Topic: Late May Astronomy Bulletin

[Clive]

GIANT METEORITE IMPACTS FORMED PARTS OF MOON’S CRUST
Royal Ontario Museum

Scientists have discovered that the formation of ancient rocks on the Moon may be directly linked to large-scale meteorite impacts. The scientists conducted new research of a unique rock collected by NASA astronauts during the 1972 Apollo 17 mission to the Moon. They found it contains mineralogical evidence that it formed at incredibly high temperatures (in excess of 2300 °C that can only be achieved by the melting of the outer layer of a planet in a large impact event. In the rock, the researchers discovered the former presence of cubic zirconia, a mineral phase often used as a substitute for diamond in jewellery. The phase would only form in rocks heated to above 2300 °C, and though it has since reverted to a more stable phase (the mineral known as baddeleyite), the crystal retains distinctive evidence of a high-temperature structure. While looking at the structure of the crystal, the
researchers also measured the age of the grain, which reveals the baddeleyite formed over 4.3 billion years ago. It was concluded that the high-temperature cubic zirconia phase must have formed before this time, suggesting that large impacts were critically important to forming new rocks on the early Moon. Fifty years ago, when the first samples were brought back from the surface of the Moon, lunar scientists raised questions about how lunar crustal rocks formed. Even today, a key question remains unanswered: how did the outer and inner layers of the Moon mix after the Moon formed? This new research suggests that large impacts over 4 billion years ago could have driven this mixing, producing the complex range of rocks seen on the surface of the Moon today. Rocks on Earth are constantly being recycled, but the Moon doesn't exhibit plate tectonics or volcanism, allowing older rocks to be preserved. By studying the Moon, we can better understand the earliest history of our planet. If large, super-heated impacts were creating rocks on the Moon, the same process was probably happening here on Earth.


PLUTO’S ATMOSPHERIC PRESSURE FALLS .
Southwest Research Institute

Pluto’s atmosphere is hard to observe from Earth. It can only be studied when Pluto passes in front of a distant star, allowing astronomers to see the effect the atmosphere has on starlight. When this happened in 2016, it confirmed that Pluto’s atmosphere was growing, a trend that astronomers had observed since 1988, when they noticed it for the first time. Now, all that has changed — Pluto’s atmosphere appears to have collapsed. The most recent occultation in July last year shows the
atmospheric pressure seems to have dropped by over 20 percent since 2016.  Astronomers have long known that Pluto’s atmosphere expands as it approaches the Sun and contracts as it recedes. When the Sun heats its icy surface, it sublimates, releasing nitrogen, methane and carbon dioxide into the atmosphere.  When it moves away, the atmosphere is thought to freeze and fall out of the sky in what must be one of the most spectacular ice storms in the solar system. Pluto reached its point of closest approach to the Sun in 1989, and has since been moving away. But its atmosphere has continued to increase to a level that is about 1/100,000 of Earth’s. Astronomers think they know why, thanks to the images sent back by the New Horizons spacecraft that flew past Pluto in 2015. These images revealed an unexpectedly complex surface with widely varying colours. A mysterious reddish cap at the north pole turned out to be coloured by organic molecules. And a large, white, ice-covered basin called Sputnik Planitia stretched across a large part of one hemisphere. Planetary geologists think Sputnik Planitia plays an important role in regulating Pluto’s atmosphere. That’s because, when it faces the Sun, it releases gas into the atmosphere. Simulations suggest that this is why Pluto’s atmosphere has continued to grow, even as it has begun to move away from the Sun.

