Topic: Late September Astronomy Bulletin

[Clive]

PLUTO FEATURES GIVEN OFFICIAL NAMES
International Astronomical Union

The IAU has assigned names to fourteen 'geo'logical features on the
surface of Pluto.  The names pay homage to underworld mythology,
pioneering space missions, historic pioneers who crossed new horizons in
exploration, and scientists and engineers associated with Pluto and the
Kuiper Belt.  This is the first set of official names of surface features
on Pluto to be approved by the IAU, the internationally recognized
authority for naming celestial bodies and their surface features.  NASA's
New Horizons team proposed the names to the IAU following the first
reconnaissance of Pluto and its moons by the New Horizons spacecraft.
Some of the names were suggested by members of the public during the 'Our
Pluto' campaign, which was launched as a partnership between the IAU, the
New Horizons project and the SETI Institute.  Other names had been used
informally by the New Horizons science team to describe the many regions,
mountain ranges, plains, valleys and craters discovered during the first
close-up look at the surfaces of Pluto and its largest moon, Charon.
More names are expected to be proposed to the IAU, both for Pluto and for
its moons.

The approved Pluto surface feature names are listed below.

Tombaugh Regio honours Clyde Tombaugh (1906-1997), the U.S. astronomer
who discovered Pluto in 1930 from the Lowell Observatory in Arizona.

Burney crater honours Venetia Burney (1918-2009), who as an 11-year-
old schoolgirl suggested the name 'Pluto' for Clyde Tombaugh's newly
discovered planet.  Later in life she taught mathematics and economics.

Sputnik Planitia is a large plain named after Sputnik 1, the first space
satellite, launched by the Soviet Union in 1957.

Tenzing Montes and Hillary Montes are mountain ranges honouring Tenzing
Norgay (1914-1986) and Sir Edmund Hillary (1919-2008), the Indian/Nepali
Sherpa and the New Zealand mountaineer who were the first people to reach
the summit of Mount Everest and return safely.

Al-Idrisi Montes honours Ash-Sharif al-Idrisi (1100-1165/66), a noted
Arab map-maker and geographer whose landmark work of medieval geography
is sometimes translated as "The Pleasure of Him Who Longs to Cross the
Horizons".

Djanggawul Fossae defines a network of long, narrow depressions named
for the Djanggawuls, three ancestral beings in indigenous Australian
mythology who travelled between the island of the dead and Australia,
creating the landscape and filling it with vegetation.

Sleipnir Fossa is named for the powerful, eight-legged horse of Norse
mythology that carried the god Odin into the underworld.

Virgil Fossae honours Virgil, one of the greatest Roman poets and Dante's
fictional guide through hell and purgatory in the Divine Comedy.

Adlivun Cavus is a deep depression named for Adlivun, the underworld in
Inuit mythology.

Hayabusa Terra is a large land mass saluting the Japanese spacecraft and
mission (2003-2010) that returned the first asteroid sample.

Voyager Terra honours the pair of NASA spacecraft, launched in 1977, that
performed the first 'grand tour' of all four giant planets.  The Voyager
spacecraft are now probing the boundary between the Sun and interstellar
space.

Tartarus Dorsa is a ridge named for Tartarus, the deepest, darkest pit of
the underworld in Greek mythology.

Elliot crater recognizes James Elliot (1943-2011), a MIT researcher who
pioneered the use of stellar occultations to study the Solar System --
leading to discoveries such as the rings of Uranus and the first
detection of Pluto's thin atmosphere.


DOES COMET MATERIAL PREDATE THE SOLAR SYSTEM?
CNRS

ESA's Rosetta mission, which ended in 2016 September, found that organic
matter made up 40% (by mass) of the nucleus of comet 67P Churyumov-
Gerasimenko, a.k.a. Chury.  Organic compounds, combining carbon,
hydrogen, nitrogen, and oxygen, are building blocks of life on Earth.
Yet, according to a group of astronomers, those organic molecules were
produced in interstellar space, well before the formation of the Solar
System.  For 70 years, scientists have known that analysis of stellar
spectra indicates unknown absorptions, throughout interstellar space,
at specific wavelengths, called the diffuse interstellar bands (DIBs). 

