SOCIETY for POPULAR ASTRONOMY Electronic News Bulletin No. 456 2017 November 5

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    SOCIETY for POPULAR ASTRONOMY Electronic News Bulletin No. 456 2017 November 5
    Here is the latest round-up of news from the Society for Popular

    Astronomy. The SPA is arguably Britain's liveliest astronomical
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    NASA has authorized a second extension of the Dawn mission at Ceres, the
    largest object in the asteroid belt between Mars and Jupiter. During
    this extension, the spacecraft, which has been orbiting Ceres since 2015
    March, will descend to lower altitudes than ever before. The spacecraft
    will continue at Ceres for the remainder of its investigation and will
    remain in a stable orbit indefinitely after its hydrazine fuel runs out.
    The Dawn flight team is studying ways to manoeuvre Dawn into a new
    elliptical orbit, which may take the spacecraft to less than 200 km from
    the surface of Ceres at closest approach. Previously, Dawn's lowest
    altitude was 385 km. A priority of the second Ceres mission extension
    is collecting data with Dawn's gamma-ray and neutron spectrometer,
    which measures the number and energy of gamma rays and neutrons. That
    information is important for understanding the composition of Ceres'
    uppermost layer and how much ice it contains. The spacecraft will also
    take visible-light images of Ceres' surface with its camera, as well as
    measurements of Ceres' mineralogy with its visible and infrared mapping
    The extended mission additionally allows Dawn to be in orbit while Ceres
    goes through perihelion, its closest approach to the Sun, which will
    occur in 2018 April. At closer proximity to the Sun, more ice on Ceres'
    surface may turn to water vapour, which may in turn contribute to the
    weak transient atmosphere detected by ESA's Herschel space observatory
    before Dawn's arrival. Building on Dawn's findings, the team has
    hypothesized that water vapour may be produced in part from energetic
    particles from the Sun interacting with ice at shallow depths in Ceres'
    surface. Scientists will combine data from ground-based observatories
    with Dawn's observations to study these phenomena further as Ceres
    approaches perihelion.


    A small, recently discovered asteroid — or perhaps a comet — appears
    to have originated from outside the Solar System, coming from somewhere
    else in our Galaxy. If so, it would be the first 'interstellar object'
    to be observed and confirmed by astronomers. The unusual object — for
    now designated A/2017 U1 — is less than 400 metres in diameter and is
    moving remarkably fast. Astronomers are using telescopes around the
    world and in space to observe this notable object, in an effort to learn
    about the origin and possibly composition of the object. A/2017 U1 was
    discovered on October 19 by the University of Hawaii's Pan-STARRS 1
    telescope during the course of its nightly search for near-Earth objects.
    Astronomers immediately realized that it was an unusual object. Its
    motion could not be explained as either a normal Solar-System asteroid
    orbit or a comet orbit. Combined data from follow-up images taken at
    the ESA telescope on Tenerife proved that the object came from outside
    the Solar System. It is the most extreme orbit that NASA scientists
    have ever seen. The object is moving extremely fast, and on such a
    trajectory that we can say with confidence that it is on its way out of
    the Solar System and will not come back. The team plotted the object's
    current trajectory and even looked into its future. A/2017 U1 came from
    the direction of the constellation Lyra, cruising through interstellar
    space at a brisk 25.5 km/s.
    The object approached the Solar System from almost directly 'above' the
    ecliptic, the approximate plane in space where the planets and most
    asteroids orbit the Sun, so it did not have close encounters with any
    of the major planets during its plunge toward the Sun. On Sept. 2, the
    small body crossed under the ecliptic plane just inside Mercury's orbit
    and then made its closest approach to the Sun on Sept. 9; answering to
    the Sun's gravity, it made a hairpin turn under our Solar System,
    passing under the Earth's orbit on Oct. 14 at a distance of about 24
    million kilometres — about 60 times the distance to the Moon. It has
    now shot back up above the plane of the planets and, travelling at
    44 km/s with respect to the Sun, is speeding toward the constellation
    Pegasus. Astronomers have long suspected that such objects should
    exist, because during the process of planet formation a lot of material
    should be ejected from planetary systems. What is surprising is that we
    have never seen interstellar objects pass through before. Since this is
    the first object of its type ever discovered, rules for naming such
    objects will need to be established by the International Astronomical


