Topic: Early November Astronomy Bulletin

[Clive]

SOFIA DISCOVERS WATER ON SURFACE OF MOON
NASA

NASA's Stratospheric Observatory for Infrared Astronomy (SOFIA) has confirmed, for the first time, water on the sunlit surface of the Moon. This discovery indicates that water may be distributed across the lunar surface, and not limited to cold, shadowed places. SOFIA has detected water molecules (H2O) in Clavius Crater, one of the largest craters visible from Earth, located in the Moon's southern hemisphere. Previous observations of the Moon's surface detected some form of hydrogen, but were unable to distinguish between water and its close chemical relative, hydroxyl (OH). Data from this location reveal water in concentrations of 100 to 412 parts per million -- roughly equivalent to a 12-ounce bottle of water -- trapped in a cubic meter of soil spread across the lunar surface. As a comparison, the Sahara desert has 100 times the amount of water than what SOFIA detected in the lunar soil. Despite the small amounts, the discovery raises new questions about how water is created and how it persists on the harsh, airless lunar surface. Water is a precious resource in deep space and a key ingredient of life as we know it.  Whether the water SOFIA found is easily accessible for use as a resource remains to be determined. Under NASA's Artemis program, the agency is eager to learn all it can about the presence of water on the Moon in advance of sending the first woman and next man to the lunar surface in 2024 and establishing a sustainable human presence there by the end of the decade. SOFIA's results build on years of previous research examining the presence of water on the Moon. When the Apollo astronauts first returned from the Moon in 1969, it was thought to be completely dry. Orbital and impactor missions over the past 20 years, such as NASA's Lunar Crater Observation and Sensing Satellite, confirmed ice in permanently shadowed craters around the Moon's poles. Meanwhile, several spacecraft -- including the Cassini mission and Deep Impact comet mission, as well as the Indian Space Research Organization's Chandrayaan-1 mission -- and NASA's ground-based Infrared Telescope Facility, looked broadly across the lunar surface and found evidence of hydration in sunnier regions. Yet those missions were unable to definitively distinguish the form in which it was present -- either H2O or OH.

SOFIA offered a new means of looking at the Moon. Flying at altitudes of up to 45,000 feet, this modified Boeing 747SP jetliner with a 106-inch diameter telescope reaches above 99% of the water vapour in Earth's atmosphere to get a clearer view of the infrared universe. Using its Faint Object infraRed CAmera for the SOFIA Telescope (FORCAST), SOFIA was able to pick up the specific wavelength unique to water molecules, at 6.1 microns, and discovered a relatively surprising concentration in sunny Clavius Crater. Several forces could be at play in the delivery or creation of this water. Micrometeorites raining down on the lunar surface, carrying small amounts of water, could deposit the water on the lunar surface upon impact. Another possibility is there could be a two-step process whereby the Sun's solar wind delivers hydrogen to the lunar surface and causes a chemical reaction with oxygen-bearing minerals in the soil to create hydroxyl. Meanwhile, radiation from the bombardment of micrometeorites could be transforming that hydroxyl into water. How the water then gets stored -- making it possible to accumulate -- also raises some intriguing questions. The water could be trapped into tiny bead-like structures in the soil that form out of the high heat created by micrometeorite impacts. Another possibility is that the water could be hidden between grains of lunar soil and sheltered from the sunlight -- potentially making it a bit more accessible than water trapped in beadlike structures. For a mission designed to look at distant, dim objects such as black holes, star clusters, and galaxies, SOFIA's spotlight on Earth's nearest and brightest neighbour was a departure from business as usual. The telescope operators typically use a guide camera to track stars, keeping the telescope locked steadily on its observing target. But the Moon is so close and bright that it fills the guide camera's entire field of view. With no stars visible, it was unclear if the telescope could reliably track the Moon. To this, in August 2018, the operators decided to try a test observation. SOFIA's follow-up flights will look for water in additional sunlit locations and during different lunar phases to learn more about how the water is produced, stored, and moved across the Moon. The data will add to the work of future Moon missions, such as NASA's Volatiles Investigating Polar Exploration Rover (VIPER), to create the first water resource maps of the Moon for future human space exploration.