DIBs are attributed to complex organic molecules that possibly constitute
the largest known reservoir of organic matter in the Universe.  Such
interstellar organic material is usually found in the same proportions.
However, very dense clouds of matter like pre-solar nebulae are
exceptions.  In the middle of those nebulae, where matter is even denser,
DIB absorptions plateau or even drop.  That is because the organic
molecules responsible for DIBs clump together there.  The clumped matter
absorbs less radiation than it did when it floated freely in space.  Such
primitive nebulae end up contracting to form solar systems like our own,
with planets and comets.

The Rosetta mission taught us that comet nuclei form by gentle accretion
of grains progressively greater in size.  First, small particles stick
together into larger grains.  Those in turn combine into larger chunks,
and so on, until they form a comet nucleus a few kilometres wide.  Thus,
the organic molecules that formerly populated the primitive nebulae --
and that are responsible for DIBs -- were probably not destroyed, but
instead incorporated into the grains making up cometary nuclei.  And there
they have remained for 4.6 billion years.  A sample-return mission would
allow laboratory analysis of cometary organic material and finally reveal
the identity of the mysterious interstellar matter underlying observed
absorption lines in stellar spectra.  If cometary organic molecules were
indeed produced in interstellar space -- and if they played a role in the
emergence of life on our planet, as scientists believe today -- might
they not also have seeded life on many other planets of our galaxy?


OTHER WORLDS THAT COULD OBSERVE THE EARTH
RAS
 
A group of scientists has turned exoplanet-hunting on its head, in a
study that instead looks at how an alien observer might be able to detect
the Earth using our own methods.  They find that at least nine exoplanets
are ideally placed to observe transits of Earth.  Thanks to facilities
and missions such as SuperWASP and Kepler, we have now discovered
thousands of planets orbiting stars other than our Sun, worlds known as
exoplanets.  The vast majority of them are found when the planets transit
in front of their host stars, allowing astronomers to see light from the
host star dim slightly at regular intervals every time the planet passes
between us and the distant star.

They identified parts of the distant sky from where various planets in
our Solar System could be seen to pass in front of the Sun (so-called
'transit zones') -- concluding that the terrestrial planets (Mercury,
Venus, Earth, and Mars) are actually much more likely to be observed than
the more distant planets (Jupiter, Saturn, Uranus, and Neptune), despite
their much larger size.  To look for worlds where civilizations would
have the best chance of observing our Solar System, the astronomers
looked for parts of the sky from which more than one planet could be seen
crossing the face of the Sun. They found that three planets at most could
be observed from anywhere outside the Solar System, and that not all
combinations of three planets are possible.  It is estimated that a
randomly positioned observer would have roughly a 1 in 40 chance of
observing at least one planet.  The probability of detecting at least two
planets would be about ten times lower, and to detect three would be a
further ten times smaller still.
 
Of the thousands of known exoplanets, the team identified sixty-eight
where observers would see one or more of the planets in our Solar System
transit the Sun.  Nine of them are well placed to observe transits of the
Earth, although none of the worlds is deemed to be habitable.  In
addition, the team estimates that there should be approximately ten
(currently undiscovered) worlds which are favourably located to detect
the Earth and are capable of sustaining life as we know it.  To date
however, no habitable planet has been discovered from which a civiliza-
tion could detect the Earth with our current level of technology.  The
ongoing K2 mission of NASA's Kepler spacecraft is to continue to hunt
for exoplanets in different regions of the sky for a few months at a
time.  Those regions are centred close to the plane of the Earth's orbit,
which means that there are many target stars located in the transit zones
of the Solar-System planets.  The team's plans for future work include
targeting the transit zones to search for exoplanets, in the hope of
finding some which could be habitable.
 
