosion of a star on Feb. 24, 1987, researchers have been searching for the squashed stellar core that should have been left behind. A group of astronomers using data from NASA space missions and ground-based telescopes may have finally found it. As the first supernova visible to the naked eye in about 400 years, Supernova 1987A (or SN 1987A for short) sparked great excitement among scientists and soon became one of the most studied objects in the sky. The supernova is located in the Large Magellanic Cloud, a small companion galaxy to our own Milky Way, only about 170,000 light-years from Earth. While astronomers watched debris explode outward from the site of the detonation, they also looked for what should have remained of the star’s core: a neutron star. Data from NASA’s Chandra X-ray Observatory and previously unpublished data from NASA’s Nuclear Spectroscopic Telescope Array (NuSTAR), in combination with data from the ground-based Atacama Large Millimeter Array (ALMA) reported last year, now present an intriguing collection of evidence for the presence of the neutron star at the centre of SN 1987A. When a star explodes, it collapses onto itself before the outer layers are blasted into space. The compression of the core turns it into an extraordinarily dense object, with the mass of the Sun squeezed into an object only about 10 miles across. These objects have been dubbed neutron stars, because they are made nearly exclusively of densely packed neutrons. They are laboratories of extreme physics that cannot be duplicated here on Earth. Rapidly rotating and highly magnetized neutron stars, called pulsars, produce a lighthouse-like beam of radiation that astronomers detect as pulses when its rotation sweeps the beam across the sky. There is a subset of pulsars that produce winds from their surfaces – sometimes at nearly the speed of light – that create intricate structures of charged particles and magnetic fields known as “pulsar wind nebulae.” With Chandra and NuSTAR, the team found relatively low-energy X-rays from SN 1987A’s debris crashing into surrounding material. The team also found evidence of high-energy particles using NuSTAR’s ability to detect more energetic X-rays.

There are two likely explanations for this energetic X-ray emission: either a pulsar wind nebula, or particles being accelerated to high energies by the blast wave of the explosion. The latter effect doesn’t require the presence of a pulsar and occurs over much larger distances from the centre of the explosion. The latest X-ray study supports the case for the pulsar wind nebula – meaning the neutron star must be there – by arguing on a couple of fronts against the scenario of blast wave acceleration. First, the brightness of the higher-energy X-rays remained about the same between 2012 and 2014, while the radio emission detected with the Australia Telescope Compact Array increased. This goes against expectations for the blast wave scenario. Next, authors estimate it would take almost 400 years to accelerate the electrons up to the highest energies seen in the NuSTAR data, which is over 10 times older than the age of the remnant. Astronomers have wondered if not enough time has passed for a pulsar to form, or even if SN 1987A created a black hole. This is more evidence supporting the idea that there is a neutron star left behind. If this is indeed a pulsar at the centre of SN 1987A, it would be the youngest one ever found. Being able to watch a pulsar essentially since its birth would be unprecedented. The authors used state-of-the-art simulations to understand how this material would absorb X-rays at different energies, enabling more accurate interpretation of the X-ray spectrum – that is, the amount of X-rays at different energies. This enables them to estimate what the spectrum of the central regions of SN 1987A is without the obscuring material. As is often the case, more data are needed to strengthen the case for the pulsar wind nebula. An increase in radio waves accompanied by an increase in relatively high-energy X-rays in future observations would argue against this idea. On the other hand, if astronomers observe a decrease in the high-energy X-rays, then the presence of a pulsar wind nebula will be corroborated. The stellar debris surrounding the pulsar plays an important role by heavily absorbing its lower-energy X-ray emission, making it undetectable at the present time. The model predicts that this material will disperse over the next few years, which will reduce its absorbing power. Thus, the pulsar emission is expected to emerge in about 10 years, revealing the existence of the neutron star. .


