THE SOCIETY FOR POPULAR ASTRONOMY Electronic News Bulletin No. 477 2018 Oct 7

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    THE SOCIETY FOR POPULAR ASTRONOMY Electronic News Bulletin No. 477 2018 October 7
    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
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    much we have to offer by visiting

    A study simulating the final stages of terrestrial planet formation shows
    that 'hit-and-run' encounters play a significant role in the acquisition of
    water by large protoplanets, like those that grew into Mars and the Earth.
    Four and a half billion years ago, the inner Solar System was a chaotic
    place with around 50-100 protoplanets ranging in size from the Moon to Mars
    that were prone to giant collisions. Bodies that formed within what is now
    the orbit of Mars contained no water, as the conditions were too hot for
    volatile material, like water or methane, to condense. For water to make
    its way onto the developing terrestrial planets, it needed to be delivered
    from outside their region via a sequence of collisions. Astronomers have
    used high-resolution simulations to track the fate of water and other
    materials through a series of different impact scenarios. Outcomes of
    collisions could include bodies sticking together, material being lost, or
    being redistributed between the two objects. The results depend on various
    factors like the speed and angle of impact, the difference in mass between
    the bodies and their total mass. They found that 'hit-and-run' collisions,
    where the impact is off-centre and the bodies have enough speed to separate
    again after the encounter, are very common. In those scenarios, tens of
    percent of water can be transferred between the colliding bodies or ejected
    and lost entirely. The smaller of the colliding pair is often modified down
    to the core and effectively stripped of water, while the more-massive body
    remains more or less unaltered. The team is now focusing on how long chains
    of successive collisions affect the evolution of a disk of planetesimals and
    protoplanets. Recent research shows that comets can only account for a
    small fraction of the terrestrial planets' water. The great collisions
    early in the Solar System's history must also be a major source. The
    results strongly suggest that astronomers need to track the water in both
    survivors following hit-and-run encounters. That will help to predict the
    properties of planets that form as the end-product of a long sequence of
    successive collisions.


    Data from the Cassini spacecraft have revealed what appear to be giant dust
    storms in equatorial regions of Saturn's moon Titan. The discovery makes
    Titan the third Solar-System body, after the Earth and Mars, where dust
    storms have been observed. The observation is helping scientists to
    understand the fascinating and dynamic environment of Saturn's largest moon.
    Titan is an intriguing world — in ways quite similar to the Earth. In
    fact, it is the only moon in the Solar System with a substantial atmosphere
    and the only celestial body other than our planet where stable bodies of
    surface liquid are known still to exist. There is one big difference,
    though: on Earth such rivers, lakes and seas are filled with water, while
    on Titan it is primarily methane and ethane that constitute the liquid
    reservoirs. In that unique cycle, the hydrocarbon molecules evaporate,
    condense into clouds and rain back onto the ground. The weather on Titan
    varies from season to season as well, just as it does on Earth. In
    particular, around the equinox — the time when the Sun crosses Titan's
    equator — massive clouds can form in tropical regions and cause powerful
    methane storms. Cassini observed such storms during several of its Titan
    When the team first observed three unusual equatorial brightenings in
    infrared images taken by Cassini around the moon's 2009 northern equinox,
    they thought that they might be the same kind of methane clouds; however, an
    investigation revealed that they were something completely different. The
    researchers were also able to rule out that the features were actually on
    the surface of Titan in the form of frozen methane rain or icy lavas. Such
    surface spots would have a different chemical signature and would remain
    visible for much longer than the bright features in this study, which were
    visible for only 11 hours to five weeks. In addition, modelling showed that
    the features must be atmospheric but still close to the surface — most
    likely forming a very thin layer of tiny solid organic particles. Since
    they were located right over the dune fields around Titan's equator, the
    only remaining explanation was that the spots were actually clouds of dust
    raised from the dunes. Organic dust is formed when organic molecules,
    formed from the interaction of sunlight with methane, grow large enough to
    fall to the surface. A member of the team said that, while this is the
    first observation ever made of a dust storm on Titan, the finding is not
    surprising. The team believes that the Huygens probe, which landed on the
    surface of Titan in 2005 January, raised a small amount of organic dust upon
    arrival owing to its powerful aerodynamic wake. But the near-surface wind
    speeds required to raise such an amount of dust as we see in these dust
    storms would have to be very strong — about five times as strong as the
    average wind speeds estimated by the Huygens measurements near the surface
    and with climate models. The existence of such strong winds generating
    massive dust storms implies that the underlying sand can be set in motion,
    too, and that the giant dunes covering Titan's equatorial regions are still
    active and continually changing. The winds could be transporting the dust
    raised from the dunes across large distances, contributing to the global
    cycle of organic dust on Titan and causing effects similar to those that can
    be observed on the Earth and Mars.

