THE SOCIETY FOR POPULAR ASTRONOMY Electronic News Bulletin No. 473 2018 August 5

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    THE SOCIETY FOR POPULAR ASTRONOMY Electronic News Bulletin No. 473 2018 August 5
    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

    European Space Agency

    Radar data collected by ESA's Mars Express point to a pond of liquid water
    buried under layers of ice and dust in the south polar region of Mars.
    Evidence for the Red Planet's watery past is prevalent across its surface
    in the form of vast dried-out river-valley networks and gigantic outflow
    channels clearly imaged by orbiting spacecraft. Orbiters, together with
    landers and rovers exploring the Martian surface, also discovered minerals
    that can only form in the presence of liquid water. But the climate has
    changed significantly over the course of the planet's 4600-million-year
    history, and liquid water cannot exist on the surface today, so scientists
    are looking underground. Early results from the 15-year-old Mars Express
    spacecraft already found that water-ice exists at the planet's poles and is
    also buried in layers interspersed with dust. The presence of liquid water
    at the base of the polar ice caps has long been suspected; after all, from
    studies on Earth, it is well known that the melting point of water decreases
    under the pressure of an overlying glacier. Moreover, the presence of salts
    on Mars could further reduce the melting point of water and keep the water
    liquid even at below-freezing temperatures. But until now evidence from the
    'Mars Advanced Radar for Subsurface and Ionosphere Sounding' instrument,
    MARSIS, the first radar sounder ever to orbit another planet, remained
    inconclusive. It has taken the persistence of scientists working with that
    subsurface-probing instrument to develop new techniques in order to collect
    as many high-resolution data as possible to confirm their exciting
    conclusion. Ground-penetrating radar uses the method of sending radar
    pulses towards the surface and timing how long it takes for them to be
    reflected back to the spacecraft, and with what strength. The properties
    of the material that lies under the ground influences the returned signal,
    which can be used to map the sub-surface topography.
    The radar investigation shows that south polar region of Mars is made of
    many layers of ice and dust down to a depth of about 1.5 km in the 200-km-
    wide area analysed in this study. A particularly bright radar reflection
    underneath the layered deposits is identified within a 20-km-wide zone.
    Analysing the properties of the reflected radar signals and considering the
    composition of the layered deposits and expected temperature profile below
    the surface, the scientists interpret the bright feature as an interface
    between the ice and a stable body of liquid water, which could be laden with
    salty, saturated sediments. For such a patch of water to be detectable by
    MARSIS, it would need to be at least several tens of centimetres thick.
    This sub-surface anomaly on Mars has radar properties matching water or
    water-rich sediments. The finding is somewhat reminiscent of Lake Vostok,
    discovered some 4 km below the ice in Antarctica on Earth. Some forms of
    microbial life are known to thrive in the Earth's sub-glacial environments,
    but underground pockets of salty, sediment-rich liquid water on Mars might
    also provide a suitable habitat, either now or in the past. Whether life
    has ever existed on Mars remains an open question, and is one that Mars
    missions, including the current European–Russian ExoMars orbiter and future
    rover, will continue to explore. Mars Express was launched 2003 June 2 and
    is due to celebrate its 15 years in Mars orbit on December 25 this year.

    Massachusetts Institute of Technology.