SUPERMASSIVE BLACK HOLES COULD FORM FROM DARK MATTER
RAS

A new theoretical study has proposed a novel mechanism for the creation of supermassive black holes from dark matter. The international team find that rather than the conventional formation scenarios involving ‘normal’ matter, supermassive black holes could instead form directly from dark matter in high density regions in the centres of galaxies. Exactly how supermassive black holes initially formed is one of the biggest problems in the study of galaxy evolution today. Supermassive black holes have been observed as early as 800 million years after the Big Bang, and how they could grow so quickly remains unexplained. Standard formation models involve normal baryonic matter – the atoms and elements that that make up stars, planets, and all visible objects – collapsing under gravity to form black holes, which then grow over time. However the new work investigates the potential existence of stable galactic cores made of dark matter, and surrounded by a diluted dark matter halo, finding that the centres of these structures could become so concentrated that they could also collapse into supermassive black holes once a critical threshold is reached. According to the model this could have happened much more quickly than other proposed formation mechanisms, and would have allowed supermassive black holes in the early Universe to form before the galaxies they inhabit, contrary to current understanding.

Another intriguing consequence of the new model is that the critical mass for collapse into a black hole might not be reached for smaller dark matter halos, for example those surrounding some dwarf galaxies. The authors suggest that this then might leave smaller dwarf galaxies with a central dark matter nucleus rather than the expected black hole. Such a dark matter core could still mimic the gravitational signatures of a conventional central black hole, whilst the dark matter outer halo could also explain the observed galaxy rotation curves. This model shows how dark matter haloes could harbour dense concentrations at their centres, which may play a crucial role in helping to understand the formation of supermassive black holes.


HIGH ENERGY NEUTRINO DETECTED AS STAR CONSUMED BY BLACK HOLE
New York University

A team of scientists has detected the presence of a high-energy neutrino -- a particularly elusive particle -- in the wake of a star's destruction as it is consumed by a black hole. Neutrinos -- as well as the process of their creation -- are hard to detect, making their discovery, along with that of Ultrahigh Energy Cosmic Rays (UHECRs), noteworthy. The origin of cosmic high-energy neutrinos is unknown, primarily because they are notoriously hard to pin down. This result would be only the second time high-energy neutrinos have been traced back to their source. Previous research found some of the earliest evidence of black holes destroying stars in what are now known as Tidal Disruption Events (TDEs). These findings set the stage for determining if TDEs could be responsible for producing UHECRs. Previously, the IceCube Neutrino Observatory located in the South Pole, reported the detection of a neutrino, whose path was later traced by the Zwicky Transient Facility at Caltech's Palomar Observatory. Specifically, its measurements showed a spatial coincidence of a high-energy neutrino and light emitted after a TDE -- a star consumed by a black hole. This suggests these star shredding events are powerful enough to accelerate high-energy particles," van Velzen explains. "Discovering neutrinos associated with TDEs is a breakthrough in understanding the origin of the high-energy astrophysical neutrinos identified by the IceCube detector at the South Pole whose sources have so far been elusive," adds Farrar, who proposed in a 2009 paper that UHECRs could be accelerated in TDEs. The neutrino-TDE coincidence also sheds light on a decades old problem: the origin of Ultrahigh Energy Cosmic Rays.


LARGEST GALAXY CLUSTER IN EARLY UNIVERSE
Instituto de Astrofísica de Canarias (IAC)

A study has found the most densely populated galaxy cluster in formation in the primitive Universe. The researchers predict that this structure, which is at a distance of 12.5 billion light years from us, will have evolved into a cluster similar to that of Virgo, a neighbour of the Local Group of galaxies to which the Milky Way belongs. Clusters of galaxies are groups of galaxies which remain together because of the action of gravity. To understand the evolution of these "cities of galaxies" scientists look for structures in formation, the so-called galaxy protoclusters, in the early Universe. In 2012 an international team of astronomers made an accurate determination of the distance of the galaxy HDF850.1, known as one of the galaxies with the highest rate of star formation in the observable Universe. To their surprise, the scientists also discovered that this galaxy, which is one of the most studied regions on the sky, known as the Hubble Deep Field/GOODS-North, is part of a group of around a dozen protogalaxies which had formed during the first thousand million years of cosmic history. Before its discovery only one other similar primordial group was known. Now, thanks to a new piece of research with the OSIRIS instrument on the Gran Telescopio Canarias (GTC, or GRANTECAN), the team has shown that it is one of the most densely populated regions populated with galaxies in the primitive Universe, and have for the first time carried out a detailed study of the physical properties of this system. Surprisingly, all the members of the cluster studied up to now, around two dozen, are galaxies with normal star formation, and that the central galaxy appears to dominate the production of stars in this structure. This recent study shows that this cluster of galaxies in formation is made up of various components, or "zones" with differences in their evolution. The astronomers predict that this structure will change gradually until it becomes a galaxy cluster similar to Virgo, the central region of the supercluster of the same name in which is situated the Local Group of galaxies to which the Milky Way belongs.