 
STAR FOUND TO CHANGE OVER DECADES
University of Notre Dame

Astronomers studying the unique binary-star system AR Scorpii have
discovered that the brightness of the system has changed over the past
decade.  The new evidence lends support to an existing theory of how the
unusual star emits energy.  AR Scorpii consists of a rapidly spinning,
magnetized white-dwarf star that mysteriously interacts with its
companion star.  The system was recently found to more than double in
brightness on time-scales of minutes and hours.  Researchers analyzed
data on the system from the Kepler Space Telescope's K2 mission taken
in 2014 before the star was known to be unusual.  The data were then
compared with archival sky-survey images going back to 2004 to look for
long-term changes in the light curve.  The binary's light curve is
unique, in that it exhibits a spike in emission every two minutes as well
as a major brightness variation over the approximately 3.5-hour orbital
period of the two stars.  One model of this system predicts long-term
variations in the way the two stars interact.  It was not known what the
time-scale of such changes might be -- whether 20 or 200 years.  By
looking at the K2 and archival data, astronomers were able to show that
in addition to hourly changes in the system, there are variations
occurring over decades.

A white dwarf is a very dense remnant of a star like the Sun.  When a
Sun-like star runs out of energy, gravity compresses its core to about
the size of the Earth but with a mass 300,000 times higher.  A teaspoon-
sized piece of a white dwarf would weigh about 15 tons.  The compression
of the star can also amplify its magnetic-field strength and its spin
rate.  The system became famous in 2016 when researchers in England
discovered that AR Scorpii, believed to be a mundane solitary star, was
actually a rapidly varying binary.  The system is remarkable, as the
white dwarf spins on its axis at an incredibly fast rate, causing flashes
in luminosity every two minutes.  The amplitude of the flashes varies
over the 3.5-hour orbital period, something that is not known to happen
in any other white-dwarf binary system.  Astronomers found that, 12 years
ago, AR Scorpii's peak brightness came a bit later in its orbit than it
does now.  That does not solve the mystery, but it is another piece to
the puzzle that is AR Scorpii.


'EXTREME' TELESCOPES FIND SECOND-FASTEST SPINNING PULSAR
NASA/Goddard Space Flight Center

By following high-energy sources mapped out by NASA's Fermi Gamma-ray
Space Telescope, the Netherlands-based Low-Frequency Array (LOFAR) radio
telescope has identified a pulsar spinning at more than 42,000
revolutions per minute, making it the second-fastest known.  A pulsar is
the core of a massive star that exploded as a supernova.  In that stellar
remnant, also called a neutron star, a mass equivalent to half a
million Earths is crushed into a magnetized, spinning ball no larger than
Washington, D.C.  The rotating magnetic field powers beams of radio waves,
visible light, X-rays and gamma rays.  If a beam happens to sweep across
the Earth, astronomers observe regular pulses of emission and classify the
object as a pulsar.  The new object, named PSR J0952-0607 -- or J0952 for
short -- is classified as a millisecond pulsar and is located between
3,200 and 5,700 light-years away in the constellation Sextans.  The pulsar
contains about 1.4 times the Sun's mass and is orbited every 6.4 hours by
a companion star that has been whittled away to less than 20 times the
mass of the planet Jupiter.  At some point in the system's history,
matter began streaming from the companion onto the pulsar, gradually
raising its spin to 707 rotations a second, or more than 42,000 rpm, and
greatly increasing its emissions.  Eventually, the pulsar began
evaporating its companion, and that process continues today.  Because of
their similarity to spiders that consume their mates, systems like J0952
are called black widow or redback pulsars, depending on how much of the
companion star remains.  Most of the known systems of that type were
found by following up Fermi unassociated sources.

