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February 2, 2019 at 9:52 pm #9752Anonymous
THE SOCIETY FOR POPULAR ASTRONOMY Electronic News Bulletin No. 485 2019 January 27
Here is the latest round-up of news from the Society for Popular
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A COLLISION IN THE ASTEROID BELT
Astronomers working with the ATLAS project in Hawaii have announced an astonishing change to asteroid 6478 Gault. The space rock had sprouted a tail. It is now gliding through the asteroid belt giving every appearance of being a comet. What happened to 6478 Gault? A clue may be found in its lineage. Asteroid Gault is a member of the Phocaea family, a swarm of rocks in the inner asteroid belt that formed as a result of inter-asteroid collisions some 2.2 billion years ago. The family gets its name from its most massive survivor, 25 Phocaea, which is about 75 km in diameter. Gault's tail may be a result of a recent collision. Researchers with the ATLAS project have looked at images of Gault in Dec. 2018 and Jan. 2019. Extrapolating its appearance backwards in time, they suggest that Gault hit another object in the asteroid belt in Nov. 2018. If that idea is correct, the tail would be debris from the crash. Asteroid Gault was discovered in 1988 by the famous astronomer couple Carolyn and Eugene Shoemaker at the Palomar Observatory in California. For the next 30 years, the 4-km wide space rock did little to attract attention. Now, astronomers around the world are eagerly monitoring 6478 Gault to see what happens next.
RAIN ON TITAN’S NORTH POLE
American Geophysical Union
Scientists have identified a reflective feature near Titan's north pole on an image taken June 7, 2016, by Cassini's near-infrared instrument, the Visual and Infrared Mapping Spectrometer. The reflective feature covered approximately 46,332 square miles, roughly half the size of the Great Lakes, and did not appear on images from previous and subsequent Cassini passes. Analyses of the short-term reflective feature suggested it likely resulted from sunlight reflecting off a wet surface. The study attributes the reflection to a methane rainfall event, followed by a probable period of evaporation. This reflective surface represents the first observations of summer rainfall on the moon's northern hemisphere. If compared to Earth's yearly cycle of four seasons, a season on Titan lasts seven Earth years. Cassini arrived at Titan during the southern summer and observed clouds and rainfall in the southern hemisphere. Climate models of Titan predicted similar weather would occur in the northern hemisphere in the years leading up to the northern summer solstice in 2017. But, by 2016, the expected cloud cover in the northern hemisphere had not appeared. This observation may help scientists gain a more complete understanding of Titan's seasons. Additional analyses suggest the methane rain fell across a relatively pebble-like surface. A rougher surface generates an amorphous pattern as the liquid settles in crevasses and gullies, while liquid falling on a smooth surface would puddle in a relatively circular pattern.
DOUBLE STAR SYSTEM FLIPS PLANETARY DISC
University of Warwick
Astronomers have found the first confirmed example of a double star system that has flipped its surrounding disc to a position that leaps over the orbital plane of those stars. The international team used the Atacama Large Millimeter/sub-millimeter Array (ALMA) to obtain high-resolution images of the Asteroid belt-sized disc. The overall system presents the unusual sight of a thick hoop of gas and dust circling at right angles to the binary star orbit. Until now this setup only existed in theorists' minds, but the ALMA observation proves that polar discs of this type exist, and may even be relatively common. Discs rich in gas and dust are seen around nearly all young stars, and we know that at least a third of the ones orbiting single stars form planets. Some of these planets end up being misaligned with the spin of the star, so astronomers have been wondering whether a similar thing might be possible for circumbinary planets. A quirk of the dynamics means that a so-called polar misalignment should be possible, but until now there was no evidence of misaligned discs in which these planets might form. The fellow researchers used ALMA to pin down the orientation of the ring of gas and dust in the system. The orbit of the binary was previously known, from observations that quantified how the stars move in relation to each other. By combining these two pieces of information they were able to establish that the dust ring was consistent with a perfectly polar orbit. This means that while the stellar orbits orbit each other in one plane, like two horses going around on a carousel, the disc surrounds these stars at right angles to their orbits, like a giant ferris wheel with the carousel at the centre.