OSIRIS-REx SPACECRAFT COLLECTS ASTEROID SAMPLE
NASA

Two days after touching down on asteroid Bennu, NASA’s OSIRIS-REx mission team received on Thursday, Oct. 22, images that confirm the spacecraft has collected more than enough material to meet one of its main mission requirements – acquiring at least grams of the asteroid’s surface material. The spacecraft captured images of the sample collector head as it moved through several different positions. In reviewing these images, the OSIRIS-REx team noticed both that the head appeared to be full of asteroid particles, and that some of these particles appeared to be escaping slowly from the sample collector, called the Touch-And-Go Sample Acquisition Mechanism (TAGSAM) head. They suspect bits of material are passing through small gaps where a mylar flap – the collector’s “lid” – is slightly wedged open by larger rocks. The team believes it has collected a sufficient sample and is on a path to stow the sample as quickly as possible. They came to this conclusion after comparing images of the empty collector head with Oct. 22 images of the TAGSAM head after the sample collection event.

The images also show that any movement to the spacecraft and the TAGSAM instrument may lead to further sample loss. To preserve the remaining material, the mission team decided to forego the Sample Mass Measurement activity originally scheduled for Saturday, Oct. 24, and cancelled a braking burn to minimize any acceleration to the spacecraft. From here, the OSIRIS-Rex team will focus on stowing the sample in the Sample Return Capsule (SRC), where any loose material will be kept safe during the spacecraft’s journey back to Earth. The TAGSAM head performed the sampling event in optimal conditions. Newly available analyses show that the collector head was flush with Bennu’s surface when it made contact and when the nitrogen gas bottle was fired to stir surface material. It also penetrated several centimetres into the asteroid’s surface material. All data so far suggest that the collector head is holding much more than 2 ounces of regolith. OSIRIS-REx remains in good health, and the mission team is finalizing a timeline for sample storage. An update will be provided once a decision is made on the sample storage timing and procedures.


TWO PLANETS FOUND AROUND RED DWARF
University of Bern

Astronomers have detected two exoplanets orbiting the star TOI-1266. The Mexico-based telescope thus demonstrates its high precision and takes an important step in the quest of finding potentially habitable worlds. Red dwarfs are the coolest kind of star. As such, they potentially allow liquid water to exist on planets that are quite close to them. In the search for habitable worlds beyond the borders of our solar system, this is a big advantage: the distance between an exoplanet and its star is a crucial factor for its detection. The closer the two are, the higher the chance that astronomers can detect the planet from Earth. One instrument, with which it is possible to study red dwarfs and their planets closely, is the Mexico-based SAINT -EX telescope, co-operated by the NCCR PlanetS. SAINT-EX is an acronym that stands for Search And characterIsatioN of Transiting EXoplanets. The project has been named in honour of Antoine de Saint-Exupéry (Saint-Ex), the famous writer, poet and aviator. The SAINT-EX Observatory is a fully robotic facility hosting a 1-metre telescope. It is equipped with instrumentation specifically suited to enable high-precision detection of small planets orbiting cool stars. Now, this specialization pays off: earlier this year, the telescope was able to detect two exoplanets orbiting the star TOI-1266, located around 120 light years from Earth.

Compared to the planets in our solar system, TOI-1266 b and c are much closer to their star -- it takes them only 11 and 19 days respectively to orbit it. However, as their host star is much cooler than the Sun, their temperatures are not very extreme: the outer planet has approximately the temperature of Venus (although it is 7 times closer to its star than Venus is to the Sun). The two planets are of similar density, possibly corresponding to a composition of about a half of rocky and metallic material and half water. This makes them about half as rocky as Earth or Venus also far rockier than Uranus or Neptune. In size, the planets clearly differ from each other. The inner planet, TOI-1266 b, measures up to a little under two-and-a-half times the Earth's diameter. This makes it a so-called "sub-Neptune." The outer planet, TOI-1266 c, is just over one-and-a-half times the size of our planet. Thus, it belongs to the category of "super-Earths." This places the two planets at the edges of the so-called radius-valley. Planets between about the radius of TOI-1266 b and c are quite rare, likely because of the effect of strong irradiation from the star, which can erode their atmospheres.  Being able to study two different types of planets in the same system is a great opportunity to better understand how these different sized planets come to be.