    For nearly a century, astronomers have puzzled over the curious variability
    of young stars residing in the Taurus-Auriga region some 450 light-years
    away. One star in particular has drawn astronomers' attention. Every few
    decades, the star's light has faded briefly before brightening again. In
    recent years, astronomers have observed the star dimming more frequently,
    and for longer periods, raising the question: what is repeatedly obscuring
    the star? The answer, astronomers believe, could shed light on some of the
    chaotic processes that take place early in a star's development. Now
    physicists have observed the star, named RW Aur A, using NASA's Chandra
    X-Ray Observatory. They have found evidence for what may have caused its
    most recent dimming event: a collision of two infant planetary bodies, which
    produced in its aftermath a dense cloud of gas and dust. As those planetary
    debris fell into the star, they generated a thick veil, temporarily obscuring
    the star's light. Computer simulations have long predicted that planets can
    fall into a young star, but astronomers have never before observed such an
    event. If their interpretation of the data is correct, this would be the
    first time that we have directly observed a young star devouring a planet
    or planets. The star's previous dimming events may have been caused by
    similar smash-ups, of either two planetary bodies or large remnants of past
    collisions that met head-on and broke apart again. Scientists who study the
    early development of stars often look to the Taurus-Auriga Dark Clouds, a
    gathering of molecular clouds, which host stellar nurseries containing
    thousands of infant stars. Young stars form from the gravitational
    collapse of gas and dust within those clouds. Very young stars, unlike our
    comparatively mature Sun, are still surrounded by rotating discs of debris,
    including gas, dust, and clumps of material ranging in size from small dust
    grains to pebbles, and possibly to fledgling planets.
    In our Solar System, we have planets, not a massive disc around the Sun.
    Such discs last for maybe 5 million to 10 million years, and in Taurus,
    there are many stars that have already lost their discs, but a few still
    have them. If you want to know what happens in the end stages of disc
    dispersal, Taurus is one of the places to look. Astronomers focus on stars
    that are young enough still to host discs. They were particularly interest-
    ed in RW Aur A, which is at the older end of the age range for young stars,
    as it is estimated to be several million years old. RW Aur A is part of a
    binary ststem: it circles another young star, RW Aur B. Both those stars
    are about the same mass as the Sun. Since 1937, astronomers have recorded
    noticeable dips in the brightness of RW Aur A at intervals of decades. Each
    dimming event appeared to last for about a month. In 2011, the star dimmed
    again, this time for about half a year. The star eventually brightened,
    only to fade again in mid-2014. In 2016 November, the star returned to its
    full luminosity. In 2017 January, RW Aur A dimmed again, and the team used
    NASA's Chandra X-Ray Observatory to record X-ray emission from the star.
    In total, Chandra recorded almost 14 hours of X-ray data from the star.
    After analyzing those data, the researchers reported several surprising
    revelations. The star's disc hosts a large amount of material; the star
    is much hotter than expected; and the disc contains much more iron than
    expected — not as much iron as is found in the Earth, but more than in,
    say, a typical moon in our Solar System. (Our own moon, however, has far
    more iron than the scientists estimated in the star's disc.) This last
    point was the most intriguing for the team. Typically, an X-ray spectrum
    of a star can show various elements, such as oxygen, iron, silicon, and
    magnesium, and the amount of each element present depends on the temperature
    within the star's disc. Here, we see a lot more iron, at least a factor of
    10 times more than before, which is very unusual, because typically stars
    that are active and hot have less iron than others, whereas this one has
    more. Where has all that iron come from? The researchers speculate that
    the excess iron may have come from one of two possible sources. The first
    is a phenomenon known as a dust pressure trap, in which small grains or
    particles such as iron can become trapped in 'dead zones' of a disc. If the
    disc's structure changes suddenly, such as when a partner star passes close
    by, the resulting tidal forces can release the trapped particles, creating
    an excess of iron that can fall into the star. The second theory is the
    more compelling one. In that scenario, excess iron is created when two
    planetesimals, or infant planetary bodies, collide, releasing a thick cloud
    of particles. If one or both planets are made partly of iron, their
    smash-up could release a large amount of iron into the star's disc and
    temporarily obscure its light as the material falls into the star. There
    are many processes that happen in young stars, but these two scenarios could
    possibly make something that looks like what the team observed. More
    observations of the star will be needed in the future, to see whether the
    amount of iron surrounding the star has changed — a measure that could help
    researchers determine the size of the iron's source. For instance, if the
    same amount of iron appears in, say, a year, that may signal that the iron
    comes from a relatively massive source, such as a large planetary collision,
    versus if there is very little iron left in the disc. Much effort currently
    goes into learning about exoplanets and how they form, so it is obviously
    very important to see how young planets could be destroyed in interactions
    with their host stars and other young planets, and what factors determine
    whether they survive.