If there were a planet or planetoid present at the inner edge of the dust ring, the ring itself would appear from the surface as a broad band rising almost perpendicularly from the horizon. The polar configuration means that the stars would appear to move in and out of the disc plane, giving objects two shadows at times. Seasons on planets in such systems would also be different. On Earth they vary throughout the year as we orbit the Sun. A polar circumbinary planet would have seasons that also vary as different latitudes receive more or less illumination throughout the binary orbit. Astronomers used to think other solar systems would form just like ours, with the planets all orbiting in the same direction around a single sun. But with the new images we see a swirling disc of gas and dust orbiting around two stars. It was quite surprising to also find that that disc orbits at right angles to the orbit of the two stars. Incredibly, two more stars were seen orbiting that disc. So if planets were born here there would be four suns in the sky!
NEW WAY SUPERMASSIVE BLACK HOLES ARE FED
American Friends of Tel Aviv University
Supermassive black holes weigh millions to billions times more than our Sun and lie at the centre of most galaxies. A supermassive black hole several million times the mass of the Sun is situated in the heart of our very own Milky Way. Despite how commonplace supermassive black holes are, it remains unclear how they grow to such enormous proportions. Some black holes constantly swallow gas in their surroundings, some suddenly swallow whole stars. But neither theory independently explains how supermassive black holes can “switch on” so unexpectedly and keep growing so fast for a long period. In February 2017, the All Sky Automated Survey for Supernovae discovered an event known as AT 2017bgt. This event was initially believed to be a “star swallowing” event, or a “tidal disruption” event, because the radiation emitted around the black hole grew more than 50 times brighter than what had been observed in 2004. However, after extensive observations using a multitude of telescopes, a team of concluded that AT 2017bgt represented a new way of “feeding” black holes. The sudden brightening of AT 2017bgt was reminiscent of a tidal disruption event. But astronomers quickly realized that this time there was something unusual. The first clue was an additional component of light, which had never been seen in tidal disruption events. The team followed this event for more than a year with telescopes on Earth and in space, and what it saw did not match anything ever seen before. Astronomers had predicted back in the 1980s that a black hole swallowing gas from its surroundings could produce the elements of light seen here. This new result is the first time the process was seen in practice. Astronomers from the U.S., Chile, Poland and the U.K. took part in the observations and analysis effort, which used three different space telescopes, including the new NICER telescope installed on board the International Space Station. One of the ultraviolet images obtained during the data acquisition frenzy turned out to be the millionth image taken by the Neil Gehrels Swift Observatory — an event celebrated by NASA, which operates this space mission.
The research team identified two additional recently reported events of black holes “switched on,” which share the same emission properties as AT 2017bgt. These three events form a new and tantalizing class of black hole re-activation. Astronomers are not yet sure about the cause of this dramatic and sudden enhancement in the black holes' feeding rate as there are many known ways to speed up the growth of giant black holes, but they typically happen during much longer timescales. The team hopes to detect many more such events, and to follow them with several telescopes working in tandem. This is the only way to complete our picture of black hole growth, to understand what speeds it up, and perhaps finally solve the mystery of how these giant monsters form.
EARLIEST MOMENTS OF A DYING STAR
University of Leicester
Astronomers have found evidence for the existence of a 'hot cocoon' of material enveloping a relativistic jet escaping a dying star. A relativistic jet is a very powerful phenomena which involves plasma jets shooting out of black holes at close to the speed of light, and can extend across millions of light years. Observations of supernova SN2017iuk taken shortly after its onset showed it expanding rapidly, at one third of the speed of light. This is the fastest supernova expansion measured to date. Monitoring of the outflow over many weeks revealed a clear difference between the initial chemical composition and that at later times. Taken together, these are indicators of the presence of the much theorised hot cocoon, filling a gap in our knowledge of how a jet of material escaping a star interacts with the stellar envelope around it and providing a potential link between two previously distinct classes of supernovae. The supernova signals the final demise of a massive star, in which the stellar core collapses and the outer layers are violently blown off. SN2017iuk belongs to a class of extreme supernovae, sometimes called hypernovae or GRB-SNe, that accompany a yet more dramatic event known as a gamma-ray burst (GRB). At stellar death, a highly relativistic, narrow beam of material can be ejected from the poles of the star which glows brightly first in gamma radiation and then across the entire electromagnetic spectrum and is known as a GRB.