NEW LIGHT ON HEAVY ELEMENTS
Michigan State University Facility for Rare Isotope Beams

Researchers have gained new insights into the cosmic origin of the heaviest elements on the periodic table from the formation of the solar system 4.6 billion years ago. Heavy elements we encounter in our everyday life, like iron and silver, did not exist at the beginning of the Universe, 13.7 billion years ago. They were created in time through nuclear reactions called nucleosynthesis that combined atoms together. In particular, iodine, gold, platinum, uranium, plutonium, and curium, some of the heaviest elements, were created by a specific type of nucleosynthesis called the rapid neutron capture process, or r process. The question of which astronomical events can produce the heaviest elements has been a mystery for decades. Today, it is thought that the r process can occur during violent collisions between two neutron stars, between a neutron star and a black hole, or during rare explosions following the death of massive stars. Such highly energetic events occur very rarely in the Universe. When they do, neutrons are incorporated in the nucleus of atoms, then converted into protons. Since elements in the periodic table are defined by the number of protons in their nucleus, the r process builds up heavier nuclei as more neutrons are captured. Some of the nuclei produced by the r process are radioactive and take millions of years to decay into stable nuclei. Iodine-129 and curium-247 are two of such nuclei that were produced before the formation of the Sun. They were incorporated into solids that eventually fell on the Earth's surface as meteorites. Inside these meteorites, the radioactive decay generated an excess of stable nuclei. Today, this excess can be measured in laboratories in order to figure out the amount of iodine-129 and curium-247 that were present in the solar system just before its formation. Why are these two r-process nuclei so special? They have a peculiar property in common: they decay at almost exactly the same rate. In other words, the ratio between iodine-129 and curium-247 has not changed since their creation, billions of years ago. This is an amazing coincidence, particularly given that these nuclei are two of only five radioactive r-process nuclei that can be measured in meteorites. With the iodine-129 to curium-247 ratio being frozen in time, like a prehistoric fossil, we can have a direct look into the last wave of heavy element production that built up the composition of the solar system, and everything within it.

Iodine, with its 53 protons, is more easily created than curium with its 96 protons. This is because it takes more neutron capture reactions to reach curium's higher number of protons. As a consequence, the iodine-129 to curium-247 ratio highly depends on the amount of neutrons that were available during their creation. The team calculated the iodine-129 to curium-247 ratios synthesized by collisions between neutron stars and black holes to find the right set of conditions that reproduce the composition of meteorites. They concluded that the amount of neutrons available during the last r-process event before the birth of the solar system could not be too high. Otherwise, too much curium would have been created relative to iodine. This implies that very neutron-rich sources, such as the matter ripped off the surface of a neutron star during a collision, likely did not play an important role. So what created these r-process nuclei? While the researchers could provide new and insightful information regarding how they were made, they could not pin down the nature of the astronomical object that created them. This is because nucleosynthesis models are based on uncertain nuclear properties, and it is still unclear how to link neutron availability to specific astronomical objects such as massive star explosions and colliding neutron stars. But the ability of the iodine-129 to curium-247 ratio to peer more directly into the fundamental nature of heavy element nucleosynthesis is an exciting prospect for the future. With this new diagnostic tool, advances in the fidelity of astrophysical simulations and in the understanding of nuclear properties could reveal which astronomical objects created the heaviest elements of the solar system. 


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