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August 18, 2019 at 8:54 am #9900Anonymous
THE SOCIETY FOR POPULAR ASTRONOMY Electronic News Bulletin No. 496 2019 August 18
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
DEAD PLANETS CAN “BROADCAST” FOR A BILLION YEARS
Scientists have determined the best candidate remnant stars to search for relics of planets, based upon the likelihood of the stars hosting surviving planetary cores and the strength of the radio signal that we can tune in to. The research assesses the survivability of planets that orbit white dwarfs, stars which have burnt all of their fuel and shed their outer layers, destroying nearby objects and removing the outer layers of planets. They have determined that the cores which result from such destruction may be detectable and could survive for long enough to be found from Earth. The first exoplanet confirmed to exist was discovered in the 1990s orbiting a pulsar, using a method that detects radio waves emitted from the star. The
researchers plan to observe white dwarfs in a similar part of the electromagnetic spectrum in the hope of achieving another breakthrough. The magnetic field between a white dwarf and an orbiting planetary core can form a unipolar inductor circuit, with the core acting as a conductor due to its metallic constituents. Radiation from that circuit is emitted as radio waves which can then be detected by radio telescopes on Earth. The effect can also be detected from Jupiter and its moon Io, which form a circuit of their own. However, the scientists needed to determine how long those cores can survive after being stripped of their outer layers. Their modelling revealed that in a number of cases, planetary cores can survive for over 100 million years and as long as a billion years.
The astronomers plan to use the results in proposals for observation time on telescopes such as Arecibo in Puerto Rico and the Green Bank Telescope in West Virginia to try to find planetary cores around white dwarfs. There is a sweet spot for detecting such planetary cores: a core too close to the white dwarf would be destroyed by tidal forces, and a core too far away would not be detectable. Also, if the magnetic field is too strong, it would push the core into the white dwarf, destroying it. Hence, we should only look for planets around those white dwarfs with weaker magnetic fields at a separation between about 3 solar radii and the Mercury–Sun distance. Nobody has ever found just the bare core of a major planet before, nor a major planet only through monitoring magnetic signatures, nor a major planet around a white dwarf. Therefore, a discovery here would represent “firsts” in three different senses for planetary systems. Astronomers will use the results of this work as guidelines for designs of radio searches for planetary cores around white dwarfs. Given the existing evidence for a
presence of planetary debris around many of them, we think that our chances for exciting discoveries are quite good. A discovery would also help reveal the history of these star systems, because for a core to have reached that stage it would have been violently stripped of its atmosphere and mantle at some point and then thrown towards the white dwarf. Such a core might also provide a glimpse into our own distant future, and how the solar system will
NEW TYPE OF PULSATING STAR DISCOVERED
Astronomers have just discovered a type of very small, very hot star that brightens and dims every few minutes as its outer layers try to maintain equilibrium. The stars have been named hot subdwarf pulsators, and they could be related to another type of rare and mysterious recently discovered star: the blue large-amplitude pulsator. Many stars pulsate, even our Sun
does on a very small scale. Those with the largest brightness changes are usually radial pulsators, 'breathing' in and out as the entire star changes size. But even though our Sun pulsates, its cycle is 11 years, and it only varies in brightness by 0.1 percent over that time frame, so it wouldn't be considered a pulsator. The brightness of pulsators can vary by up to 10 per cent owing to changes in size and temperature. The four new stars the team identified in data from the Zwicky Transient Facility survey pulsate on time-scales between 200 and 475 seconds, varying in brightness by around 5 per cent. Such a change in brightness can be produced by eclipsing binaries, so they needed to be ruled out before the stars could be classed as a new type. Once the research team had done that, they realised they could be looking at a new class of subdwarf B stars.
Subdwarf B stars are interesting. They are tiny for stars — maybe 10 percent of the size of the Sun. But they are dense, too. Into that small diameter, they squeeze between 20 and 50 per cent of the Sun's mass. They burn very hot, towards the blue end of the spectrum, between 20,000 and 40,000 Kelvin. So they're also very bright. It's thought that they form along the evolutionary path of a star up to eight times the mass of the Sun as it dies. When these stars run out of hydrogen to fuse in their cores, they start fusing helium, ballooning out into red giants. A subdwarf B star is what happens when the outer hydrogen layers of a red giant are stripped away before helium fusion begins – possibly by a binary companion, but the exact mechanisms are unknown. So there you have a tiny, hot, dense blue star. And some of them do pulsate. The V361 Hya class have a pressure oscillation mode, which means their pulsations are produced by internal pressure fluctuations in the star. The V1093 Her class are gravity-mode pulsators, produced by gravity waves (not to be confused with gravitational waves). The researchers are still looking into the exact mechanism behind the oscillations of hot subdwarf pulsators, but believe it may be unstable radial modes produced by something called the iron kappa mechanism, whereby a buildup of iron in the star produces an energy layer that results in a pulsation. They also believe another difference could be what's happening in their cores. Subdwarf B stars are generally considered to be fusing helium,
either in their core, or a shell around the core. But the researchers believe that hot subdwarf pulsators lost their outer material before the helium was hot and dense enough for fusion. They also found that the pulsation resembles that of blue large-amplitude pulsators, a type of star just discovered in 2017. That means that the two types of stars could be related.