The simulations are complicated by Sputnik Planitia’s colour, which determines the amount of light it absorbs, and this in turn is influenced by ice formation in ways that are hard to predict. Nevertheless, these same simulations suggest that, since 2015, Sputnik Planitia should have begun to cool, causing the atmosphere to condense into ice. However, the models suggest that Pluto’s atmosphere ought to have shrunk by less than 1 percent since 2016, not the 20 percent observed by the Japanese team. So there may be some other factor at work that is accelerating Pluto’s atmospheric collapse. The result must also be treated with caution. The effect of Pluto’s atmosphere on distant starlight is small and hard to observe with the 60 centimetre reflecting telescope that the team used. They say the various sources of error in their measurement make it only marginally significant. Better observations from larger telescopes are desperately needed. But this is unlikely to happen anytime soon. As well as moving away from the sun, Pluto is moving out of the galactic plane, making stellar occultations much rarer and with less bright stars.  That means the chances to make better observations in the future will be few and far between. The team concludes with a plea for astronomers to observe Pluto with bigger, more sensitive telescopes, preferably those with diameters measured in metres. Until then, Pluto’s vanishing atmosphere will remain something of a mystery.


BREAKTHROUGH STUDY OF STELLAR PULSATIONS
NASA/Goddard Space Flight Center

Astronomers have detected elusive pulsation patterns in dozens of young, rapidly rotating stars thanks to data from NASA's Transiting Exoplanet Survey Satellite (TESS). The discovery will revolutionize scientists' ability to study details like the ages, sizes and compositions of these stars -- all members of a class named for the prototype, the bright star Delta Scuti. Delta Scuti stars clearly pulsate in interesting ways, but the patterns of those pulsations have so far defied understanding. To use a musical analogy, many stars pulsate along simple chords, but Delta Scuti stars are complex, with notes that seem to be jumbled. TESS has shown us that's not true for all of them. Geologists studying seismic waves from earthquakes figured out Earth's internal structure from the way the reverberations changed speed and direction as they travelled through it. Astronomers apply the same principle to study the interiors of stars through their pulsations, a field called asteroseismology. Sound waves travel through a star's interior at speeds that change with depth, and they all combine into pulsation patterns at the star's surface.  Astronomers can detect these patterns as tiny fluctuations in brightness and use them to determine the star's age, temperature, composition, internal structure and other properties. Delta Scuti stars are between 1.5 and 2.5 times the Sun's mass.  They're named after Delta Scuti, a star visible to the human eye in the southern constellation Scutum that was first identified as variable in 1900. Since then, astronomers have identified thousands more like Delta Scuti, many with NASA's Kepler space telescope, another planet-hunting mission that operated from 2009 to 2018. But scientists have had trouble interpreting Delta Scuti pulsations. These stars generally rotate once or twice a day, at least a dozen times faster than the Sun.  The rapid rotation flattens the stars at their poles and jumbles the pulsation patterns, making them more complicated and difficult to decipher. To determine if order exists in Delta Scuti stars' apparently chaotic pulsations, astronomers needed to observe a large set of stars multiple times with rapid sampling. TESS monitors large swaths of the sky for 27 days at a time, taking one full image every 30 minutes with each of its four cameras. This observing strategy allows TESS to track changes in stellar brightness caused by planets passing in front of their stars, which is its primary mission, but half-hour exposures are too long to catch the patterns of the more rapidly pulsating Delta Scuti stars. Those changes can happen in minutes.

But TESS also captures snapshots of a few thousand pre-selected stars -- including some Delta Scuti stars -- every two minutes. When the team began sorting through the measurements, it found a subset of Delta Scuti stars with regular pulsation patterns. Once they knew what to look for, they searched for other examples in data from Kepler, which used a similar observing strategy. They also conducted follow-up observations with ground-based telescopes. In total, they identified a batch of 60 Delta Scuti stars with clear patterns. Pulsations in the well-behaved Delta Scuti group fall into two major categories, both caused by energy being stored and released in the star. Some occur as the whole star expands and contracts symmetrically. Others occur as opposite hemispheres alternatively expand and contract. Bedding's team inferred the alterations by studying each star's fluctuations in brightness. The data have already helped settle a debate over the age of one star, called HD 31901, a member of a recently discovered stream of stars orbiting within our galaxy. Scientists placed the age of the overall stream at 1 billion years, based on the age of a red giant they suspected belonged to the same group. A later estimate, based on the rotation periods of other members of the stellar stream, suggested an age of only about 120 million years. The team used the TESS observations to create an asteroseismic model of HD 31901 that supports the younger age. The set of 60 stars has clear patterns because they're younger than other Delta Scuti stars, having only recently settled into producing all of their energy through nuclear fusion in their cores. The pulsations occur more rapidly in the fledgling stars. As the stars age, the frequency of the pulsations slows, and they become jumbled with other signals. Another factor may be TESS's viewing angle.  Theoretical calculations predict that a spinning star's pulsation patterns should be simpler when its rotational pole faces us instead of its equator. The team's TESS data set included around 1,000 Delta Scuti stars, which means that some of them, by chance, must be viewed close to pole-on.


