THE SOCIETY FOR POPULAR ASTRONOMY Electronic News Bulletin No. 481 2018 Dec 2.00

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    THE SOCIETY FOR POPULAR ASTRONOMY Electronic News Bulletin No. 481 2018 December 2

    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

    University of Texas at Austin

    Today, most of the water on Mars is locked away in frozen ice caps. But
    billions of years ago it flowed freely across the surface, forming rushing
    rivers that emptied into craters, forming lakes and seas. New research has
    found evidence that sometimes the lakes would accumulate so much water that
    they overflowed and burst from their basins, creating catastrophic floods
    that carved canyons very rapidly, perhaps in a matter of weeks. From
    studies of rock formations from satellite images, scientists know that
    hundreds of craters across the surface of Mars were once filled with water.
    More than 200 of those 'paleo-lakes' have outlet canyons, tens to hundreds
    of kilometres long and several kilometres wide, carved by water flowing from
    the ancient lakes. However, until this study, it was unknown whether the
    canyons were gradually carved over millions of years or carved rapidly by
    single floods.
    Using high-resolution photos taken by the Mars Reconnaissance Orbiter
    satellite, the researchers examined the topography of the outlets and the
    crater rims and found a correlation between the size of the outlet and the
    volume of water expected to be released during a large flooding event.
    If the outlet had instead been gradually whittled away over time, the
    relationship between water volume and outlet size probably would not hold.
    In total, the researchers examined 24 paleo-lakes and their outlet canyons
    across the Red Planet. One of the paleo-lakes examined in the study, Jezero
    Crater, is a potential landing site for NASA's Mars 2020 rover mission to
    look for signs of past life. The team proposed the crater as a landing site
    on the basis of prior studies that found that it held water for long periods
    in Mars' past. While massive floods flowing from Martian craters might
    sound like a scene from a science-fiction novel, a similar process occurs on
    Earth when lakes dammed by glaciers break through their icy barriers. The
    researchers found that the similarity is more than superficial. As long as
    gravity is accounted for, floods create outlets with similar shapes whether
    on Earth or Mars. Although big floods on Mars and Earth are governed by the
    same mechanics, they fit into different geological paradigms. On Earth, the
    slow-and-steady motion of tectonic plates dramatically changes the planet's
    surface over millions of years. In contrast, the lack of plate tectonics on
    Mars means that cataclysmic events — like floods and asteroid impacts —
    quickly create changes that can amount to near-permanent changes in the


    In 2017 November, scientists pointed the Spitzer Space Telescope at the
    object known as 'Oumuamua — the first known interstellar object to visit
    the Solar System. The infrared Spitzer was one of many telescopes pointed
    at 'Oumuamua in the weeks after its discovery that October. 'Oumuamua was
    too faint for Spitzer to detect when it looked more than two months after
    the object's closest approach to the Earth in early September. However,
    the 'non-detection' puts a new limit on how large the strange object can
    be. The new size limit is consistent with the findings of a research paper
    published earlier this year, which suggested that outgassing was responsible
    for the slight changes in 'Oumuamua's speed and direction as it was tracked
    last year. The authors of that paper concluded that the expelled gas acted
    like a small thruster gently pushing the object. That determination was
    dependent on 'Oumuamua being relatively smaller than typical Solar-System
    comets. (The conclusion that 'Oumuamua experienced outgassing suggested
    that it was composed of frozen gases, similar to a comet.)
    'Oumuamua was first detected by the University of Hawaii's Pan-STARRS 1
    telescope on Haleakala, Hawaii (the object's name is a Hawaiian word meaning
    'visitor from afar arriving first), in 2017 October while the telescope was
    surveying for near-Earth asteroids. Subsequent detailed observations
    conducted by multiple ground-based telescopes and the Hubble Space Telescope
    detected the sunlight reflected off 'Oumuamua's surface. Large variations
    in its brightness suggested that 'Oumuamua is highly elongated and probably
    less than 800 metres in its longest dimension. But Spitzer tracks asteroids
    and comets using the infrared energy, or heat, that they radiate, which can
    provide more specific information about an object's size than optical
    observations of reflected sunlight alone would.
    The fact that 'Oumuamua was too faint for Spitzer to detect sets a limit on
    the object's total surface area. However, since the non-detection can not
    be used to infer shape, the size limits are presented as what 'Oumuamua's
    diameter would be if it were spherical. Using three separate models that
    make different assumptions about the object's composition, Spitzer's
    non-detection limited 'Oumuamua's 'spherical diameter' to 440 metres, 140
    metres or perhaps as little as 100 metres. The wide range of results stems
    from the assumptions about 'Oumuamua's composition, which influences how
    visible (or faint) it would appear to Spitzer were it a particular size.
    The new study also suggests that 'Oumuamua may be up to 10 times more
    reflective than the comets in the Solar System — a surprising result,
    according to the paper's authors. Because infrared light is largely heat
    radiation produced by 'warm' objects, it can be used to determine the
    temperature of a comet or asteroid; in turn, that can be used to determine
    the reflectivity of the object's surface — what scientists call albedo.
    An object with low reflectivity retains more heat than an object with high
    reflectivity, so a lower temperature means a higher albedo. A comet's
    albedo can change throughout its lifetime. When it passes close to the Sun,
    a comet's ice warms and turns directly into a gas, sweeping dust and dirt
    off the comet's surface and revealing more reflective ice. 'Oumuamua had
    been travelling through interstellar space for millions of years, far from
    any star that could refresh its surface. But it may have had its surface
    refreshed through such 'outgassing' when it made an extremely close approach
    to the Sun, a little more than five weeks before it was discovered. In
    addition to sweeping away dust and dirt, some of the released gas may have
    covered the surface of 'Oumuamua with a reflective coat of ice and snow —
    a phenomenon that has also been observed in comets in the Solar System.
    'Oumuamua is now on its way out of the Solar System — almost as far from
    the Sun as Saturn's orbit — and is well beyond the reach of any existing