    Southwest Research Institute

    Scientists have studied an unusual pair of asteroids and discovered that
    their existence points to an early planetary rearrangement in the Solar
    System. Those bodies, called Patroclus and Menoetius, are targets of NASA's
    upcoming Lucy mission. They are around 70 miles across and orbit around one
    another as they collectively circle the Sun. They constitute the only large
    binary known in the population of ancient bodies referred to as the Trojan
    asteroids. The two swarms of Trojans orbit at roughly the same distance
    from the Sun as Jupiter, one swarm orbiting ahead of, and the other behind,
    the planet. The Trojans were probably captured during a dramatic period of
    dynamic instability when a skirmish between the Solar System's giant planets
    — Jupiter, Saturn, Uranus and Neptune — occurred. That shake-up pushed
    Uranus and Neptune outwards, where they encountered a large primordial
    population of small bodies thought to be the source of today's Kuiper Belt
    objects, which orbit at the edge of the Solar System. Many small bodies of
    that primordial Kuiper Belt were scattered inwards, and a few of those
    became trapped as Trojan asteroids. A key issue with that Solar-System
    evolution model, however, has been when it took place. The scientists
    demonstrate that the very existence of the Patroclus–Menoetius pair
    indicates that the dynamic instability among the giant planets must have
    occurred within the first 100 million years of the Solar-System formation.
    Recent models of small-body formation suggest that that type of binaries are
    left-overs from the very earliest times of the Solar System, when pairs of
    small bodies could form directly from a collapsing cloud of 'pebbles'.
    Observations of today's Kuiper Belt show that binaries like those were quite
    common in ancient times. Only a few of them now exist within the orbit of
    Neptune. The question is how to interpret the survivors. Had the
    instability been delayed many hundreds of millions of years, as suggested by
    some Solar-System evolution models, collisions within the primordial
    small-body disc would have disrupted such relatively fragile binaries,
    leaving none to be captured in the Trojan population. Earlier dynamical
    instabilities would have left more binaries intact, increasing the likeli-
    hood that at least one would have been captured in the Trojan population.
    The team created new models that show that the existence of the Patroclus–
    Menoetius binary strongly indicates an earlier instability. That early
    dynamical-instability model has important consequences for the terrestrial
    planets, particularly regarding the origin of large impact craters on the
    Moon, Mercury and Mars that formed approximately 4 billion years ago. The
    impactors that made those craters are less likely to have been flung in from
    the outer regions of the Solar System. That could imply that they were made
    by small-body leftovers of the terrestrial-planet formation process. This
    work underscores the importance of the Trojan asteroids in illuminating the
    history of the Solar System. Much more will be learned about Patroclus–
    Menoetius binary when NASA's Lucy mission surveys the pair in 2033, at the
    conclusion of a 12-year mission to tour both Trojan swarms.