HOT WIND DETECTED NEAR BLACK HOLE
University of Southampton
Astrophysicists have detected a very hot, dense outflowing wind close to a black hole at least 25,000 light-years from Earth. The gas (ionised helium and hydrogen) was emitted in bursts which repeated every 8 minutes, the first time this behaviour has been seen around a black hole. The object was Swift J1357.2-0933 which was first discovered as an X-ray transient — a
system that exhibits violent outbursts — in 2011. Such transients all consist of a low-mass star, similar to our Sun, and a compact object, which can be a white dwarf, neutron star or black hole. I this case, Swift J1357.2-0933 has a black hole compact object which is at least 6 times the mass of our Sun. Material from the normal star is pulled by the compact object into a disc in between the two. Massive outbursts occur when the material in the disc becomes hot and unstable and it releases copious amounts of energy. What was particularly unusual about this system was that ground-based telescopes had revealed that its optical brightness displayed
periodic dips in its output and that the period of these dips slowly changed from around 2 minutes to about 10 minutes as the outburst evolved. Such strange behaviour has never been seen in any other object. The cause of these remarkable, fast dips has been a hot topic of scientific debate ever since their discovery. So it was with great excitement that astronomers
greeted the second outburst of this object in mid-2017, presenting an opportunity to study its strange behaviour in greater detail. Scientists recognised that key to getting the answer was to obtain optical spectra a number of times during each dip cycle, essentially studying how the colour changed with time. But with the object about 10,000 times fainter than the faintest star visible to the naked eye and the dip period of only around 8 minutes, a very big telescope had to be used. So, they used SALT, the Southern African Large Telescope, the largest optical telescope in the southern hemisphere.
Not only does SALT have the necessary huge collecting area (it has a 10m diameter mirror), but it is operated in a 100% queue-scheduled way by resident staff astronomers, meaning that it can readily respond to unpredictable transient events. This was perfect for Swift J1357.2-0933, and SALT obtained more than an hour of spectra, with one taken every 100 seconds. The results from these spectra were stunning. They showed ionised helium in absorption, which had never been seen in such systems before. This indicated that it must be both dense and hot — around 40,000 degrees. More remarkably, the spectral features were blue-shifted (due to the Doppler effect), indicating that they were blowing towards us at about 600km/s. But
what was really astonishing was the discovery that these spectral features were visible only during the optical dips in the light-curve. We have interpreted this quite unique property as due to a warp or ripple in the inner accretion disc that orbits the black hole on the dipping timescale. The warp is very close to the black hole at just 1/10 the radius of the disc. What is driving this matter away from the black hole? It is almost certainly the radiation pressure of the intense X-rays generated close to the black hole. But it has to be much brighter than we see directly, suggesting that the material falling on to the black hole obscures it from direct view, like clouds obscuring the Sun. This occurs because we happen to be viewing the binary system from a vantage point where the disc appears edge-on and rotating blobs in this disc obscure our view of the central black hole. Interestingly there are no eclipses by the companion star seen in either the optical or X-ray as might be expected. This is explained by it
being very small, and constantly in the shadow of the disc. This inference comes from detailed theoretical modelling of winds being blown off accretion discs that was undertaken using supercomputer calculations. This object has remarkable properties amongst an already interesting group of objects that have much to teach us about the end-points of stellar evolution and the formation of compact objects. We already know of a couple of dozen black
hole binary systems in our Galaxy, which all have masses in the 5-15 solar mass range, and the single black hole at our Galactic Centre is around 4 million solar masses. They all grow by the accretion of matter that we have witnessed so spectacularly in this object. We also know that a substantial fraction of the accreting material is being blown away. When that happens
from the supermassive black holes at the centres of galaxies, those powerful winds and jets can have a huge impact on the rest of the galaxy.