    The Cassini spacecraft ended its journey on Sept. 15 with an intentional
    plunge into the atmosphere of Saturn, but analysis continues on the
    mountain of data the spacecraft sent during its long 'life'. The
    spacecraft's Ion and Neutral Mass Spectrometer (INMS) returned a lot
    of direct measurements of the components in Saturn's upper atmosphere,
    which stretches almost to the rings. From those observations, the team
    sees evidence that molecules from the rings are raining down onto the
    atmosphere. That influx of material from the rings was expected, but
    INMS data show hints of ingredients more complex than just water, which
    makes up the bulk of the rings' composition. In particular, the
    instrument detected methane, a volatile molecule that scientists would
    not expect to be abundant in the rings or found so high in Saturn's
    Chief among the questions that scientists hope to answer by using data
    from Cassini is the age and origin of the rings. Theoretical modelling
    has shown that, without forces to confine them, the rings would spread
    out over hundreds of millions of years — much younger than Saturn
    itself. Such spreading happens because faster-moving particles that
    orbit closer to Saturn occasionally collide with slower particles on
    slightly farther-out orbits. When that happens, some momentum from the
    faster particles is transferred to the slower particles, speeding the
    latter up in their orbit and causing them to move farther out. The
    inverse happens to the faster, inner particles. Previous research had
    shown that gravitational tugs from the moon Mimas are solely responsible
    for halting the outward spread of Saturn's B ring — that ring's outer
    edge is defined by the dark region known as the Cassini Division. Ring
    scientists had thought that the small moon Janus was responsible for
    confining the outer edge of the A ring, but a new modelling study shows
    that the A ring's outward creep is kept in check by a confederation of
    moons, including Pan, Atlas, Prometheus, Pandora, Janus, Epimetheus and


    A 'citizen scientist' was the first to detect tell-tale signs that a
    comet was orbiting a distant star monitored by the Kepler space
    observatory. The discovery marks the first time that the presence of an
    object as small as a comet has been inferred by observing dips in the
    intensity of light from a star. Such dips usually signal crossings of
    planets or other objects in front of the star, which briefly block a
    small fraction of its light. In this case things were different: the
    researchers were able to pick out the comet's tail, a trail of gas and
    dust, which blocked about 0.1 per cent of the star's light as the comet
    streaked by. The data came from the Kepler space telescope, a stellar
    observatory that was launched in 2009. For four years, the spacecraft
    monitored about 200,000 stars for dips in brightness caused by transit-
    ing exoplanets. To date, the mission has identified and confirmed more
    than 2,400 exoplanets, mostly orbiting stars in the constellation
    Cygnus, with the help of automated algorithms that quickly sift through
    the data, looking for the characteristic dips. The smallest exoplanets
    detected thus far measure about one-third the diameter of the Earth.
    Comets, in comparison, are only the size of a small city at their
    largest, making them much more difficult to detect. But on March 18
    this year Thomas Jacobs, an amateur astronomer who makes it his hobby to
    comb through Kepler's data, was able to pick out several curious light
    patterns amid the noise. Jacobs is part of the 'Planet Hunters'
    citizen-scientist project established by Yale University, which enlists
    amateur astronomers in the search for exoplanets. The idea was that the
    human eye might be able to notice things that a computer would miss.
    Astronomers could name 10 types of objects that those people have found
    in the Kepler data but that algorithms could not find, because of the
    pattern-recognition capability of the human eye. During the search, the
    amateur observed three unusual dips in the light coming from KIC
    3542116, a faint star located 800 light-years away – he flagged the
    events and alerted a professional astronomer with whom he had collab-
    orated in the past to interpret his findings. A further three transits
    were subsequently found. The asymmetry in the light curves resembled
    disintegrating planets, with long trails of debris that would continue
    to block a bit of light as the planet moves away from the star.
    However, such disintegrating planets orbit their star, transiting
    repeatedly. In contrast, no such periodic pattern had been observed in
    the transits identified. The only kind of body that could do the same
    thing and not repeat is one that probably gets destroyed in the end. In
    other words, instead of repeatedly orbiting the star, the objects must
    have transited, then ultimately flown too close to the star, and
    vaporised. The only thing that fits the bill, and has a small enough
    mass to be destroyed, is a comet.