The LOFAR discovery also hints at the potential to find a new population
of ultra-fast pulsars.  Theorists say pulsars could rotate as fast as
72,000 rpm before breaking apart, yet the fastest spin known -- by PSR
J1748-2446ad, reaching nearly 43,000 rpm -- is just 60 per cent of the
theoretical maximum.  Perhaps pulsars with faster periods simply can't
form.  But the gap between theory and observation may also result from the
difficulty in detecting the fastest rotators.  During its nine years in
orbit, Fermi has played a role in the discovery of more than 100 pulsars,
either through direct detection of gamma-ray pulses or radio follow-up of
unassociated sources.


EARLY BLACK HOLES
University of California - Los Angeles

A long-standing question in astrophysics is whether the Universe's very
first black holes came into existence less than a second after the Big
Bang or whether they formed only millions of years later during the
deaths of the earliest stars.  Now there is a compellingly simple new
theory suggesting that black holes could have formed very shortly after
the Big Bang, long before stars began to shine.  Astronomers have
previously suggested that such so-called primordial black holes could
account for all or some of the Universe's mysterious dark matter and that
they might have seeded the formation of the supermassive black holes that
exist at the centres of galaxies.  The new theory proposes that primordial
black holes might help create many of the heavier elements found in
nature.  The researchers began by considering that a uniform field of
energy pervaded the Universe shortly after the Big Bang.  Scientists
expect that such fields existed in the distant past.  After the Universe
rapidly expanded, that energy field would have separated into clumps. 
Gravity would cause those clumps to attract one another and merge
together.  The UCLA researchers proposed that some small fraction of
the growing clumps became dense enough to become black holes.  Their
hypothesis is fairly generic, and it does not rely on the 'unlikely
coincidences' that undermine other theories explaining primordial black
holes.  They suggest that it is possible to search for such primordial black
holes using astronomical observations.  One method involves measuring the
very tiny changes in a star's brightness that result from the gravitational
effects of a primordial black hole passing between the Earth and that star. 
Earlier this year, U.S. and Japanese astronomers published a paper on their
discovery of one star in a nearby galaxy that brightened and dimmed
precisely as if a primordial black hole were passing in front of it.  In a
separate study, astronomers proposed that primordial black holes might play
an important role in the formation of heavy elements such as gold, silver,
platinum and uranium, which could be ongoing in our galaxy and others. 
The origin of those heavy elements has long been a mystery to researchers.

The UCLA research suggests that a primordial black hole occasionally
collides with a neutron star.  When that happens, the primordial black
hole consumes the neutron star from the inside, a process that takes
about 10,000 years.  As the neutron star shrinks, it spins even faster,
eventually causing small fragments to detach and fly off.  Those fragments
of neutron-rich material may be the sites in which neutrons fuse into
heavier and heavier elements.  However, the probability of a neutron star
capturing a black hole is rather low, which is consistent with
observations of only some galaxies being enriched in heavy elements. 
The theory that primordial black holes collide with neutron stars to create
heavy elements also explains the observed lack of neutron stars in the
centre of the Milky Way galaxy, a long-standing mystery in astrophysics.


PULSAR REVEALS UNIQUE TIMING GLITCH
RAS
 
The discovery of the largest timing irregularity yet observed in a pulsar
is the first confirmation that pulsars in binary systems exhibit the
strange phenomenon known as a 'glitch'.  Pulsars are one possible result
of the final stages of evolution of massive stars.  Such stars end their
'lives' in huge supernova explosions, ejecting their stellar materials
outwards into space and leaving behind an extremely dense and compact
object, which could be a white dwarf, a neutron star or a black hole. 
If a neutron star is left, it may have a very strong magnetic field and
rotate extremely quickly, emitting a beam of light that can be observed
when the beam points towards the Earth, in much the same way as a light-
house beam sweeps past an observer.  To the observer on Earth, it looks
as though the star is emitting pulses of light, hence the name 'pulsar'.
Now a group of scientists in Turkey has discovered a sudden change in the
rotation speed of the peculiar pulsar SXP 1062.  Jumps in frequency,
known as 'glitches', are commonly seen in isolated pulsars, but have
so far never been observed in binary pulsars (pulsars orbiting with a
companion white dwarf or neutron star) such as SXP 1062.  SXP 1062 is
located in the Small Magellanic Cloud, a satellite galaxy of our own
Milky Way galaxy, and one of our nearest intergalactic neighbours at
200,000 light-years away.  That pulsar is particularly interesting, since
as well as orbiting its partner star as part of a binary pair, it is also
still surrounded by the remnants of the supernova explosion which created
it.
 
