Topic: Late October Astronomy Bulletin

[Clive]

DUNG BEETLES NAVIGATE USING THE MILKY WAY

Spaceweather.com

In 2009, entomologists made an astonishing discovery. Nocturnal dung beetles (Scarabaeus satyrus) can navigate using the Milky Way. Although the compound eyes of beetles cannot resolve individual stars, this species can see the Milky Way as a stripe across the sky and perhaps even sense features within it such as the galactic center and lanes of stardust. Dung beetles are nature's sanitation crew. Whenever a pile of brown material is dumped in the forest, dung beetles converge to clean up the mess. Each beetle sculpts a dung ball, which they roll away in a straight line. Far from the pile, the ball will be buried and eaten, and sometimes used as bedding for dung beetle eggs. Dung beetles are combative. If two beetles leaving the pile bump into one other, they can get into a brutal wrestling match often ending with overhead judo-style full body throws. During the day they steer by the Sun. Dung beetles can see polarization patterns in the daytime sky, and use these patterns to hold course. A single patch of blue sky is sufficient. The trick works at night, too. Dung beetles are the only known creatures who can see the polarization of moonlight, which is 100 million times weaker than daylight polarization. Studies show that dung beetles walk straight as accurately at night as during the day, even when the Moon is a faint crescent. But what happens when there's no Sun or Moon? In the early 2000s, this question troubled two pioneers of dung beetle research of Lund University in Sweden. To find the answer, they took some beetles to the planetarium at the University of the Witwatersrand in Johannesburg, South Africa, and projected the Milky Way onto the domed ceiling. The beetles saw it, and navigated.

Scientists have built a rudimentary planetarium just for dung beetles. It uses LED lights to mimic the Milky Way as beetles see it through their compound eyes. In 2017 they found that dung beetles were able to distinguish between north and south arms of the Milky Way, sensing intensity contrasts as low as 13%. This threshold puts features such as the galactic centre in Sagittarius and the Great Rift in Cygnus theoretically within range of beetle senses. The team described how urban lights wipe out the Milky Way, reduce the polarization of Moonlight by 60% to 70%, and "create anthropogenic celestial cues." The last item is worst of all. Spotlights and brightly lit buildings mesmerize beetles who suddenly ignore the sky and make a beeline for manmade bulbs. Researchers believe they are only scratching the surface of this field with potentially thousands of species watching the stars. Everything from simple light bulbs to sophisticated satellite megaconstellations may be affecting these members of our ecosystem.

NEWLY FORMED CRATERS LOCATED ON MARS :

University of Maryland

Researchers with NASA's InSight mission located four new craters created by impacts on the surface of Mars. Using data from a seismometer and visuals acquired from the Mars Reconnaissance Orbiter, the team successfully calculated and confirmed the impact locations. This is the first time that researchers have been able to capture the dynamics of an impact on Mars. As space projectiles enter the planetary atmosphere and impact the ground, the projectiles trigger acoustic waves (sound waves that travel through fluid or gas) and seismic waves (waves that travel through a solid medium). The team used these waves, measured by the SEIS (Seismic Experiment for Interior Structure) instrument on InSight, to estimate the approximate locations of resulting impact sites, observing the unique physics that dictated the projectiles' movements. The team then matched their approximations to visuals provided by high-resolution cameras, confirming the sites and accuracy of the team's modelling. These findings demonstrate how planetary seismology (the study of quakes and related events like volcanic eruptions) can be used to identify sources of seismic activity. This ability may help researchers measure how often new impacts occur in the inner solar system, where both Mars and Earth reside -- an observation essential to understanding the population of near-Earth objects like asteroids or rock fragments that may pose a danger to Earth. Additionally, using images to determine the precise location of these impacts makes their associated acoustic and seismic waves invaluable for studying the Martian atmosphere and interior. With a better understanding of marsquake locations, scientists will be able to gather essential information about the planet, such as the size and solidity of its core or its heating processes. Geophysicists anticipate that new advances in planetary seismology will allow them to better investigate underlying tectonic activities and other sources of seismic activity within Mars.