BETELGEUSE 25% CLOSER TO EARTH THAN PREVIOUSLY THOUGHT
Australian National University

Betelgeuse caused quite a kerfuffle among astronomers in late 2019 and early 2020 when the star's brightness dropped precipitously. There was much speculation that it might explode, but astronomers knew this was incredibly unlikely. What they were more concerned with was why the star suddenly dimmed so dramatically: What was going on in the upper layers of this enormous star that could make such a huge difference in its light? It seems pretty clear now that the red supergiant expelled an enormous cloud of dust, which blocks visible light, dimming it substantially. It's not clear why this happened and why on such a huge scale. Massive stars like Betelgeuse have complicated effects going on in their upper layers which can cause the star to physically pulsate, getting bigger and smaller over time. It's likely some other event happened in the star (perhaps a rising plume of hot gas) coupled with the normal pulsation, causing the creation of the dust that dimmed it. This event, and the efforts to understand it, reminded astronomers that we really don't understand what's going inside this star very well, and in fact there are still basic facts about it we don't know! For example, its size, mass, age, and distance are extremely difficult to determine. However, a group of astronomers took a look at some old data of the star to measure its brightness changes, and by feeding them into some complicated models were able to get new estimates of these characteristics. Some of what they found is similar to older estimates, but their estimate of the size of the star has been revised downward quite a bit. The old estimates were it being 1.5 billion kilometres or so across, while the new one is now just over a billion. That's still enormous, but a lot smaller than previous thinking. But if it's smaller, that means in turn it must be closer to us. They get a number of about 530 light years, 25% closer than previous estimates. However we are still in no danger if it goes supernova. At their new estimated distance it's still way too far away to hurt us if it explodes.

The astronomers wanted to investigate Betelgeuse using physical models of how gas flows inside the star, and how seismic waves travel through it, because these depend on the internal structure of the star (a little like how seismic waves traveling through the Earth tell geologists about the structure inside our planet). The change in brightness in the star over time depends on these factors as well. The astronomers needed better brightness observations of Betelgeuse to feed their models, so they turned to an unusual source: the Solar Mass Ejection Imager, a detector on the Coriolis mission. This spacecraft was designed to observe the Sun and understand activity that can interfere with communications. The instrument looked at the Sun, but also observed many bright stars including Betelgeuse. The astronomers reprocessed the data from that mission, providing measurements that filled a gap in observations in the late 2000s, giving them better data to use. Betelgeuse is known to pulsate on multiple timescales, including one of several years' length and another of just over a year. Using the new data they found those periods to be 6.5 years and 416 days. The models use that data, and also need things like the initial mass of the star, its age, and so on. Many of these numbers aren't pinned down, so the calculations are run with the input variables varied a little bit each time to see how the end results are changed. So if the initial mass of Betelgeuse is thought to be 18 –21 times the Sun then the models are run with a mass of 18, then 18.1 (say), then 18.2, and so on. The results can then be analyzed statistically to see how robust they are. And that's where the surprises came in. They find the size of the star dropped from something like 1100 times wider than the Sun to just 764 times wider.  That's a big drop.