TRUE COLOURS OF QUASARS
Astronomers have identified a rare moment in the life of some of the Universe's most energetic objects. Quasars were first observed 60 years ago, but their origins still remain a mystery. Now researchers have observed what they suggest is a “brief transition phase” in the development of these galactic giants that could shed light on how quasars and their host galaxies evolve. Quasars are powered by the energy from supermassive black holes at their centres as they feed on surrounding gases. They are thousands of times brighter than galaxies like our Milky Way and the majority are blue in colour. However, a significant number are red as they are viewed through huge clouds of dust and gas that obscure them from view. The conventional view of red quasars is that they are actually blue quasars that are angled
away from our line-of-sight. Instead, the team has ruled this model out and have shown that red quasars are likely to be the result of a brief, but violent, phase in the evolution of galaxies when the black hole ejects a large amount of energy into the surrounding clouds of dust and gas. This injection of energy blows away the dust and gas to reveal a blue quasar.
Observations using radio telescopes support this theory by showing that black holes at the centre of red quasars produce a greater amount of radio emission than those at the centre of blue quasars. How quasars develop has been the cause of significant uncertainty. What the results suggest is that quasars undergo a brief transition phase, changing colour from red to blue, when they emerge from the deep shroud of dust and gas surrounding them. Astronomers believe we are seeing a rare but important step in the life of these galactic beasts during galaxy evolution when their black holes are starting to shape their environments. The researchers studied 10,000 red and blue quasars as they would have been seen seven to 11 billion years ago when the Universe was relatively young using archival data from the Sloan
Digital Sky Survey and the Very Large Array radio astronomy observatory. They say their research could also tell us more about galaxy evolution. They expect that during this transition phase the energy from the super-massive black hole will burn off the gas needed to form stars. Without the gas the galaxy cannot continue to grow, so what we are possibly seeing
is the start of a quasar effectively ending the life of the galaxy by destroying the very thing it needs to survive. The researchers say the next step in their research is to use more in-depth data to understand the finer details of this transition phase.
ANCIENT MASSIVE GALAXIES
University of Tokyo
Astronomers used the combined power of multiple astronomical observatories around the world and in space to discover a treasure-trove of previously unknown ancient massive galaxies. This is the first multiple discovery of its kind and such an abundance of this type of galaxy defies current models of the Universe. These galaxies are also intimately connected with supermassive black holes and the distribution of dark matter. The Hubble Space Telescope gave us unprecedented access to the previously unseen Universe, but even it is blind to some of the most fundamental pieces of the cosmic puzzle. Astronomers wanted to see some things they long suspected may be out there but which Hubble could not show them. Newer generations of astronomical observatories have finally revealed what they sought. This is the first time that such a large population of massive galaxies was confirmed during the first 2 billion years of the 13.7-billion-year life of the Universe. These were previously invisible to us. This finding contravenes current models for that period of cosmic evolution and will help
to add some details, which have been missing until now. But how can something as big as a galaxy be invisible to begin with? The light from these galaxies is very faint with long wavelengths invisible to our eyes and undetectable by Hubble. So astronomers turned to the Atacama Large Millimeter/submillimeter Array (ALMA), which is ideal for viewing these kinds of things. Even though these galaxies were the largest of their time, the light from them is not only weak but also stretched due to their immense distance. As the Universe expands, light passing through becomes stretched, so visible light becomes longer, eventually becoming infrared. The amount of stretching allows astronomers to calculate how far away something is, which also tells you how long ago the light you're seeing was emitted from the thing in question.
It was hard to convince others these galaxies were as old as suspected. Initial suspicions about their existence came from the Spitzer Space Telescope's infrared data but ALMA has sharp eyes and revealed details at submillimetre wavelengths, the best wavelength to peer through dust present in the early Universe. Even so, it took further data from the Very Large
Telescope in Chile to really prove we were seeing ancient massive galaxies where none had been seen before. Another reason these galaxies appear so weak is because larger galaxies, even in the present day, tend to be shrouded in dust, which obscures them more than their smaller galactic siblings. And what does the discovery of these massive galaxies imply? The
more massive a galaxy, the more massive the supermassive black hole at its heart. So the study of these galaxies and their evolution will tell us more about the evolution of supermassive black holes, too. Massive galaxies are also intimately connected with the distribution of invisible dark matter. This plays a role in shaping the structure and distribution of galaxies. Theoretical researchers will need to update their theories now. What's also interesting is how these 39 galaxies are different from our own. If our solar system were inside one of them and you were to look up at the sky on a clear night, you would see something quite different to the
familiar pattern of the Milky Way. For one thing, the night sky would appear far more majestic. The greater density of stars means there would be many more stars close by appearing larger and brighter. But conversely, the large amount of dust means farther-away stars would be far less visible, so the background to these bright close stars might be a vast dark void. As this is the first time such a population of galaxies has been discovered, the implications of their study are only now being realized. There may be many surprises yet to come.