EARTH-LIKE PLANETS UNLIKELY TO BE ANOTHER “PALE BLUE DOT”

RAS

When searching for Earth-like worlds around other stars, instead of looking for the ‘pale blue dot’ described by Carl Sagan, new research suggests that a hunt for dry, cold ‘pale yellow dots’ might have a better chance of success. The near balance of land-to-water that has helped life flourish on Earth could be highly unusual, according to a Swiss-German study. Astronomers studied how the evolution and cycles of continents and water could shape the development of terrestrial exoplanets. Results from their models suggest that planets have approximately an 80 percent probability of being mostly covered by land, with 20 percent likely to be mainly oceanic worlds. Barely one percent of the outcomes had an Earth-like distribution of land and water. The team’s numerical models suggest that the average surface temperatures would not be too different, with perhaps a 5° Celsius variation, but that the land-to-ocean distribution would affect the planets’ climates. An ocean world, with less than 10 percent land, would likely be moist and warm, with a climate similar to the Earth in the tropic and subtropic epoch that followed the asteroid impact that caused the extinction of the dinosaurs. The continental worlds, with less than 30 percent oceans, would feature colder, drier and harsher climates. Cool deserts might occupy in the inner parts of landmasses, and overall they would resemble our Earth sometime during the last Ice Age, when extensive glaciers and ice-sheets developed. On Earth, the growth of continents by volcanic activity and their erosion by weathering is approximately balanced. Life based on photosynthesis thrives on land, where it has direct access to solar energy. The oceans provide a huge reservoir of water that enhances rainfall and prevent the present climate from becoming too dry.

BIG PLANETS GET HEAD START IN PANCACKE-THIN NURSERIES

RAS

Astronomers have observed a remarkably thin disc of dust and gas around a young star, and found that its structure accelerated the process of grains clumping together to form planets.

Planets only have a limited opportunity to form before the disc of gas and dust, their nursery, is dissipated by radiation from their parent star. The initial micron-sized particles composing the disc must grow rapidly to larger millimetre-sized grains, the building blocks of planets. In this thin disc, we can see that the large particles have settled into a dense midplane, due to the combined effect of stellar gravity and interaction with the gas, creating conditions that are extremely favourable for planetary growth. Using the Atacama Large Millimeter Array (ALMA) in Chile, the team obtained very high resolution images of the proto-planetary disc Oph163131, located in a nearby star-forming region called Ophiuchus. Their observations showed that, while disc is twice the diameter of our Solar System, at its outer edge the bulk of the dust is concentrated vertically in a layer only half the distance from Earth to the Sun. This makes it one of the thinnest planetary nurseries observed to date.

Looking at proto-planetary discs edge-on gives a clear view of the vertical and radial dimensions, so that we can disentangle the dust evolution processes at work. ALMA gave astronomers the first look at the distribution of millimetre-sized grains in this disc. Their concentration into such a thin layer was a surprise, as previous Hubble Space Telescope (HST) observations of finer, micron-sized particles showed a region extending almost 20 times higher. Simulations by the team based on the observations show that the seeds of gas-giant planets, which must be at least 10 Earth-masses, can form in the outer part of the disc in less than 10 million years. This is within the typical lifetime of a planetary nursery before it dissipates. Thin planet nurseries appear to be favourable for forming big planets, and may even facilitate planets forming at large distance from the central star.

HOT GAS BUBBLE ORBITS MILKY WAY’S SUPERMASSIVE BLACK HOLE

ESO

Using the Atacama Large Millimeter/submillimeter Array (ALMA), astronomers have spotted signs of a ‘hot spot’ orbiting Sagittarius A*, the black hole at the centre of our galaxy. The finding helps us better understand the enigmatic and dynamic environment of our supermassive black hole. Astronomers think they are looking at a hot bubble of gas zipping around Sagittarius A* on an orbit similar in size to that of the planet Mercury, but making a full loop in just around 70 minutes. This requires a mind blowing velocity of about 30% of the speed of light. By chance, some of the observations were done shortly after a burst or flare of X-ray energy was emitted from the centre of our galaxy, which was spotted by NASA’s Chandra Space Telescope. These kinds of flares, previously observed with X-ray and infrared telescopes, are thought to be associated with so-called ‘hot spots’, hot gas bubbles that orbit very fast and close to the black hole. What is really new and interesting is that such flares were so far only clearly present in X-ray and infrared observations of Sagittarius A*. Here we see for the first time a very strong indication that orbiting hot spots are also present in radio observations. Perhaps these hot spots detected at infrared wavelengths are a manifestation of the same physical phenomenon: as infrared-emitting hot spots cool down, they become visible at longer wavelengths, like the ones observed by ALMA and the EHT. The flares were long thought to originate from magnetic interactions in the very hot gas orbiting very close to Sagittarius A*, and the new findings support this idea. Now we find strong evidence for a magnetic origin of these flares and our observations give us a clue about the geometry of the process. The new data are extremely helpful for building a theoretical interpretation of these events.