But there's more. We can measure Betelgeuse's apparent size on the sky — it's so enormous it's not just an unresolved dot, but can be seen as an actual disk. The size of that disk depends on the distance to the star and its actual, physical size. Now that they had the physical size, they could get the distance, and they found that it's about 530 light years away, when earlier numbers are more like 640. That's a lot closer! Getting the distance to Betelgeuse is surprisingly hard. It's too bright for the Gaia observatory to observe, and other methods yield different values. 530 light years is consistent with old measurements from the Hipparcos satellite, for example, but disagrees with newer ones using radio observatories. Another interesting outcome of their calculations is that for all this to work, Betelgeuse has to be fusing helium into carbon in its core. Stars like the Sun fuse hydrogen into helium, and massive stars go through that phase in a few million years. After that they fuse helium, which lasts for about 100,000 years before moving onto to carbon fusion. After that the steps take shorter and shorter times, until the star tries to fuse iron, in which case it goes Supernova. So their models indicate it's still in the early stages of helium fusion, which in turn means it won't explode for a long, long time.
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STAR CLUSTERS ONLY TIP OF ICEBERG
University of Vienna

Star clusters have been part of the Imaginarium of human civilization for millennia.  The brightest star clusters to Earth, like the Pleiades, are readily visible to the naked eye. A team of astronomers has now revealed the existence of massive stellar halos, termed coronae, surrounding local star clusters. Clusters form big families of stars that can stay together for large parts of their lifetime. Today, we know of roughly a few thousand star clusters in the Milky Way, but we only recognize them because of their prominent appearance as rich and tight groups of stars. Given enough time, stars tend to leave their cradle and find themselves surrounded by countless strangers, thereby becoming indistinguishable from their neighbours and hard to identify. Our Sun is thought to have formed in a star cluster but has left its siblings behind a long time ago. Thanks to the ESA Gaia spacecraft's precise measurements, astronomers at the University of Vienna have now discovered that what we call a star cluster is only the tip of the iceberg of a much larger and often distinctly elongated distribution of stars. Measurements reveal the vast numbers of sibling stars surrounding the well-known cores of the star clusters for the first time.  It appears that star clusters are enclosed in rich halos, or coronae, more than 10 times as large as the original cluster, reaching far beyond our previous guesses.  The tight groups of stars we see in the night sky are just a part of a much larger entity.

To find the lost star siblings, the research team developed a new method that uses machine learning to trace groups of stars which were born together and move jointly across the sky. The team analyzed 10 star clusters and identified thousands of siblings far away from the centre of the compact clusters, yet clearly belonging to the same family. An explanation for the origin of these coronae remains uncertain, yet the team is confident that their findings will redefine star clusters and aid our understanding of their history and evolution across cosmic time. The star clusters investigated were thought to be well-known prototypes, studied for more than a century, yet it seems we have to start thinking bigger. The discovery will have important implications for our understanding of how the Milky Way was built, cluster by cluster, but also implications for the survival rate of proto-planets far from the sterilizing radiation of massive stars in the centres of clusters. Dense star clusters with their massive but less dense coronae might not be a bad place to raise infant planets after all.


STAR CLUSTER BREAKS METAL-POOR RECORD
W. M. Keck Observatory

In a surprising discovery, astronomers using two Maunakea Observatories -- W. M. Keck Observatory and Canada-France-Hawaii Telescope (CFHT) -- have found a globular star cluster in the Andromeda Galaxy that contains a record-breaking low amount of metals. The stars in the cluster, called RBC EXT8, have on average 800 times less iron than our Sun and are three times more iron-poor than the previous globular cluster record-holder. RBC EXT8 is also extremely deficient in magnesium.  A globular cluster is a large, dense collection of thousands to millions of ancient stars that move together as a tight-knit group through a galaxy. Until now, astronomers thought large globular clusters had to contain a considerable amount of heavy elements. Hydrogen and helium are the two main elements created after the Big Bang. Heavier elements such as iron and magnesium formed later. Finding a massive globular cluster like RBC EXT8 that is extremely impoverished in metals defies current formation models, calling into question some of our ideas about the birth of stars and galaxies in the young Universe. The finding shows that massive globular clusters could form in the early Universe out of gas with only a small 'sprinkling' of elements other than hydrogen and helium. This is surprising because such pristine gas was thought to be in building blocks too small to form such massive star clusters. This discovery is exciting because the idea of a 'metallicity floor' for globular clusters, that must contain some minimum amount of heavy metals, underpinned so much of our thinking about how these very old star clusters
formed in the early Universe. The researchers observed RBC EXT8 using Keck Observatory's High-Resolution Echelle Spectrometer (HIRES) in October of 2019. The globular cluster was not originally on the program, but the team had a couple of hours of observing time left and decided to aim the Keck I telescope at the cluster, whose stellar content had not yet been studied in detail. The team made spectroscopic observations to determine RBC EXT8's metal content and used three archive images from CFHT to determine its size and estimate its mass. Their remarkable result came as quite a surprise.


