THE SOCIETY FOR POPULAR ASTRONOMY Electronic News Bulletin No. 487 2019 April 14

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    THE SOCIETY FOR POPULAR ASTRONOMY Electronic News Bulletin No. 487 2019 April 14

    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 http://www.popastro.com/

    SPUN UP ASTEROID COMING APART
    NASA/Goddard Space Flight Center

    A small asteroid has been caught in the process of spinning so fast that
    it is throwing off material, according to new data from the Hubble Space
    Telescope. Images show two narrow, comet-like tails of dusty debris
    streaming from the asteroid (6478) Gault. Each tail represents an episode
    in which the asteroid gently shed its material — key evidence that Gault is
    beginning to come apart. Discovered in 1988, the 4-kilometre-wide asteroid
    has been observed repeatedly, but the debris tails are the first evidence of
    disintegration. Gault is 344 million kilometres from the Sun. For the
    roughly 800,000 known asteroids between Mars and Jupiter, astronomers
    estimate that this type of event in the asteroid belt is rare, occurring
    roughly once a year. Watching an asteroid become unglued gives astronomers
    the opportunity to study the makeup of such space rocks without sending a
    spacecraft to sample them. Gault is only the second asteroid whose
    disintegration has been strongly linked to a process known as the YORP
    effect. (YORP stands for “Yarkovsky-O'Keefe-Radzievskii-Paddack”, the names
    of four scientists who contributed to the concept.) When sunlight heats an
    asteroid, infrared radiation escaping from its warmed surface carries off
    angular momentum as well as heat. That process creates a tiny torque that
    can cause the asteroid continually to spin faster. When the resulting
    centrifugal force starts to overcome gravity, the asteroid's surface becomes
    unstable, and landslides may send dust and rubble drifting into space at a
    couple of miles per hour. The researchers estimate that Gault could have
    been slowly spinning up for more than 100 million years.

    SATURN'S RINGS COAT TINY MOONS
    NASA

    New findings have emerged about five tiny moons nestled in and near Saturn's
    rings. The new research, from data gathered by six of Cassini's instruments
    before its mission ended in 2017, is a clear confirmation that dust and ice
    from the rings accretes onto the moons embedded within and near the rings.
    Scientists also found the moon surfaces to be highly porous, further
    confirming that they were formed in multiple stages as ring material settled
    onto denser cores that might be remnants of a larger object that broke
    apart. The porosity also helps explain their shape: rather than being
    spherical, they are blobby and ravioli-like, with material stuck around
    their equators. Of the satellites studied, the surfaces of those closest
    to Saturn — Daphnis and Pan — are the most altered by ring materials.
    The surfaces of the moons Atlas, Prometheus and Pandora, farther out from
    Saturn, have ring material as well — but they are also coated with the
    bright icy particles and water vapour from the plume spraying out of
    Enceladus. (A broad outer ring of Saturn, known as the E ring, is formed
    by the icy material that fans out from Enceladus' plume.)
    The key puzzle piece was a data set from Cassini's Visible and Infrared
    Mapping Spectrometer (VIMS), which collected light visible to the human eye
    and also infrared light of longer wavelengths. It was the first time that
    Cassini was close enough to create a spectral map of the surface of the
    innermost moon Pan. By analyzing the spectra, VIMS was able to learn about
    the composition of materials on all five moons. VIMS saw that the ring
    moons closest to Saturn appear the reddest, similar to the colour of the
    main rings. Scientists don't yet know the composition of the material
    that appears red, but they believe that it's likely to be a mix of organics
    and iron. The moons just outside the main rings, on the other hand, appear
    more blue, similar to the light from Enceladus' icy plumes.

    MOST PROMISING EXOPLANETS IN HUNT FOR LIFE
    Cornell University :