ALMA allows astronomers to study polarised radio emission from Sagittarius A*, which can be used to unveil the black hole’s magnetic field. The team used these observations together with theoretical models to learn more about the formation of the hot spot and the environment it is embedded in, including the magnetic field around Sagittarius A*. Their research provides stronger constraints on the shape of this magnetic field than previous observations, helping astronomers uncover the nature of our black hole and its surroundings. The observations confirm some of the previous discoveries made by the GRAVITY instrument at ESO’s Very Large Telescope (VLT), which observes in the infrared. The data from GRAVITY and ALMA both suggest the flare originates in a clump of gas swirling around the black hole at about 30% of the speed of light in a clockwise direction in the sky, with the orbit of the hot spot being nearly face-on. The team is also hoping to be able to directly observe the orbiting gas clumps with the EHT, to probe ever closer to the black hole and learn more about it.



GAMMA RAYS FROM NEIGHBOURING GALAXY

Universiteit van Amsterdam

A team of researchers, including UvA physicists and astronomers, has studied gamma rays caused by the Sagittarius Dwarf, a small neighbouring galaxy of our Milky Way. They showed that all the observed gamma radiation can be explained by millisecond pulsars, and can therefore not be interpreted as a smoking gun signature for the presence of dark matter. The centre of our galaxy is blowing a pair of colossal bubbles of gamma radiation (the magenta structures in the image above) that span a whopping 50,000 light-years across. Discovered with the Fermi Gamma-ray Space Telescope about a decade ago, the source of this hourglass-shaped phenomenon remains unclear. These Fermi bubbles are patched with a few enigmatic substructures of very bright gamma-ray emission. One of the brightest spots, the so-called Fermi cocoon, is found in the southern lobe and was originally thought to be due to past outbursts from the Galaxy's supermassive black hole. A team analyzed data from the GAIA and Fermi space telescopes to reveal that the Fermi cocoon is actually due to emission from the Sagittarius dwarf galaxy. This satellite galaxy of the Milky Way is seen through the Fermi bubbles from our position on Earth . Due to its tight orbit around our galaxy and previous passages through the galactic disk, it has lost most of its interstellar gas and many of its stars have been ripped from its core into elongated streams. Given that the Sagittarius dwarf is completely quiescent -- it has no gas, and no stellar nurseries -- there are only a few possibilities for its gamma-ray emission, including

(1) a population of unknown millisecond pulsars or

(2) dark matter annihilations.

Millisecond pulsars are the remnants of certain types of stars, significantly more massive than the Sun, that are in close binary systems, and now blast out cosmic particles as a result of their extreme rotational energies. The electrons fired by millisecond pulsars collide with low-energy photons of the Cosmic Microwave Background, propelling them to become high-energy gamma radiation. Collaborators have convincingly demonstrated that the gamma-ray cocoon is explained by millisecond pulsars in the Sagittarius dwarf, and that the dark matter hypothesis is strongly disfavoured. This discovery sheds light on millisecond pulsars as efficient accelerators of highly energetic electrons and positrons. It also suggests that similar physical processes could be ongoing in other dwarf satellite galaxies of the Milky Way. This is highly significant because dark matter researchers have long believed that an observation of gamma rays from a dwarf satellite would be a smoking gun signature for dark matter annihilation. This study compels a reassessment of the high energy emission capabilities of quiescent stellar objects, such as dwarf spheroidal galaxies, and their role as prime targets for dark matter annihilation searches.