    Astronomers recently discovered that spots on the surface of a super-
    giant star are driving huge spiral structures in its stellar wind.
    Massive stars are responsible for producing the heavy elements that are
    included in life on Earth. At the end of their lives they scatter the
    material into interstellar space in catastrophic explosions called
    supernovae – without those dramatic events, our Solar System would never
    have formed. Zeta Puppis is an evolved massive supergiant star. It is
    about sixty times the mass of the Sun, and seven times hotter at the
    surface. Massive stars are rare, and usually found in binary or
    multiple systems. Zeta Puppis is special however, because it is a
    single massive star, moving through space alone, at a velocity of about
    60 km/s. One theory is that Zeta Puppis has interacted with a binary or
    a multiple system in the past, and been thrown out into space at an
    incredible velocity. Using a network of nano-satellites from the
    BRIght Target Explorer (BRITE) space mission, astronomers monitored
    the brightness of the surface of Zeta Puppis over a six-month period,
    and simultaneously monitored the behaviour of its stellar wind from
    several ground-based professional and amateur observatories.

    The observations revealed a repeated pattern every 1.78 days, both at
    the surface of the star and in the stellar wind. The periodic signal
    turns out to reflect the rotation of the star through bright spots tied
    to its surface, which are driving large-scale spiral-like structures in
    the wind, dubbed 'co-rotating interaction regions' or CIRs. By studying
    a spectral line of ionized helium in the star's wind, astronomers
    clearly saw some 'S' patterns caused by areas of CIRs induced in the wind
    by the bright surface spots. In addition to the 1.78-day periodicity,
    the research team also detected random changes on time-scales of hours
    at the surface of Zeta Puppis, strongly correlated with the behaviour of
    small regions of higher density in the wind (known as clumps) that
    travel outwards from the star. The results are exciting because there
    is evidence, for the first time, of a direct link between surface varia-
    tions and wind clumping, both random in nature. After several decades
    of puzzling over the potential link between the surface variability of
    very hot massive stars and their wind variability, these results are a
    significant breakthrough in massive-star research, essentially owing to
    the BRITE nanosats and the large contribution by amateur astronomers.
    The physical origins of the bright surface spots and the random
    brightness variations discovered in Zeta Puppis remain unknown at this
    point, and will be the subject of further investigations, probably
    requiring many more observations by space observatories, large ground-
    based facilities, and small telescopes alike.

    ESA/Hubble Information Centre

    The Hubble space telescope has observed for the first time the source
    of a gravitational wave, created by the merger of two neutron stars.
    The merger created a kilonova — an object predicted by theory decades
    ago — that ejects heavy elements such as gold and platinum into space.
    The event also provides the strongest evidence yet that short-duration
    gamma-ray bursts are caused by mergers of neutron stars. This discovery
    is the first glimpse of multi-messenger astronomy, bringing together
    gravitational waves and electromagnetic radiation. On 2017 August 17
    the Laser Interferometer Gravitational-Wave Observatory (LIGO) and
    the Virgo Interferometer both alerted astronomical observers all round
    the world about the detection of a gravitational-wave event named
    GW170817. About two seconds after the detection of the gravitational
    wave, ESA's INTEGRAL telescope and NASA's Fermi gamma-ray space
    telescope observed a short gamma-ray burst from the same direction.
    In the night following the initial discovery, a fleet of telescopes
    started a hunt to locate the source of the event. Astronomers found
    it in the lenticular galaxy NGC 4993, about 130 million light-years
    away. A point of light was shining where nothing was visible before,
    and that set off one of the largest multi-telescope observing campaigns
    ever — among them the Hubble space telescope. Several different teams
    of scientists used Hubble over the fortnight following the gravitational
    -wave event alert to observe NGC 4993. Using Hubble's high-resolution
    imaging capabilities they managed to get the first observational proof
    for a kilonova, the visible counterpart of the merging of two extremely
    dense objects — most likely two neutron stars. Such mergers were first
    suggested more than 30 years ago, but this marks the first reasonably
    firm observation of such an event. The distance to the merger makes the
    source both the closest gravitational-wave event detected so far and
    also one of the closest gamma-ray burst sources ever seen.
    Hubble captured images of the galaxy in visible and infrared light,
    witnessing a new bright object within NGC 4993 that was brighter than a
    nova but fainter than a supernova. The images showed that the object
    faded noticeably over the six days of the Hubble observations. Using
    Hubble's spectroscopic capabilities the teams also found indications of
    material being ejected by the kilonova as fast as one-fifth of the speed
    of light. Astronomers were surprised at just how closely the behaviour
    of the kilonova matched predictions. It looked nothing like known
    supernovae, and so confidence was soon very high that this really was a
    kilonova. Connecting kilonovae and short gamma-ray bursts to neutron-
    star mergers has so far been difficult, but with the multitude of
    detailed observations following the detection of the gravitational-wave
    event GW170817 has now finally verified the connection. The spectrum
    of the kilonova looked exactly the way that theoretical physicists had
    predicted the outcome of the merger of two neutron stars would appear.
    It ties this object to the gravitational-wave source beyond all
    reasonable doubt. The infrared spectra taken by Hubble also showed
    several broad features that could be identified with some of the
    heaviest elements in nature. The observations may accordingly help to
    solve another long-standing question in astronomy — the origin of heavy
    chemical elements, like gold and platinum. In the merger of two neutron
    stars, the conditions appear to be just right for their production.
    Now, astronomers won't just look at the light from an object, as we've
    done for hundreds of years, but also listen to it. Gravitational waves
    provide us with complementary information about objects which are very
    hard to study by observations only of electromagnetic waves. So pairing
    gravitational waves with electromagnetic radiation will help astronomers
    to understand some of the most extreme events in the Universe.