STARS AND PLANETS GROW AS SIBLINGS
Max-Planck-Gesellschaft

Astronomers have found compelling evidence that planets start to form while infant stars are still growing. The high-resolution image obtained with the Atacama Large Millimeter/submillimeter Array (ALMA) shows a young proto-stellar disk with multiple gaps and rings of dust. This new result shows the youngest and most detailed example of dust rings acting as cosmic cradles, where the seeds of planets form and take hold. An international team of scientists targeted the proto-star IRS 63 with the ALMA radio observatory. This system is 470 light years from Earth and located deep within the dense L1709 interstellar cloud in the Ophiuchus constellation.  Proto-stars as young as IRS 63 are still swaddled in a large and massive blanket of gas and dust called an envelope, and the proto-star and disk feed from this reservoir of material. In systems older than 1,000,000 years, after the proto-stars have finished gathering most of their mass, rings of dust have been previously detected in great numbers. IRS 63 is different: at under 500,000 years old, it is less than half the age of other young stars with dust rings and the proto-star will still grow significantly in mass. Astronomers used to think that stars entered adulthood first and then were the mothers of planets that came later. But now it seems that proto-stars and planets grow and evolve together from early times, like siblings.

Planets face some serious obstacles during their earliest stages of formation. They have to grow from tiny dust particles, smaller than household dust here on Earth.  However, even after the dust clumps together to form a planet embryo, the still-forming planet could disappear by spiralling inwards and being consumed by the central proto-star. If planets do start to form very early and at large distances from the proto-star, they may better survive this process. The team found that there is about 0.5 Jupiter masses of dust in the young disk of IRS 63 further than 20 au from its centre (at a distance similar to the Uranus orbit in our solar system). That is not counting the amount of gas, which could add up to 100 times more material. It takes at least 0.03 Jupiter masses of solid material to form a planet core that will efficiently accrete gas and grow to form a giant gas planet. There is growing evidence that Jupiter may have actually formed much farther out in the Solar System, beyond the Neptune orbit, and then migrated inwards to its present location.Similarly, the dust surrounding IRS 63 shows that there is enough material far from the proto-star and at a stage young enough that there is a chance for this Solar System analogue to form planets in the way that Jupiter is suspected to have formed.


GALAXY HAD BROADSIDE COLLISION WITH MILKY WAY
Rensselaer Polytechnic Institute

Nearly 3 billion years ago, a dwarf galaxy plunged into the centre of the Milky Way and was ripped apart by the gravitational forces of the collision. Astrophysicists say that the merger produced a series of tell-tale shell-like formations of stars in the vicinity of the Virgo constellation, the first such "shell structures" to be found in the Milky Way. The finding offers further evidence of the ancient event, and new possible explanations for other phenomena in the galaxy. Astronomers identified an unusually high density of stars called the Virgo Overdensity about two decades ago. Star surveys revealed that some of these stars are moving toward us while others are moving away, which is also unusual, as a cluster of stars would typically travel in concert. Based on emerging data, astrophysicists proposed in 2019 that the overdensity was the result of a radial merger, the stellar version of a T-bone crash. The newly announced shell structures are planes of stars curved, like umbrellas, left behind as the dwarf galaxy was torn apart, literally bouncing up and down through the centre of the galaxy as it was incorporated into the Milky Way, an event the researchers have named the "Virgo Radial Merger." Each time the dwarf galaxy stars pass quickly through the galaxy centre, slow down as they are pulled back by the Milky Way's gravity until they stop at their farthest point, and then turn around to crash though the centre again, another shell structure is created.  Simulations that match survey data can be used to calculate how many cycles the dwarf galaxy has endured, and therefore, when the original collision occurred.