TELESCOPE SEES SIGNS OF PLANET BIRTH
ESO

Observations made with the Very Large Telescope (ESO’s VLT) have revealed the telltale signs of a star system being born. Around the young star AB Aurigae lies a dense disc of dust and gas in which astronomers have spotted a prominent spiral structure with a ‘twist’ that marks the site where a planet may be forming. The observed feature could be the first direct evidence of a baby planet coming into existence. Thousands of exoplanets have been identified so far, but little is known about how they form. Astronomers know planets are born in dusty discs surrounding young stars, like AB Aurigae, as cold gas and dust clump together. The new observations provide crucial clues to help scientists better understand this process. Astronomers need to observe very young systems to really capture the moment when planets form. But until now they had been unable to take sufficiently sharp and deep images of these young discs to find the ‘twist’ that marks the spot where a baby planet may be coming to existence. The new images feature a stunning spiral of dust and gas around AB Aurigae, located 520 light-years away from Earth in the constellation of Auriga. Spirals of this type signal the presence of baby planets, which ‘kick’ the gas, creating “disturbances in the disc in the form of a wave, somewhat like the wake of a boat on a lake. As the planet rotates around the central star, this wave gets shaped into a spiral arm. The very bright yellow ‘twist’ region close to the centre of the new AB Aurigae image, which lies at about the same distance from the star as Neptune from the Sun, is one of these disturbance sites where the team believe a planet is being made. Observations of the AB Aurigae system made a few years ago with the Atacama Large Millimeter/submillimeter Array (ALMA), provided the first hints of ongoing planet formation around the star. In the ALMA images, scientists spotted two spiral arms of gas close to the star, lying within the disc’s inner region. Then, in 2019 and early 2020, astronomers set out to capture a clearer picture by turning the SPHERE instrument on ESO’s VLT in Chile toward the star. The SPHERE images are the deepest images of the AB Aurigae system obtained to date.

With SPHERE's imaging system, astronomers could see the fainter light from small dust grains and emissions coming from the inner disc. They confirmed the presence of the spiral arms first detected by ALMA and also spotted another remarkable feature, a ‘twist’, that points to the presence of ongoing planet formation in the disc.  The twist is expected from some theoretical models of planet formation. It corresponds to the connection of two spirals — one winding inwards of the planet’s orbit, the other expanding outwards — which join at the planet location. They allow gas and dust from the disc to accrete onto the forming planet and make it grow.  ESO is constructing the 39-metre Extremely Large Telescope, which will draw on the cutting-edge work of ALMA and SPHERE to study extrasolar worlds. This telescope will allow astronomers to get even more detailed views of planets in the making and
see directly and more precisely how the dynamics of the gas contributes to the formation of planets.