    NASA's new Transiting Exoplanet Survey Satellite (TESS) is designed to
    ferret out habitable exoplanets, but with hundreds of thousands of sunlike
    and smaller stars in its camera views, which of those stars could host
    planets like our own? TESS will observe 400,000 stars across the whole sky
    to catch a glimpse of a planet transiting across the face of its star, one
    of the primary methods by which exoplanets are identified. A team of
    astronomers from Cornell University, Lehigh University and Vanderbilt
    University has identified the most promising targets for this search in the
    new 'TESS Habitable Zone Star Catalogue', published in Astrophysical Journal
    Letters. The catalogue identifies 1,822 stars for which TESS is sensitive
    enough to observe Earth-like planets just a bit larger than the Earth that
    receive radiation from their star equivalent to what Earth receives from the
    Sun. For 408 stars, TESS can glimpse a planet just as small as Earth, with
    similar irradiation, in one transit alone. Confirming that an exoplanet has
    been observed and determining the distance between it and its star requires
    detecting two transits across the star. The 1,822 stars the researchers
    have identified in the catalogue are ones from which TESS could detect two
    planetary transits during its mission. Those orbital periods place them
    squarely in the habitable zone of their star. The habitable zone is the
    area around a star at which water can be liquid on a rocky planet's surface,
    therefore considered ideal for sustaining life. As the researchers note,
    planets outside the habitable zone could certainly harbour life, but it
    would be extremely difficult to detect any signs of life on such frozen
    planets without flying there.
    The catalogue also identifies a subset of 227 stars for which TESS can not
    only probe for planets that receive the same irradiation as Earth, but for
    which TESS can also probe out farther, covering the full extent of the
    habitable zone all the way to cooler Mars-like orbits. That will allow
    astronomers to probe the diversity of potentially habitable worlds around
    hundreds of cool stars during the TESS mission's lifetime. The stars
    selected for the catalogue are bright, cool dwarfs, with temperatures
    roughly between 2,700 and 5,000 degrees Kelvin. The stars in the catalogue
    are selected because of their brightness; the closest are approximately 6
    light-years from Earth. A total of 137 stars in the catalogue are within
    the continuous viewing zone of NASA's James Webb Space Telescope, now under
    construction. Webb will be able to observe them to characterize planetary
    atmospheres and search for signs of life in their atmospheres. Planets
    that TESS identifies may also make excellent targets for observations by
    ground-based extremely large telescopes currently being built, the
    researchers note, as the brightness of their host stars would make them
    easier to characterize.

    ARE BROWN DWARFS FAILED STARS OR SUPER PLANETS?
    University of Heidelberg

    Brown dwarfs fill the 'gap' between stars and the much smaller planets —
    two very different types of astronomical objects. But how they originate
    has yet to be fully explained. Astronomers have discovered that the star v
    Ophiuchi in the Milky Way is being orbited by two brown dwarfs, which in all
    probability formed along with the star from a gas and dust disc, just as
    planets do. Brown dwarfs orbit either one star or travel in isolation in
    the vast expanse of the Milky Way. Their masses — at least 13 times more
    than that of the Jupiter — is sufficient to generate, at least temporarily,
    energy in their cores through nuclear fusion. They are not sufficiently
    massive, however, to ignite hydrogen in their cores and hence to create
    their own light. The heat they continue to radiate after formation is how
    astronomers are able to locate them. It is estimated that up to 100 billion
    brown dwarfs make their home in the Milky Way. Yet it remains unclear how
    they form — whether they are 'failed' stars or possibly even super-planets.
    The recent discoveries could provide an answer. Astronomers analysed the
    variations in radial velocity of the star v Ophiuchi for 11 years. The star
    has a mass slightly greater than two and half times that of the Sun, and is
    located approximately 150 light years from Earth in the constellation
    Ophiuchus. The team noticed a certain pattern in the measurements, similar
    to those caused by orbiting planets or binary stars, which is usually
    nothing out of the ordinary. But in this case, in-depth analysis of the data
    revealed something extraordinary: apparently, v Ophiuchi is being orbited by
    two brown dwarfs with an orbital period of approximately 530 and 3,185 days,
    which puts them in a 6:1 resonant configuration. So the brown dwarf closer
    to v Ophiuchi orbits its star exactly six times while the other, more
    distant brown dwarf completes only one orbit. This discovery sheds
    completely new light on the evolution of brown dwarfs. Do they develop
    exclusively like normal stars in interstellar clouds, or can they also form
    in the so-called protoplanetary disc of gas and dust that surrounds the
    parent star in the early phase of its formation? The 6:1 resonance is a
    strong indication for the latter scenario. Only then could the orbits of
    the newly developing brown dwarfs adjust to a stable resonance over millions
    of years. That is what the extensive dynamic analyses for possible
    configurations of the vOphiuchi system suggest. This superplanetary system
    is the first of its kind as well as the first sure sign that brown dwarfs
    can form in a protoplanetary disc and the team hope for other such
    discoveries that may one day allow astronomers to clarify how many of the
    'failed stars' are actually more massive siblings of Jupiter and Saturn.