A NEW LENS FOR SPACE TELESCOPES
University of Arizona
U.S astronomers are currently fabricating mirrors for the largest and most advanced earth-based telescope: the Giant Magellan Telescope. But there are size constraints, ranging from the mirror's own weight, which can distort images, to the size of freeways and underpasses that are needed to transport finished pieces. Such giant mirrors are reaching their physical limits. Scientists are developing a new technology to replace mirrors in space telescopes. If they succeed, they will be able to vastly increase the light-collecting power of telescopes, and among other science, study the atmospheres of 1,000 potentially earth-like planets for signs of life. The researchers intend to deploy a fleet of 35 14-metre-wide spherical telescopes, each individually more powerful than the Hubble Space Telescope. Each unit will contain an 8.5-metre diameter lens, which will be used for astronomical observations. When combined, the telescope array will be powerful enough to characterize 1,000 extrasolar planets from as far away as 1,000 light years. Even NASA's most ambitious space telescope missions are
designed to study a handful of potentially Earth-like extrasolar planets. The Hubble mirror is 2.4 meters in diameter and the James Webb Space Telescope mirror is 6.5 meters in diameter. Both were designed for different purposes and before exoplanets were even discovered.
Telescope mirrors collect light — the larger the surface, the more starlight they can catch. But no one can build a 50-metre mirror. So scientists came up with Nautilus, which relies on lenses, and instead of building an impossibly huge 50-metre mirror, they plan on building a whole array of identical smaller lenses to collect the same amount of light. The lenses were inspired by lighthouse lenses — large but lightweight — and include additional tweaks such as precision carving with diamond-tipped tools. The patented design, which is a hybrid between refractive and diffractive lenses, make them more powerful and suitable for planet hunting.
Because the lenses are lighter than mirrors, they are less expensive to launch into space and can be made quickly and cheaply using a mould. They are also less sensitive to misalignments, making telescopes built with this technology much more economical. Nautilus telescopes also don't require any fancy observing technique. In the last few decades, computers, electronics and data-collection instruments have all become smaller, cheaper, faster and more efficient. Mirrors, on the other hand, are exceptions to this growth as they haven't seen big cost reductions.
DARK MATTER MAY BE OLDER THAN BIG BANG
Johns Hopkins University
Dark matter, which researchers believe make up about 80% of the Universe's mass, is one of the most elusive mysteries in modern physics. What exactly it is and how it came to be is a mystery, but a new Johns Hopkins University study now suggests that dark matter may have existed before the Big Bang. The study revealed a new connection between particle physics and astronomy. If dark matter consists of new particles that were born before the Big Bang, they affect the way galaxies are distributed in the sky in a unique way. This connection may be used to reveal their identity and make conclusions about the times before the Big Bang too. While not much is known about its origins, astronomers have shown that dark matter plays a
crucial role in the formation of galaxies and galaxy clusters. Though not directly observable, scientists know dark matter exists by its gravitation effects on how visible matter moves and is distributed in space. For a long time, researchers believed that dark matter must be a leftover substance from the Big Bang. Researchers have long sought this kind of dark matter, but so far all experimental searches have been unsuccessful. If dark matter were truly a remnant of the Big Bang, then in many cases researchers should have seen a direct signal of dark matter in different particle physics experiments already.
Using a new, simple mathematical framework, the study shows that dark matter may have been produced before the Big Bang during an era known as the cosmic inflation when space was expanding very rapidly. The rapid expansion is believed to lead to copious production of certain types of particles called scalars. So far, only one scalar particle has been discovered, the famous Higgs boson. We do not know what dark matter is, but if it has anything to do with any scalar particles, it may be older than the Big Bang. With the proposed mathematical scenario, we don't have to assume new types of interactions between visible and dark matter beyond gravity, which we already know is there. While the idea that dark matter existed before the Big Bang is not new, other theorists have not been able to come up with calculations that support the idea. The new study shows that researchers have always overlooked the simplest possible mathematical scenario for dark matter's origins. The new study also suggests a way to test the origin of dark matter by observing the signatures dark matter leaves on the distribution of matter in the Universe. While this type of dark matter is too elusive to be found in particle experiments, it can reveal its presence in astronomical observations. We will soon learn more about the origin of dark matter when the Euclid satellite is launched in 2022. It's going to be very exciting to see what it will reveal about dark matter and if its findings can be used to peak into the times before the Big Bang.
Bulletin compiled by Clive Down
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