The pulsar is thought to pull in the leftover material from the supernova
explosion, feeding on it in a process known as accretion.  The team
believes that the size of the glitch is due to the gravitational influence
of its companion star and the accretion of the surrounding remnant
material, which together exert large forces on the crust of the neutron
star.  When the forces are no longer sustainable, a rapid change in
internal structure transfers momentum to the crust, changing the rotation
of the pulsar very suddenly and producing a glitch.  The size of the
glitch indicates that the interiors of neutron stars in binary systems
may be quite different from the interiors of isolated neutron stars.  The
work will be followed up with NASA's 'Neutron-Star Interior Composition
Explorer' (NICER) mission, launched in June this year.  The team hopes
that the finding may lead to a better understanding of the interiors of
neutron stars, putting new constraints on the neutron-star equation of
state.


SUPERNOVA ANALYSIS REFRAMES DARK-ENERGY DEBATE
RAS
 
The accelerating expansion of the Universe may not be real, but could
just be an apparent effect, according to a new study by a group at the
University of Canterbury in Christchurch, New Zealand, which finds the fit
of Type Ia supernovae to a model universe with no dark energy to be very
slightly better than the fit to the standard dark energy model.  Dark
energy has been assumed to form roughly 70% of the present material
content of the Universe. However, that mysterious quantity is really
just a place-holder for unknown physics.  Current models of the Universe
require a dark-energy term to explain the observed acceleration in the
rate at which the Universe is expanding.  Scientists base that conclusion
on measurements of the distances to supernova explosions in distant
galaxies, which appear to be further away than they should be if the
Universe?s expansion were not accelerating.  However, just how
statistically significant that signature of cosmic acceleration is has
been hotly debated in the past year.  The previous debate pitted the
standard Lambda Cold Dark Matter (CDM) cosmology against an empty
universe whose expansion neither accelerates nor decelerates.  Both of
those models, though, assume a simplified 100-year-old cosmic expansion
law -- Friedmann's equation.  Friedmann's equation assumes an expansion
identical to that of a featureless soup, with no complicating structure.
However, the present Universe actually contains a complex cosmic web of
galaxy clusters in sheets and filaments that surround and thread vast
empty voids.

It is believed that the past debate missed an essential point; if dark
energy does not exist then a likely alternative is that the average
expansion law does not follow Friedmann's equation.  Rather than
comparing the standard CDM cosmological model with an empty Universe,
the new study compares the fit of supernova data in CDM to a different
model, called the 'timescape cosmology'.  That has no dark energy.  Instead,
clocks carried by observers in galaxies differ from the clock that best
describes average expansion once the lumpiness of structure in the
Universe becomes significant.  Whether or not one infers accelerating
expansion then depends crucially on the clock used.  The timescape
cosmology was found to give a slightly better fit to the largest
supernova data catalogue than the CDM cosmology.  Unfortunately the
statistical evidence is not yet strong enough to rule definitively in
favour of one model or the other, but future missions such as ESA's
Euclid satellite will be able to distinguish between the standard cosmo-
logy and other models, and help scientists to decide whether dark energy
is real or not.  Deciding that not only requires more data, but also
better understanding of the properties of supernovae which currently
limit the precision with which they can be used to measure distances. 
On that score, the new study shows significant unexpected effects which
are missed if only one expansion law is applied.  Consequently, even as
a toy model the timescape cosmology provides a powerful tool to test our
current understanding, and casts new light on our most profound cosmic
questions.