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THE SOCIETY FOR POPULAR ASTRONOMY Electronic News Bulletin No. 520 2020 July 26

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THE SOCIETY FOR POPULAR ASTRONOMY Electronic News Bulletin No. 520 2020 July 26

Here is the latest round-up of news from the Society for Popular Astronomy.  The SPA is arguably Britain's liveliest astronomical society, with members all over the world. We accept subscription payments online at our secure site and can take credit and debit cards. You can join or renew via a secure server or just see how much we have to offer by visiting www.popastro.com

Center for Astrophysics, Harvard & Smithsonian (CfA)

Scientists at Harvard University and the Black Hole Initiative (BHI) have developed a new method to find black holes in the outer solar system, and along with it, determine once-and-for-all the true nature of the hypothesized Planet Nine. The paper highlights the ability of the future Legacy Survey of Space and Time (LSST) mission to observe accretion flares, the presence of which could prove or rule out Planet Nine as a black hole. The team has developed the new method to search for black holes in the outer solar system, based on flares that result from the disruption of intercepted comets. The study suggests that the LSST has the capability to find black holes by observing for accretion flares resulting from the impact of small Oort cloud objects. In the vicinity of a black hole, small bodies that approach it will melt as a result of heating from the background accretion of gas from the interstellar medium onto the black hole. Once they melt, the small bodies are subject to tidal disruption by the black hole, followed by accretion from the tidally disrupted body onto the black hole. Because black holes are intrinsically dark, the radiation that matter emits on its way to the mouth of the black hole is our only way to illuminate this dark environment. Future searches for primordial black holes could be informed by the new calculation. This method can detect or rule out trapped planet-mass black holes out to the edge of the Oort cloud. It could be capable of placing new limits on the fraction of dark matter contained in primordial black holes.  The upcoming LSST is expected to have the sensitivity required to detect accretion flares, while current technology isn't able to do so without guidance. LSST has a wide field of view, covering the entire sky again and again, and searching for transient flares. Other telescopes are good at pointing at a known target but we do not know exactly where to look for Planet Nine. We only know the broad region in which it may reside. LSST's ability to survey the sky twice per week is extremely valuable. In addition, its unprecedented depth will allow for the detection of flares resulting from relatively small impactors, which are more frequent than large ones.

The new paper focuses on the famed Planet Nine as a prime first candidate for detection. The subject of much speculation, most theories suggest that Planet Nine is a previously undetected planet, but it may also flag the existence of a planet-mass black hole. Planet Nine is a compelling explanation for the observed clustering of some objects beyond the orbit of Neptune. If the existence of Planet Nine is confirmed through a direct electromagnetic search, it will be the first detection of a new planet in the solar system in two centuries, not counting Pluto. A failure to detect light from Planet Nine -- or other recent models, such as the suggestion to send probes to measure gravitational influence -- would make the black hole model intriguing. There has been a great deal of speculation concerning alternative explanations for the anomalous orbits observed in the outer solar system. One of the ideas put forth was the possibility that Planet Nine could be a grapefruit-sized black hole with a mass of five to ten times that of the Earth. The focus on Planet Nine is based both in the unprecedented scientific significance that a hypothetical discovery of a planet-mass black hole in the solar system would hold as well as the continued interest in understanding what's out there. It immediately raises questions: why is it there? How did it obtain its properties? Did it shape the solar system history? Are there more like it?


With the help of citizen scientists, astronomers have discovered two highly unusual brown dwarfs, balls of gas that are not massive enough to power themselves the way stars do. Participants in the NASA-funded Backyard Worlds: Planet 9 project helped lead scientists to these bizarre objects, using data from NASA's Near-Earth Object Wide-Field Infrared Survey Explorer (NEOWISE) satellite along with all-sky observations collected between 2009 and 2011 under its previous moniker, WISE.  Backyard Worlds: Planet 9 is an example of "citizen science," a collaboration between professional scientists and members of the public. Scientists call the newly discovered objects "the first extreme T-type subdwarfs." They weigh about 75 times the mass of Jupiter and clock in at roughly 10 billion years old. These two objects are the most planet-like brown dwarfs yet seen among the Milky Way's oldest population of stars. Astronomers hope to use these brown dwarfs to learn more about exoplanets, which are planets outside our solar system. The same physical processes may form both planets and brown dwarfs. These two special brown dwarfs have highly unusual compositions. When viewed in particular wavelengths of infrared light, they look like other brown dwarfs, but at others they do not resemble any other stars or planets that have been observed so far. Scientists were surprised to see they have very little iron, meaning that, like ancient stars, they have not incorporated iron from star births and deaths in their environments. A typical brown dwarf would have as much as 30 times more iron and other metals than these newly discovered objects. One of these brown dwarfs seems to have only about 3% as much iron as our Sun.

Scientists expect very old exoplanets would have a low metal content, too. A central question in the study of brown dwarfs and exoplanets is how much does planet formation depend on the presence of metals like iron and other elements formed by multiple earlier generations of stars. The fact that these brown dwarfs seem to have formed with such low metal abundances suggests that maybe we should be searching harder for ancient, metal-poor exoplanets, or exoplanets orbiting ancient, metal-poor stars. The study first noticed one of the unusual brown dwarfs, called WISE 1810, in 2016, but it was in a crowded area of the sky and was difficult to confirm. With the help of a tool called WiseView, created by Backyard Worlds it was confirmed that the object was moving quickly, which is a good indication that an object is a nearby celestial body like a planet or brown dwarf. The second unusual brown dwarf, WISE 0414, was discovered by a group of citizen scientists who combed through hundreds of images taken by WISE looking for moving objects, which are best detected with the human eye. Astronomers followed up to determine their physical properties and confirm that they are indeed brown dwarfs. The discovery of these two unusual brown dwarfs suggests astronomers may be able to find more of these objects in the future.