    University of Michigan

    Scientists have deduced that the Andromeda galaxy, our closest large
    galactic neighbour, shredded and cannibalized a massive galaxy two
    billion years ago. Even though it was mostly shredded, that massive galaxy
    left behind a rich trail of evidence: an almost invisible halo of stars
    larger than the Andromeda galaxy itself, an elusive stream of stars and a
    separate enigmatic compact galaxy, M32. Discovering and studying that
    decimated galaxy will help astronomers understand how disc galaxies like the
    Milky Way evolve and survive large mergers. The disrupted galaxy, named
    M32p, was the third-largest member of the Local Group of galaxies, after the
    Milky Way and Andromeda galaxies. Using computer models, the astronomers
    were able to piece together the evidence, revealing this long-lost sibling
    of the Milky Way. Scientists have long known that the nearly invisible
    large haloes of stars surrounding galaxies contain the remnants of smaller
    cannibalized galaxies. A galaxy like Andromeda was expected to have
    consumed hundreds of its smaller companions. Researchers thought that would
    make it difficult to learn about any single one of them. Using new computer
    simulations, the scientists were able to understand that even though many
    companion galaxies were consumed by Andromeda, most of the stars in the
    Andromeda's outer faint halo were mostly contributed by shredding a single
    large galaxy. They realized they could use that information of Andromeda's
    outer stellar halo to infer the properties of the largest of the shredded
    galaxies. Astronomers have been studying the Local Group — the Milky Way,
    Andromeda and their companions — for so long that it was rather shocking to
    realize that the Milky Way had a large sibling, and we never knew about it.
    The galaxy called M32p, which was shredded by the Andromeda galaxy, was at
    least 20 times larger than any galaxy which merged with the Milky Way over
    the course of its lifetime. M32p would have been massive, making it the
    third-largest galaxy in the Local Group after Andromeda and the Milky Way
    galaxies. The scientists say that this idea might also solve a long-
    standing mystery — the formation of Andromeda's enigmatic satellite galaxy
    M32. They suggest that the compact and dense M32 is the surviving centre of
    the Milky Way's long-lost sibling, like the indestructible stone in a plum.
    While M32 looks like a compact example of an old, elliptical galaxy, it
    actually has lots of young stars. It is one of the most compact galaxies
    in the Universe. The researchers say that their study may alter the
    traditional understanding of how galaxies evolve. They realized that the
    Andromeda's disc survived an impact with a massive galaxy, which would
    question the common wisdom that such large interactions would destroy discs
    and form an elliptical galaxy. The timing of the merger may also explain
    the thickening of the disc of the Andromeda galaxy as well as a burst of
    star formation two billion years ago, a finding which was independently
    reached by French researchers earlier this year. The Andromeda Galaxy, with
    a spectacular burst of star formation, would have looked very different two
    billion years ago.