NEW CLASS OF COSMIC EXPLOSIONS
National Radio Astronomy Observatory

Astronomers have found two objects that, added to a strange object discovered in 2018, constitute a new class of cosmic explosions. The new type of explosion shares some characteristics with supernova explosions of massive stars and with the explosions that generate gamma-ray bursts (GRBs), but still has distinctive differences from each. The saga began in June of 2018 when astronomers saw a cosmic blast with surprising characteristics and behaviour. The object, dubbed AT2018cow ("The Cow"), drew worldwide attention from scientists and was studied extensively. While it shared some characteristics with supernova explosions, it differed in important aspects, particularly its unusual initial brightness and how rapidly it brightened and faded in just a few days. In the meantime, two additional blasts -- one from 2016 and one from 2018 -- also showed unusual characteristics and were being observed and analyzed. The two new explosions are called CSS161010 (short for CRTS CSS161010 J045834-081803), in a galaxy about 500 million light-years from Earth, and ZTF18abvkwla ("The Koala"), in a galaxy about 3.4 billion light-years distant. Both were discovered by automated sky surveys (Catalina Real-time Transient Survey, All-Sky Automated Survey for Supernovae, and Zwicky Transient Facility) using visible-light telescopes to scan large areas of sky nightly. Two teams of astronomers followed up those discoveries by observing the objects with the National Science Foundation's Karl G. Jansky Very Large Array (VLA). Both teams also used the Giant Metrewave Radio Telescope in India and the team studying CSS161010 used NASA's Chandra X-ray Observatory. Both objects gave the observers surprises. An astronomer at Caltech, lead author of the study on ZTF18abvkwla, immediately noted that the object's radio emission was as bright as that from a gamma-ray burst. Another study at Northwestern on CSS161010 found that the object had launched an "unexpected" amount of material into interstellar space at more than half the speed of light. In both cases, the follow-up observations indicated that the objects shared features in common with AT2018cow.  The scientists concluded that these events, called Fast Blue Optical Transients (FBOTs), represent, along with AT2018cow, a type of stellar explosion significantly different from others

FBOTs probably begin, the astronomers said, the same way as certain supernovae and gamma-ray bursts -- when a star much more massive than the Sun explodes at the end of its "normal" atomic fusion-powered life. The differences show up in the aftermath of the initial explosion. In the "ordinary" supernova of this type, called a core-collapse supernova, the explosion sends a spherical blast wave of material into interstellar space. If, in addition to this, a rotating disk of material briefly forms
around the neutron star or black hole left after the explosion and propels narrow jets of material at nearly the speed of light outward in opposite directions, these jets can produce narrow beams of gamma rays, causing a gamma-ray burst. The rotating disk, called an accretion disk, and the jets it produces, are called an "engine" by astronomers. FBOTs, the astronomers concluded, also have such an engine. In their case, unlike in gamma-ray bursts, it is enshrouded by thick material. That material probably was shed by the star just before it exploded, and may have been pulled from it by a binary companion. When the thick material near the star is struck by the blast wave, it causes the bright visible-light burst soon after the explosion that initially made these objects appear so unusual. That bright burst also is why astronomers call these blasts "fast blue optical transients. This is one of the characteristics that distinguished them from ordinary supernovae. As the blastwave from the explosion collides with the material around the star as it travels outwards, it produces radio emission. This very bright emission was the important clue that proved that the explosion was powered by an engine. The shroud of dense material means that the progenitor star is different from those leading to gamma-ray bursts.  The astronomers said that in the Cow and in CSS161010, the dense material included hydrogen, something never seen in in gamma-ray bursts.

Using the W.M. Keck Observatory, the astronomers found that both CSS 161010 and ZTF18abvkwla, like the Cow, are in small, dwarf galaxies. The dwarf galaxy properties might allow some very rare evolutionary paths of stars" that lead to these distinctive explosions. Although a common element of the FBOTs is that all three have a 'central engine,' the astronomers caution that the engine also could be the result of stars being shredded by black holes, though they consider supernova-type explosions to be the more likely candidate. Observations of more FBOTs and their environments will answer this question. To do that, the scientists say they will need to use telescopes covering a wide range of wavelengths, as they have done with the first three objects. While FBOTs have proven rarer and harder to find than some of us were hoping, in the radio band they're also much more luminous than
astronomers had guessed, allowing them to provide quite comprehensive data even on events that are far away.