Until now, astronomers have been unable to study the earliest moments in the development of a supernova of this kind (a GRB-SN), but SN2017iuk was fortuitously close-by — at roughly 500 million light years from Earth — and the GRB light was underluminous, allowing the SN itself to be detectable at early times. When the first sets of data came in there was an unusual component to the light that looked very blue, prompting a monitoring campaign to see if its origin could be determined by following the evolution and taking detailed spectra. The gamma-ray burst itself looked quite weak, so astronomers could see other processes that were going on around the newly-formed jet which are normally drowned out. The idea of a cocoon of thermalised gas created by the relativistic jet as it drills out of the star had been proposed and implied in other cases, but here was the evidence that was needed to pin down the existence of such a structure. A coordinated approach using a suite of space- and ground-based observatories was required to monitor the supernova over 30 days and at many wavelengths. The event was first detected using the Neil Gehrels Swift Observatory. Swift is a NASA space mission in which the University of Leicester is one of three partners, and hosts its UK data centre. Data obtained with the Gravitational-wave Optical Transient Observatory (GOTO) helped to track the supernova light, while spectroscopy was obtained through dedicated observing programmes including initiatives by the STARGATE Collaboration. The relativistic jet punches out through the star as if it was a bullet being fired out from the inside of an apple. What has been seen for the first time is all the debris that explodes out after the bullet.
Speeds of up to 115,000 kilometres per second were measured for the expanding supernova for approximately one hour after its onset. A different chemical composition was found for the early expanding supernova when compared with the more iron-rich later ejecta. The team concluded that just hours after the onset the ejecta is coming from the interior, from a hot cocoon created by the jet. Existing supernova production models proved insufficient to account for the large amount of high velocity material measured. The team developed new models which incorporated the cocoon component and found these were an excellent match. SN2017iuk also provides a long-sought link between the supernova that accompany GRBs, and those that do not: in lone supernovae, high speed outflows have also been seen, with velocities reaching 50,000 kilometres per second, which can originate in the same cocoon scenario but escape of the relativistic GRB jet is somehow thwarted. Core-collapse supernovae without GRBs are usually found much later after their onset, giving scientists very little chance of detecting any signatures of a hot cocoon, whilst cocoon features in GRB-associated supernovae are usually hidden by the bright, relativistic jet. The rare case of SN2017iuk has opened a window onto the earliest stages of this type of supernova phenomenon, allowing the elusive cocoon structure to be observed.
RUSSIA’S SPACE TELESCOPE FAILURE
Russia's only space radio telescope is no longer responding to commands from Earth. Some of the Spektr-R satellite's communication systems had stopped working but it was still transmitting scientific data according to the RIA Novosti news agency. The telescope has been operational way beyond its expected five-year lifespan, Specialists had repeatedly tried and failed to fix the lost connection. The head of research for the Spektr-R project, said the link went down on the morning of 11 January, but added that “there is still hope”.
CHINA’S MOON PLANTS ARE DEAD
Just 24 hours after releasing photographs of the first ever plants grown on the moon, China revealed the tender green shoots were all dead. The cotton plants had been the only seeds to sprout inside their aluminium container, known as a “Moon surface micro-ecological circle”, which cost more than £1.15m. But the probe is now in lunar night, and temperatures have fallen too low for life to survive. The Chang’e-4 probe, which landed on the far-side of the Moon on 3 January, apparently entered “sleep mode” as the first lunar night after the probe’s landing fell. It is not clear why, if the Chinese space agency knew the falling of the lunar night would kill the plants. Other reports have suggested the pod was supposed to last three months and create a self-sustaining environment for life away from our planet. The moon lander was carrying soil, cotton seeds, rock cress, rapeseed and potato seeds, yeast and fruit fly eggs. China said the rapeseed and potato seeds had also germinated, but the cotton seeds were first to sprout. It also said the 3kg aluminium container was designed to maintain a temperature of between 1 and 30 degrees, allow in natural light and feed the plants with water and a nutrient solution.
Bulletin compiled by Clive Down
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