    Hiroshima University

    Near the centre of the constellation of Cygnus is a star orbiting the first
    black hole ever discovered. Together, they form a binary system known as
    Cygnus X-1. The black hole is also one of the brightest sources of X-rays
    in the sky. However, the geometry of matter that gives rise to that light
    was uncertain. The research team revealed that information from a new
    technique called X-ray polarimetry. Taking a picture of a black hole is not
    easy. For one thing, it is not yet possible to observe a black hole because
    light cannot escape it. Rather, instead of observing the black hole itself,
    scientists can observe light coming from matter close to the black hole. In
    the case of Cygnus X-1, thst matter comes from the star that closely orbits
    the black hole. Most light that we see, such as from the Sun, vibrates in
    many directions. Polarization filters light so that it vibrates in one
    direction. That is how snow goggles with polarized lenses let skiers see
    more easily where they are going down the mountain — they work because the
    filter cuts out light reflecting off the snow. It's the same situation with
    hard X-rays around a black hole. However, hard X-rays and gamma rays coming
    from near the black hole penetrate the filter. There are no such 'goggles'
    for those rays, so we need another special kind of treatment to direct and
    measure that scattering of light. The team needed to determine where the
    light was coming from and where it was being scattered. In order to make
    both of those measurements, they launched an X-ray polarimeter on a balloon
    called PoGO+. From there, the team could piece together what fraction of
    hard X-rays reflected off the accretion disc and identify the matter shape.
    Two competing models describe how matter near a black hole can look in a
    binary system such as Cygnus X-1: the lamp-post and the extended model. In
    the lamp-post model, the corona is compact and bound closely to the black
    hole. Photons bend toward the accretion disc, resulting in more reflected
    light. In the extended model, the corona is larger and spread around the
    vicinity of the black hole. In that case, the reflected light by the disc
    is weaker. Since light did not bend that much under the strong gravity of
    the black hole, the team concluded that the black hole fitted the extended-
    corona model. With that information, the researchers can uncover more
    characteristics about black holes. One example is their spin. The effects
    of spin can modify the space-time surrounding the black hole. Spin could
    also provide clues to the evolution of the black hole. It could be slowing
    down in speed since the beginning of the Universe, or it could be accumu-
    lating matter and spinning faster.


    In 2013, ESA's Planck mission unveiled a new image of the cosmos: an all-
    sky survey of the microwave radiation produced at the beginning of the
    Universe. That first light emitted by the Universe provides a wealth of
    information about its content, its rate of expansion, and the primordial
    fluctuations in density that were the precursors of the galaxies. The
    Planck consortium has now published the full and final version of those
    data on the ESA website. With its increased reliability and its data on
    the polarisation of relic radiation, the Planck mission corroborates the
    standard cosmological model with unrivalled precision for those parameters,
    even if some anomalies still remain. For that work the Planck consortium
    called upon some three hundred researchers, in particular from CNRS, CNES
    (the French national space agency), CEA (the French Alternative Energies and
    Atomic Energy Commission) and several universities in France.
    Launched in 2009, the Planck satellite mapped the cosmic microwave back-
    ground, microwave radiation emitted 380,000 years after the Big Bang, when
    the Universe was still a hot, almost completely homogeneous gas. Tiny
    variations in its temperature provide information about its content, its
    rate of expansion and the properties of the primordial fluctuations that
    gave rise to the galaxies. An initial analysis of the data set was
    published in 2015, in the form of eight all-sky maps that included the
    polarisation of the cosmic microwave background, which determines how the
    waves that make up light vibrate on tiny scales. That key information
    bears the imprint of the last interaction between light and matter in the
    primordial Universe. However, only a preliminary analysis had been carried
    out on it.
    The polarisation of relic radiation produces a signal 50 to 100 times weaker
    than that of its temperature and 10 to 20 times weaker than that emitted by
    the polarised emission of Galactic dust. Thanks to its HFI (High Frequency
    Instrument), the Planck satellite nonetheless obtained an extremely precise
    map of primordial polarisation across the entire sky. This was a world
    first and provides us with a wealth of information. Comprehensive,
    definitive and more reliable, the data published on 2018 July 17 confirm
    the preliminary findings, supporting a model which provides an excellent
    description of the content of the Universe in terms of ordinary matter, cold
    dark matter and dark energy (whose nature is unknown), with an inflation
    phase at its very beginning. That cosmological model can now be derived
    using temperature or polarisation data independently, with comparable
    accuracy. It considerably reinforces the standard model of cosmology,
    however surprising that may be. The results are described in a set of a
    dozen scientific papers, involving around 300 researchers. However, some
    anomalies and limitations remain. In particular, the rate of expansion of
    the Universe differs by a few per cent depending on whether the data from
    the Hubble Space Telescope or from the Planck mission are used. That
    question is still an open one, and a lot of telescopes will be marshalled
    in an attempt to resolve the issue.