    NASA/Goddard Space Flight Center

    An unusual infrared light emission from a nearby neutron star detected by
    the Hubble Space Telescope could indicate new features never before seen.
    One possibility is that there is a dusty disk surrounding the neutron star;
    another is that there is an energetic wind coming off the object and
    slamming into gas in the interstellar space that the neutron star is
    ploughing through. Although neutron stars are generally studied in radio
    and high-energy emissions, such as X-rays, this study demonstrates that new
    and interesting information about neutron stars can also be gained by
    studying them in infrared light. The observation could help astronomers
    understand better the evolution of neutron stars — the incredibly dense
    remnants after a massive star explodes as a supernova. Neutron stars are
    also called pulsars because their very fast rotation (typically fractions of
    a second, in this case 11 seconds) causes time-variable emission from light-
    emitting regions. This particular neutron star belongs to a group of seven
    nearby X-ray pulsars — nicknamed 'the Magnificent Seven' — that are hotter
    than they ought to be considering their ages and available energy reservoir
    provided by the loss of rotation energy. Astronomers observed an extended
    area of infrared emissions around this neutron star — named RX J0806.4-4123
    — the total size of which translates into about 200 astronomical units at
    the assumed distance of the pulsar. This is the first neutron star in which
    an extended signal has been seen only in infrared light. The researchers
    suggest two possibilities that could explain the extended infrared signal
    seen by Hubble. The first is that there is a disk of material — possibly
    mostly dust — surrounding the pulsar.
    Another theory is that there could be what is known as a 'fallback disk' of
    material that coalesced around the neutron star after the supernova. Such a
    disk would be composed of matter from the progenitor massive star. Its
    subsequent interaction with the neutron star could have heated the pulsar
    and slowed its rotation. If confirmed as a supernova fallback disk, this
    result could change our general understanding of neutron-star evolution.
    The second possible explanation for the extended infrared emission from this
    neutron star is a 'pulsar wind' nebula. A pulsar wind nebula would require
    the neutron star to exhibit a pulsar wind. A pulsar wind can be produced
    when particles are accelerated in the electrical field that is produced by
    the fast rotation of a neutron star with a strong magnetic field. As the
    neutron star travels through the interstellar medium faster than the speed
    of sound, a shock can form where the interstellar medium and the pulsar wind
    interact. The shocked particles would then emit synchrotron radiation,
    causing the extended infrared signal that we see. Typically, pulsar-wind
    nebulae are seen in X-rays and an infrared-only pulsar wind nebula would be
    very unusual and exciting.

    Purdue University

    The explosions of supernovae can be so bright that they outshine their host
    galaxies. They take months or years to fade away, and sometimes the gaseous
    remains of the explosion slam into hydrogen-rich gas and temporarily become
    bright again — but could they remain luminous without any outside inter-
    ference? As large stars explode, their interiors collapse down to a point
    at which all their particles become neutrons. If the resulting neutron star
    has a magnetic field and rotates fast enough, it may develop into a pulsar
    wind nebula. That is most likely what happened to SN 2012au. We know that
    supernova explosions produce such types of rapidly rotating neutron stars,
    but we never saw direct evidence of it at this unique time frame. This is a
    key moment when the pulsar-wind nebula is bright enough to act like a light
    bulb illuminating the explosion's outer ejecta. SN 2012au was already known
    to be extraordinary — and weird — in many ways. Although the explosion
    was not bright enough to be termed a 'superluminous' supernova, it was
    extremely energetic and long-lasting, and dimmed in a similarly slow light
    curve. Superluminous supernovae are a hot topic in transient astronomy.
    They are potential sources of gravitational waves and black holes, and
    astronomers think that they might be related to other kinds of explosions,
    like gamma-ray bursts and fast radio bursts. Researchers want to understand
    the fundamental physics behind them, but they are difficult to observe
    because they are relatively rare and happen so far away.

    International Centre for Radio Astronomy Research

    Two of the closest galaxies to the Milky Way — the Large and Small
    Magellanic Clouds — may have had a third companion. New research describes
    how another 'luminous' galaxy was probably engulfed by the Large Magellanic
    Cloud some three to five billion years ago. Most stars in the Large
    Magellanic Cloud revolve clockwise around the centre of that galaxy. But,
    unusually, some stars revolve anti-clockwise. For a while, it was thought
    that those stars might have come from its companion galaxy, the Small
    Magellanic Cloud. Astronomers used computer modelling to simulate galaxy
    mergers. What they found is that in such a merging event, you actually can
    get quite strong counter-rotation after a merger takes place. That is
    consistent with what we see when we actually observe the galaxies. From the
    southern hemisphere the Magellanic Clouds can be seen in the night sky with
    the naked eye and have been observed by ancient cultures for thousands of
    years. The Large Magellanic Cloud is a relatively small 160,000 light-years
    away from us, while the Small Magellanic Cloud is around 200,000 light-years
    away. The finding could help to resolve a problem that has perplexed
    astronomers for years — why stars in the Large Magellanic Cloud are
    generally either very old or very young. Galaxies often contain large star
    clusters, which consist of many stars that are all of quite similar ages and
    made in similar environments. In the Milky Way, the star clusters are all
    very old, but in the Large Magellanic Cloud, there are very old clusters as
    well as ones that are very young — but nothing in between. That is known
    as the 'age-gap' problem. Because in the Large Magellanic Cloud we see star
    formation starting again, that could be indicative of a galaxy merger taking
    place. The finding could also help to explain why the Large Magellanic
    Cloud appears to have a thick disk.