THE ODDS OF LIFE EMERGING BEYOND OUR PLANET
Columbia University

We know from the geological record that life started relatively quickly, as soon as our planet's environment was stable enough to support it. We also know that the first multicellular organism, which eventually produced today's technological civilization, took far longer to evolve, approximately 4 billion years. But despite knowing when life first appeared on Earth, scientists still do not understand how life occurred, which has important implications for the likelihood of finding life elsewhere in the Universe. Now astronomers show how an analysis using a statistical technique called Bayesian inference could shed light on how complex extraterrestrial life might evolve in alien worlds. To conduct the analysis, astronomers used the chronology of the earliest evidence for life and the evolution of humanity. They asked how often we would expect life and intelligence to re-emerge if Earth's history were to repeat, re-running the clock over and over again. They framed the problem in terms of four possible answers: Life is common and often develops intelligence, life is rare but often develops intelligence, life is common and rarely develops intelligence and, finally, life is rare and rarely develops intelligence. This method of Bayesian statistical inference -- used to update the probability for a hypothesis as evidence or information becomes available -- states prior beliefs about the system being modelled, which are then combined with data to cast probabilities of outcomes. The technique is akin to betting odds. It encourages the repeated testing of new evidence against your position, in essence a positive feedback loop of refining your estimates of likelihood of an event.

From these four hypotheses, astronomers used Bayesian mathematical formulas to weigh the models against one another. In Bayesian inference, prior probability distributions always need to be selected, but a key result here is that when one compares the rare-life versus common-life scenarios, the common-life scenario is always at least nine times more likely than the rare one. The analysis is based on evidence that life emerged within 300 million years of the formation of the Earth's oceans as found in carbon-13-depleted zircon deposits, a very fast start in the context of Earth's lifetime. The ratio is at least 9:1 or higher, depending on the true value of how often intelligence develops. The conclusion is that if planets with similar conditions and evolutionary timelines to Earth are common, then the analysis suggests that life should have little problem spontaneously emerging on other planets. And what are the odds that these extraterrestrial lives could be complex, differentiated and intelligent? Here, the inquiry is less assured, finding just 3:2 odds in favour of intelligent life. This result stems from humanity's relatively late appearance in Earth's habitable window, suggesting that its development was neither an easy nor ensured process. If we played Earth's history again, the emergence of intelligence is actually somewhat unlikely. The odds in the study aren't overwhelming, being quite close to 50:50, and the findings should be treated as no more than a gentle nudge toward a hypothesis. The analysis can't provide certainties or guarantees, only statistical probabilities based on what happened here on Earth. Yet encouragingly, the case for a universe teeming with life emerges as the favoured bet. The search for intelligent life in worlds beyond Earth should be by no means discouraged.


NASA NAMES NEXT SPACE TELESCOPE
Science Alert

NASA has named a powerful new space telescope after the woman who masterminded the existence of such observatories in the first place. Dr. Nancy Grace Roman spent 21 years at NASA developing and launching space-based observatories that studied the Sun, deep space, and Earth's atmosphere. She most famously worked to develop the concepts behind the Hubble Space Telescope, which just spent its 30th year in orbit. Roman earned the nickname "mother of Hubble" for her role in pushing for that telescope. When it launched in 1990, Hubble became the first of NASA's "great observatories," which are designed to push the limits of human knowledge about the cosmos. Roman also served as NASA's first Chief of Astronomy, making her the first woman to hold an executive position at the agency. She died in 2018. Roman "had huge influence in all of astronomy and space. Roman's work led space astronomy to where it is today.  NASA plans to launch the new telescope, which was originally called the Wide Field InfraRed Survey Telescope (WFIRST), into Earth's orbit in the mid-2020s. Over its five-year lifetime, the Roman Space Telescope will measure light from a billion galaxies and survey the inner Milky Way with the hope of finding about 2,600 new planets and photographing them. The new observatory will have a field of view 100 times greater than Hubble's. Each of its photos will be equivalent to about 100 Hubble images' worth of pixels.