    The nearest single star to the Sun hosts an exo-planet at least 3.2 times
    as massive as the Earth — a so-called super-Earth. One of the largest
    observing campaigns to date using data from a world-wide array of tele-
    scopes, including the planet-hunting HARPS instrument, have revealed this
    frozen, dimly lit world. The newly discovered planet is the second-closest
    known exo-planet to the Earth. Barnard's star is the fastest-moving star in
    the night sky. The planet, designated Barnard's Star b, now steps in as the
    second-closest known exo-planet to the Earth. The gathered data indicate
    that the planet could be a super-Earth, having a mass at least 3.2 times
    that of the Earth, and orbits its host star in roughly 233 days. Barnard's
    Star, the planet?s host star, is a red dwarf, a cool, low-mass star, which
    only dimly illuminates this newly-discovered world. Light from Barnard's
    Star provides its planet with only 2% of the energy the Earth receives from
    the Sun. Despite being relatively close to its parent star — at a distance
    only 0.4 times that between Earth and the Sun — the exoplanet lies close to
    the snow line, the region where volatile compounds such as water condense
    into solid ice. That freezing, shadowy world could have a temperature of
    170 K, making it inhospitable for life as we know it. Named for astronomer
    E. E. Barnard, Barnard's Star is the closest single star to the Sun. While
    the star itself is ancient — probably twice the age of our Sun — and
    relatively inactive, it also has the fastest apparent motion of any star in
    the night sky. Super-Earths are the most common type of planet to form
    around low-mass stars such as Barnard's Star, lending credibility to its
    newly discovered planetary candidate. Furthermore, current theories of
    planetary formation predict that the snow line is the ideal location for
    such planets to form.
    Previous searches for a planet around Barnard's Star have had disappointing
    results — this recent breakthrough was possible only by combining measure-
    ments from several high-precision instruments mounted on telescopes all over
    the world. The astronomers used the Doppler effect to find the exo-planet
    candidate. While the planet orbits the star, its gravitational pull causes
    the star to wobble. HARPS can detect changes in the star's velocity as
    small as 3.5 km/h — about walking pace. That radial-velocity method has
    never been used previously to detect a similar super-Earth-type exo-planet
    in such a large orbit around its star.