    DARK MATTER NOT MADE FROM TINY BLACK HOLES
    Kavli Institute for the Physics and Mathematics of the Universe

    Scientists know that 85 per cent of the matter in the Universe is made up
    of dark matter. Its gravitational force prevents stars in our Milky Way
    from flying apart. However, attempts to detect such dark-matter particles
    by underground experiments, or accelerator experiments including the
    world's largest accelerator, the Large Hadron Collider, have so far failed.
    That has led scientists to consider Hawking's 1974 theory of the existence
    of primordial black holes, born shortly after the Big Bang, and his
    speculation that they could make up a large fraction of the elusive dark
    matter that scientists are trying to discover today. An international
    team of researchers has used the gravitational lensing effect to look for
    primordial black holes between the Earth and the Andromeda galaxy.
    Gravitational lensing, an effect first suggested by Einstein, manifests
    itself as the bending of light rays coming from a distant object such as
    a star due to the gravitational effect of an intervening massive object
    such as a primordial black hole. In extreme cases, such light bending
    causes the background star to appear much brighter than it really is.
    However, gravitational lensing effects are very rare events because it
    requires a star in the Andromeda galaxy, a primordial black hole acting
    as the gravitational lens, and an observer on Earth to be exactly in
    line with one another. So to maximize the chances of capturing an event,
    the researchers used the Hyper Suprime-Cam digital camera on the Subaru
    telescope in Hawaii, which can image the whole of the Andromeda galaxy
    in one shot.
    Taking into account how fast primordial black holes are expected to move
    in interstellar space, the team took multiple images to try to catch the
    flicker of a star as it brightens for a period of a few minutes to hours
    due to gravitational lensing. From 190 consecutive images of the Andromeda
    galaxy taken over seven hours during one clear night, the team scoured the
    data for potential gravitational-lensing events. If dark matter consists of
    primordial black holes of a given mass, in this case masses lighter than the
    Moon, the researchers expected to find about 1000 events. But after careful
    analyses, they could identify only one case. The team's results showed that
    primordial black holes can contribute no more than 0.1 per cent of all dark-
    matter mass. Therefore it is unlikely that the theory is true. The
    researchers are now planning to develop their analysis of the Andromeda
    galaxy further. One new theory that they will investigate is to find
    whether binary black holes discovered by the gravitational-wave detector
    LIGO are in fact primordial black holes.

    BLACK HOLE IMAGE MAKES HISTORY
    NASA

    A black hole and its shadow have been captured in an image for the first
    time, an historic feat, by an international network of radio telescopes
    called the Event Horizon Telescope (EHT). A black hole is an extremely
    dense object from which no light can escape. Anything that comes within a
    black hole's 'event horizon', its point of no return, will be consumed,
    never to re-emerge, because of the black hole's unimaginably strong
    gravity. By its very nature, a black hole cannot be seen, but the hot disc
    of material that encircles it shines bright. Against a bright backdrop,
    such as that disc, a black hole appears to cast a shadow. The image shows
    the shadow of the supermassive black hole in the centre of Messier 87 (M87),
    an elliptical galaxy some 55 million light-years away. That black hole is
    6.5 US-billion times the mass of the Sun. Catching its shadow involved
    eight ground-based radio telescopes around the globe, operating together
    as if they were one telescope the size of our entire planet. While NASA
    observations did not directly trace out the historic image, astronomers
    used data from NASA's Chandra and NuSTAR satellites to measure the X-ray
    brightness of M87's jet. Scientists used that information to compare their
    models of the jet and disc around the black hole with the EHT observa-
    tions. Other insights may come as researchers continue to pore over these
    data.
    There are many remaining questions about black holes that the coordinated
    NASA observations may help to answer. Mysteries linger about why particles
    get such a huge energy boost around black holes, forming dramatic jets that
    surge away from the poles of black holes at nearly the speed of light.
    When material falls into the black hole, where does the energy go? NASA
    space telescopes have previously studied a jet extending more than 1,000
    light-years away from the centre of M87. The jet is made of particles
    travelling near the speed of light, shooting out at high energies from close
    to the event horizon. The EHT was designed in part to study the origin of
    that jet and others like it. A blob of matter in the jet called HST-1,
    discovered by Hubble astronomers in 1999, has undergone a mysterious cycle
    of brightening and dimming. Chandra, NuSTAR and Swift, as well as NASA's
    Neutron star Interior Composition Explorer (NICER) experiment on the Inter-
    national Space Station, also looked at the black hole at the centre of our
    own Milky Way galaxy, called Sagittarius A*, in coordination with EHT.

    Bulletin compiled by Clive Down
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