A supernova may seem like a pretty final fate, but now astronomers have discovered a star that apparently survived this explosive process. It wasn't without consequence, however - the star was kicked out of a tight binary orbit and flung across the galaxy. The star in question is a white dwarf called SDSS J1240+6710, which was discovered in 2015. At the time, astronomers noted that the object had an unusual composition. While white dwarfs typically have atmospheres made mostly of hydrogen and helium, this one had neither - instead it was made up of an odd combination of oxygen, neon, magnesium and silicon. On closer inspection with the Hubble Space Telescope, astronomers from the University of Warwick have found that the star is much stranger than previously thought. The researchers spotted signs of carbon, sodium and aluminium in its atmosphere, and its mass was found to be very small - only about 40 percent that of the Sun. Weirder still, this star is absolutely pelting through the Milky Way. The scientists measured its velocity and found that it's traveling at around 900,000 km/h. Piecing together these bizarre characteristics, the team came to one conclusion: somehow the star underwent a partial supernova, but survived. This would have burned away the hydrogen and helium, which were missing, and produced the carbon, sodium and aluminium, which were detected.

It couldn't have been a full supernova though. First of all, the star is still intact, but secondly the astronomers detected no iron, nickel, chromium or manganese, all of which should have been produced in a full-blown explosion. The star's unusually low mass could also be explained by an explosion blowing the rest of it away. And finally, the blast would be responsible for its insane speed. Altogether, the team says that the white dwarf once belonged to a binary pair, before some kind of partial supernova sent both stars shooting off in different directions. This isn't the only star to disobey what we thought were the rules about supernovae. In recent years several stars have been seen to completely vanish without the trademark explosion.  Another appears to have picked up the slack and gone supernova several times over a few decades.

Cardiff University

Cardiff University scientists say they are closer to understanding how a supermassive black hole (SMBH) is born thanks to a new technique that has enabled them to zoom in on one of these enigmatic cosmic objects in unprecedented detail. Scientists are unsure as to whether SMBHs were formed in the extreme conditions shortly after the big bang, in a process dubbed a 'direct collapse', or were grown much later from 'seed' black holes resulting from the death of massive stars. If the former method were true, SMBHs would be born with extremely large masses -- hundreds of thousands to millions of times more massive than our Sun -- and would have a fixed minimum size. If the latter were true then SMBHs would start out relatively small, around 100 times the mass of our Sun, and start to grow larger over time by feeding on the stars and gas clouds that live around them. Astronomers have long been striving to find the lowest mass SMBHs, which are the missing links needed to decipher this problem. The team has pushed the boundaries, revealing one of the lowest-mass SMBHs ever observed at the centre of a nearby galaxy, weighing less than one million times the mass of our Sun.  The SMBH lives in a galaxy that is familiarly known as "Mirach's Ghost," due to its close proximity to a very bright star called Mirach, giving it a ghostly shadow. The findings were made using a new technique with the Atacama Large Millimeter/submillimeter Array (ALMA), a state-of-the-art telescope situated high in the Chilean Andes that is used to study light from some of the coldest objects in the Universe. The SMBH in Mirach's Ghost appears to have a mass within the range predicted by 'direct collapse' models. Astronomers know it is currently active and swallowing gas, so some of the more extreme 'direct collapse' models that only make very massive SMBHs cannot be true. This on its own is not enough to definitively tell the difference between the 'seed' picture and 'direct collapse' -- we need to understand the statistics for that -- but this is a massive step in the right direction.

Black holes are objects that have collapsed under the weight of gravity, leaving behind small but incredibly dense regions of space from which nothing can escape, not even light. An SMBH is the largest type of black hole that can be hundreds of thousands, if not billions, of times the mass of the Sun. It is believed that nearly all large galaxies, such as our own Milky Way, contain an SMBH located at its centre.  SMBHs have also been found in very distant galaxies as they appeared just a few hundred million years after the big bang. This suggests that at least some SMBHs could have grown very massive in a very short time, which is hard to explain according to models for the formation and evolution of galaxies. All black holes grow as they swallow gas clouds and disrupt stars that venture too close to them, but some have more active lives than others. Looking for the smallest SMBHs in nearby galaxies could therefore help us reveal how SMBHs start off. In their study, the international team used brand new techniques to zoom further into the heart of a small nearby galaxy, called NGC404, than ever before, allowing them to observe the swirling gas clouds that surrounded the SMBH at its centre. The ALMA telescope enabled the team to resolve the gas clouds in the heart of the galaxy, revealing details only 1.5 light years across, making this one of the highest resolution maps of gas ever made of another galaxy. Being able to observe this galaxy with such high resolution enabled the team to overcome a decade's worth of conflicting results and reveal the true nature of the SMBH at the galaxy's centre. The study demonstrates that with this new technique astronomers can really begin to explore both the properties and origins of these mysterious objects. If there is a minimum mass for a supermassive black hole, it hasn't been found yet.

Bulletin compiled by Clive Down
(c) 2020 The Society for Popular Astronomy

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