    For the first time astronomers have detected gravitational waves from a
    merged, hyper-massive neutron star. Gravitational waves were predicted by
    Albert Einstein in his General Theory of Relativity in 1915. The waves
    are disturbances in space-time, generated by rapidly moving masses, which
    propagate out from the source. By the time the waves reach the Earth,
    they are incredibly weak and their detection requires extremely sensitive
    equipment. It took scientists until 2016 to announce the first observation
    of gravitational waves, using the Laser Interferometer Gravitational Wave
    Observatory (LIGO) detector. Since that seminal result, gravitational waves
    have been detected on a further six occasions. One of those, GW170817,
    resulted from the merger of two stellar remnants known as neutron stars.
    Those objects form after stars much more massive than the Sun explode
    as supernovae, leaving behind cores of material packed to extraordinary
    densities. At the same time as the burst of gravitational waves from the
    merger, observatories detected emission in gamma rays, X-rays, ultraviolet,
    visible light, infrared and radio waves — an unprecedented observing
    campaign that confirmed the location and nature of the source. The initial
    observations of GW170817 suggested that the two neutron stars merged into a
    black hole, an object with a gravitational field so powerful that not even
    light can escape its grasp. Astronomers set out to check that, using a
    novel technique to analyze the data from LIGO and the Virgo gravitational
    wave detector sited in Italy. Their detailed analysis shows the H1 and L1
    detectors in LIGO, which are separated by more than 3,000 kilometres,
    simultaneously picked up a descending 'chirp' lasting around 5 seconds.
    Significantly, the chirp started between the end of the initial burst of
    gravitational waves and a subsequent burst of gamma rays. Its low frequency
    (less than 1 KHz, reducing to 49 Hz) suggests that the merged object spun
    down to become a larger neutron star, rather than a black hole. There are
    other objects like that, with their total mass matching known neutron-star
    binary pairs. But the team has now confirmed their origin. Gravitational-
    wave astronomy, and eking out the data from every detection, will take a
    further step forward next year, when the Japanese Kamioka Gravitational-Wave
    Detector (KAGRA) comes online.

    NASA/Goddard Space Flight Center

    Astronomers may have finally uncovered the long-sought progenitor to a
    specific type of exploding star by sifting through Hubble Space Telescope
    archival data. The supernova, called a Type Ic, is thought to detonate
    after its massive-star precursor has shed or been stripped of its outer
    layers of hydrogen and helium. Such stars could be among the most massive
    known — at least 30 times more massive than the Sun. Even after shedding
    some of their material late in life, they are expected to be big and bright.
    So it was a mystery why astronomers had not been able to find any such stars
    in pre-explosion images. Finally, in 2017, astronomers got lucky. A nearby
    star ended its life as a Type Ic supernova. Two teams of astronomers pored
    through the archive of Hubble images to uncover the putative precursor star
    in pre-explosion photos taken in 2007. The supernova, catalogued as SN
    2017ein, appeared near the centre of the 'nearby' spiral galaxy NGC 3938,
    located roughly 65 million light-years away. That potential discovery could
    yield insight into stellar evolution, including how the masses of stars are
    distributed when they are born in batches. An analysis of the object's
    colours shows that it is blue and extremely hot. On the basis of that
    assessment, both teams suggest two possibilities for the source's identity.
    The progenitor could be a single massive star between 45 and 55 times the
    mass of the Sun. Another idea is that it could have been a massive binary-
    star system in which one of the stars was between 60 and 80 solar masses and
    the other roughly 48 Suns. In that latter scenario, the stars are orbiting
    closely and interact with one another. The more massive star is stripped of
    its hydrogen and helium layers by the close companion, and eventually
    explodes as a supernova.
    The possibility of a massive double-star system is a surprise. Expectations
    on the identity of the progenitors of Type Ic supernovae have been a puzzle.
    Astronomers have known that the supernovae were deficient in hydrogen and
    helium, and initially proposed that some massive stars shed material in a
    strong wind (a stream of charged particles) before they exploded. When they
    did not find the progenitor stars, which should have been extremely massive
    and bright, they suggested a second method to produce the exploding stars
    that involves a pair of close-orbiting, lower-mass binary stars. In that
    scenario, the more massive star is stripped of its hydrogen and helium by
    its companion. But the 'stripped' star is still massive enough eventually
    to explode as a Type Ic supernova. Type Ic supernovae are just one class
    of exploding star. They account for about 20 per cent of massive stars that
    explode from the collapse of their cores. Astronomers caution that they
    won't be able to confirm the source's identity until the supernova fades in
    about two years. The astronomers hope to use either Hubble or the upcoming
    NASA James Webb Space Telescope to see whether the candidate progenitor star
    has disappeared or has significantly dimmed. They will also be able to
    separate the supernova's light from that of stars in its environment to
    calculate a more accurate measurement of the object's brightness and mass.

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