The new paper identifies two shell structures in the Virgo Overdensity and two in the Hercules Aquila Cloud region, based on data from the Sloan Digital Sky Survey, the European Space Agency's Gaia space telescope, and the LAMOST telescope in China. Computer modelling of the shells and the motion of the stars indicates that the dwarf galaxy first passed through the galactic centre of the Milky Way 2.7 billion years ago. Most if not all of those stars appear to be "immigrants," stars that formed in smaller galaxies that were later pulled into the Milky Way. As the smaller galaxies coalesce with the Milky Way, their stars are pulled by so-called "tidal forces," the same kind of differential forces that make tides on Earth, and they eventually form a long cord of stars moving in unison within the halo. Such tidal mergers are fairly common and have formed the subject of much research over the past two decades. More violent "radial mergers" are considered far less common.  There are other galaxies, typically more spherical galaxies, that have a very pronounced shell structure, so astronomers know that these things happen, but they had looked in the Milky Way and hadn't seen really obvious, gigantic shells. As they modelled the movement of the Virgo Overdensity, they began to consider a radial merger. And then they realized that it's the same type of merger that causes these big shells. It just looks different because we're inside the Milky Way, so we have a different perspective, and also this is a disk galaxy and we don't have as many examples of shell structures in disk galaxies. The finding poses potential implications for a number of other stellar phenomena, including the Gaia Sausage, a formation of stars believed to have resulted from the merger of a dwarf galaxy between 8 and 11 billion years ago. Previous work supported the idea that the Virgo Radial Merger and the Gaia Sausage resulted from the same event; the much lower age estimate for the Virgo Radial Merger means that either the two are different events or the Gaia Sausage is much younger and could not have caused the creation of the thick disk of the Milky Way, as previously claimed. A recently discovered spiral pattern in position and velocity data for stars close to the Sun, sometimes called the Gaia Snail, and a proposed event called the Splash, may also be associated with the Virgo Radial Merger. There are lots of potential tie-ins to this finding. The Virgo Radial Merger opens the door to greater understanding of other phenomena that we see and don't fully understand, and that could very well have been affected by something having fallen right through the middle of the galaxy less than 3 billion years ago.


JAMES WEBB TELESCOPES WILL UNCOVER GALAXIES
University of Melbourne

Two new studies from the University of Melbourne will help the largest, most powerful and complex space telescope ever built to uncover galaxies never before seen by humanity. Powerful lights called 'quasars' are the brightest objects in the Universe. Powered by supermassive black holes up to a trillion times the mass of our Sun, they outshine entire galaxies of billions of stars. Simulations show that while even NASA's Hubble Space Telescope can't see galaxies currently hidden by these quasars, the James Webb Telescope will be able to get past the glare. Although quasars are known to reside at the centres of galaxies, it has been difficult to tell what those galaxies are like and how they compare to galaxies without quasars. The University of Melbourne team collaborated with researchers from the US, China, Germany, and The Netherlands to use the Hubble Space Telescope to try to observe these galaxies. They then used a state-of-the-art computer simulation called BlueTides, which was developed by a team led by ASTRO 3D distinguished visitor, Tiziana Di Matteo, from Carnegie Mellon University in Pittsburgh, Pennsylvania, US. The team used these simulations to determine what Webb's cameras would see if the observatory studied these distant systems. They found that distinguishing the host galaxy from the quasar would be possible, although still challenging due to the galaxy's small size on the sky. They also found that the galaxies hosting quasars tended to be smaller than average, spanning only about 1/30 the diameter of the Milky Way despite containing almost as much mass as our galaxy. The host galaxies are surprisingly tiny compared to the average galaxy at that point in time.