    Observations made with the Very Large Telescope have for the first time
    revealed the effects predicted by Einstein's general relativity on the
    motion of a star passing through the extreme gravitational field near the
    super-massive black hole in the centre of the Milky Way. That long-sought
    result represents the climax of a 26-year-long observational campaign using
    ESO's telescopes in Chile. Obscured by thick clouds of absorbing dust, the
    closest supermassive black hole to the Earth lies 26,000 light-years away at
    the centre of the Milky Way. That gravitational monster, which has a mass
    four million times that of the Sun, is surrounded by a small group of stars
    orbiting around it at high speed. That extreme environment — the strongest
    gravitational field in our galaxy — makes it the perfect place to explore
    gravitational physics, and particularly to test Einstein?s general theory of
    relativity. New infrared observations from the GRAVITY, SINFONI and NACO
    instruments on ESO's Very Large Telescope (VLT) have now allowed astronomers to follow one of the stars, called S2, as it passed very close to the
    black hole during 2018 May. At the closest point the star was at a distance
    of less than 20 billion kilometres from the black hole and moving at a
    speed of more than 25 million kilometres per hour — almost three per cent
    of the speed of light. The team compared the position and velocity measure-
    ments from GRAVITY and SINFONI respectively, along with previous observa-
    tions of S2 using other instruments, with the predictions of Newtonian
    gravity, general relativity and other theories of gravity. The new results
    are inconsistent with Newtonian predictions and in excellent agreement with
    the predictions of general relativity.
    The new measurements clearly reveal an effect called gravitational red-
    shift. Light from the star is stretched to longer wavelengths by the very
    strong gravitational field of the black hole. And the change in the
    wavelength of light from S2 agrees precisely with that predicted by
    Einstein's theory of general relativity. This is the first time that such
    a deviation from the predictions of the simpler Newtonian theory of gravity
    has been observed in the motion of a star around a supermassive black hole.
    The team used SINFONI to measure the velocity of S2 towards and away from
    the Earth and the GRAVITY instrument in the VLT Interferometer (VLTI) to make precise measurements of the changing position of S2 in order to define the
    shape of its orbit. GRAVITY creates such sharp images that it can reveal the
    motion of the star from night to night as it passes close to the black hole,
    26 000 light-years from Earth. The first observations of S2 with GRAVITY,
    about two years ago, already showed that it would be the ideal black hole
    laboratory. During the close passage, scientists could even detect the
    faint glow around the black hole on most of the images, which allowed them
    to follow the star precisely in its orbit, ultimately leading to the
    detection of the gravitational redshift in the spectrum of S2. More than
    one hundred years after he published his paper setting out the equations of
    general relativity, Einstein has been proved right once more — and in a
    much more extreme laboratory than he could have possibly imagined! Here in
    the Solar System we can only test the laws of physics now and under certain
    circumstances. So it's very important in astronomy to also check that those
    laws are still valid where the gravitational fields are very much stronger.
    Continuing observations are expected to reveal another relativistic effect
    very soon — a small rotation of the star's orbit, known as Schwarzschild
    precession — as S2 moves away from the black hole.