    ESA/Hubble Information Centre

    Using the Hubble space telescope, astronomers have discovered that the
    brightest galaxies within galaxy clusters 'wobble' relatively to the
    clusters' centres of mass. That unexpected result is inconsistent with
    predictions made by the current standard model of dark matter. With
    further analysis it may provide insights into the nature of dark matter,
    perhaps even indicating that new physics is at work. Dark matter
    constitutes just over 25 per cent of all matter in the Universe but
    cannot be directly observed, making it one of the biggest mysteries in
    modern astronomy. Invisible haloes of such elusive dark matter enclose
    galaxies and galaxy clusters alike. The latter are massive groupings of
    up to a thousand galaxies immersed in hot intergalactic gas. Such
    clusters have very dense cores, each containing a massive galaxy called
    the 'brightest cluster galaxy' (BCG). The standard model of dark
    matter (cold dark matter model) predicts that once a galaxy cluster has
    returned to a 'relaxed' state after experiencing the turbulence of a
    merging event, the BCG does not move from the cluster's centre. It is
    held in place by the enormous gravitational influence of dark matter.
    But now, a team of Swiss, French, and British astronomers has analysed
    ten galaxy clusters observed with the Hubble telescope, and found that
    their BCGs are not fixed at the centre as expected. The Hubble data
    indicate that they are 'wobbling' around the centre of mass of each
    cluster long after the galaxy cluster has returned to a relaxed state
    following a merger. In other words, the centre of the visible parts of
    each galaxy cluster and the centre of the total mass of the cluster —
    including its dark matter halo — are offset, by as much as 40,000
    That indicates that, rather than a dense region in the centre of the
    galaxy cluster, as predicted by the cold dark matter model, there is a
    much shallower central density. That is a striking signal of exotic
    forms of dark matter right at the heart of galaxy clusters. The
    wobbling of the BCGs could be analysed only because the galaxy clusters
    studied also act as gravitational lenses. They are so massive that they
    warp space-time enough to distort light from more distant objects behind
    them. That effect, called strong gravitational lensing, can be used to
    make a map of the dark matter associated with the cluster, enabling
    astronomers to work out the exact position of the centre of mass and
    then measure the offset of the BCG from that centre. If the 'wobbling'
    is not an unknown astrophysical phenomenon and is in fact the result of
    the behaviour of dark matter, then it is inconsistent with the standard
    model of dark matter and can only be explained if dark-matter particles
    can interact with each other — a strong contradiction to the current
    understanding of dark matter. That may indicate that new fundamental
    physics is required to solve the mystery of dark matter.

    Bulletin compiled by Clive Down
    (c) 2017 The Society for Popular Astronomy

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