    Astronauts on a mission to Mars would be exposed to at least 60% of the
    total radiation dose limit recommended for their career during the journey
    itself to and from the planet, according to data from the ESA-Roscosmos
    ExoMars Trace Gas Orbiter. The Trace Gas Orbiter began its scientific
    mission at Mars in April, and while its primary goals are to provide the
    most detailed inventory of Martian atmospheric gases to date — including
    those that might be related to active geological or biological processes —
    its radiation monitor has been collecting data since launch in 2016. The
    Liulin-MO dosimeter of the 'Fine-Resolution Epithermal Neutron Detector'
    (FREND) provided data on the radiation doses recorded during the orbiter's
    six-month interplanetary cruise to Mars, and since the spacecraft reached
    orbit around the planet. On Earth, a strong magnetic field and a thick
    atmosphere protect us from the unceasing bombardment of galactic cosmic
    rays, fragments of atoms from outside the Solar System that travel at close
    to the speed of light and are highly penetrating for biological material.
    In space that has the potential to cause serious damage to people, including
    radiation sickness, an increased lifetime risk for cancer, central-nervous-
    system effects, and degenerative diseases, which is why ESA is researching
    ways to protect astronauts on long space missions. The ExoMars measure-
    ments cover a period of declining solar activity, corresponding to a high
    radiation dose. Increased activity of the Sun can deflect the galactic
    cosmic rays, although very large solar flares and eruptions can themselves
    be dangerous to astronauts. Radiation doses accumulated by astronauts in
    interplanetary space would be several hundred times larger than the doses
    accumulated by people over the same time period on Earth, and several times
    larger than the doses of astronauts and cosmonauts working on the Inter-
    national Space Station.


    Irish astronomers are about to gain access to the world's most advanced
    ground-based astronomical telescopes following the signature of Ireland's
    Accession Agreement in Dublin. By joining ESO, Ireland adds to its
    already rich astronomical history, stretching back centuries. For several
    decades in the 19th century, Ireland hosted the world's largest telescope
    — the Leviathan of Parsonstown — a 1.8-metre reflecting telescope at
    Birr Castle (whose grounds are now home to I-LOFAR, port of a Europe-wide
    low-frequency radio telescope). Ireland?s vibrant research community and
    high-tech industrial sector have supported ESO membership for many years,
    and will now gain access to a range of instrumentation and industrial
    opportunities as a result of ESO membership. ESO is the foremost inter-
    governmental astronomy organisation in Europe and the world's most
    productive ground-based astronomical observatory by far. It has 16 Member
    States: Austria, Belgium, the Czech Republic, Denmark, France, Finland,
    Germany, Ireland, Italy, the Netherlands, Poland, Portugal, Spain, Sweden,
    Switzerland and the United Kingdom, along with the host state of Chile and
    with Australia as a Strategic Partner. It operates three unique world-class
    observing sites in Chile: La Silla, Paranal and Chajnantor. At Paranal, ESO
    operates the Very Large Telescope and its world-leading Very Large Telescope
    Interferometer as well as two survey telescopes, VISTA working in the infra-
    red and the visible-light VLT Survey Telescope. ESO is also a major partner
    in two facilities on Chajnantor, APEX and ALMA, the largest astronomical
    project in existence. And on Cerro Armazones, close to Paranal, ESO is
    building the 39-metre Extremely Large Telescope, the ELT, which will become
    the world's biggest telescope.

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