    SAPeople News

    South Africa has unveiled a new super radio telescope that will study galaxy
    formation, a first phase of what will be the world's largest telescope. The
    64-dish MeerKAT telescope in the Northern Cape region of South Africa will
    be integrated into a multinational Square Kilometre Array (SKA). When fully
    operational, the SKA telescope will be 50 times more powerful than current
    telescopes. The telescope will be the largest of its own kind in the world,
    with image-resolution quality exceeding the Hubble Space Telescope by a
    factor of 50. The SKA will comprise a forest of 3,000 dishes spread over
    a total area of a square kilometre across remote terrain in several African
    countries, as well as Australia, to allow astronomers to see deeper into
    space with unparalleled detail. The SKA, which is expected to be fully
    operational by 2030, will explore exploding stars, black holes and traces of
    the universe's origins some 14 billion years ago. The telescope is being
    built by an international consortium, including Australia, Britain, Canada,
    China, India, Italy, New Zealand, Sweden and the Netherlands. Other African
    countries involved are Botswana, Ghana, Kenya, Madagascar, Mauritius,
    Mozambique, Namibia and Zambia. Last month, scientists linked a powerful
    optical telescope, MeerLITCH, built 200 km south of Carnarvon, with the
    MeerKAT to allow for simultaneous optical and radio study of cosmic events
    as they occur.


    Astronomers using ALMA and NOEMA have made the first definitive detection of a radioactive molecule in interstellar space. The radioactive part of the
    molecule is an isotope of aluminium. The observations reveal that the
    isotope was dispersed into space after the collision of two stars, that left
    behind a remnant known as CK Vulpeculae. This is the first time that a
    direct observation has been made of that element from a known source.
    Previous identifications of that isotope have come from the detection of
    gamma rays, but their precise origin had been unknown. The team used the
    Atacama Large Millimeter/submillimeter Array (ALMA) and the NOrthern
    Extended Millimeter Array (NOEMA) to detect a source of the radioactive
    isotope aluminium-26. The source, known as CK Vulpeculae, was first seen
    in 1670 and at the time it appeared to observers as a bright, red 'new
    star'. Though initially visible with the naked eye, it quickly faded and
    now requires powerful telescopes to see it, a dim central star surrounded
    by a halo of glowing material flowing away from it. 348 years after the
    initial event was observed, the remains of this explosive stellar merger
    have led to the clear and convincing signature of a radioactive version of
    aluminium, known as aluminium-26. This is the first unstable radioactive
    isotope definitively detected outside the Solar System. Unstable isotopes
    have an excess of nuclear energy and eventually decay into a stable form.
    The team detected the unique spectral signature of molecules made up of
    aluminium-26 and fluorine (26AlF) in the debris surrounding CK Vulpeculae,
    which is about 2000 light-years away. As these molecules spin and tumble
    through space, they emit a distinctive fingerprint of millimetre-wavelength
    light, a process known as rotational transition. Astronomers consider that
    to be the 'gold standard' for detections of molecules. The observation of
    that particular isotope provides fresh insights into the merger process that
    created CK Vulpeculae. It also demonstrates that the deep, dense, inner
    layers of a star, where heavy elements and radioactive isotopes are forged,
    can be churned up and cast out into space by stellar collisions.
    The astronomers also determined that the two stars that merged were of
    relatively low mass, one being a red giant star with a mass somewhere
    between 0.8 and 2.5 times that of our Sun. Being radioactive, aluminium-26
    will decay to become more stable, and in that process one of the protons in
    the nucleus decays into a neutron. During the process, the excited nucleus
    emits a photon with very high energy, which we observe as a gamma ray.
    Previously, detections of gamma-ray emissions have shown that around two
    solar masses of aluminium-26 are present across the Milky Way, but the
    process that created the radioactive atoms was unknown. Furthermore, owing
    to the way that gamma rays are detected, their precise origin was also
    largely unknown. At the same time, however, the team has concluded that
    the production of aluminium-26 by objects similar to CK Vulpeculae is
    unlikely to be the major source of aluminium-26 in the Milky Way. The mass
    of aluminium-26 in CK Vulpeculae is roughly a quarter of the mass of Pluto,
    and given that these events are so rare, it is highly unlikely that they are
    the sole producers of the isotope in the Milky Way galaxy. This leaves the
    door open for further studies into such radioactive molecules.

    Bulletin compiled by Clive Down
    (c) 2018 The Society for Popular Astronomy
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