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Early August Astronomy Bulletin
« on: August 08, 2018, 16:17 »
European Space Agency
Radar data collected by ESA's Mars Express point to a pond of liquid water
buried under layers of ice and dust in the south polar region of Mars.
Evidence for the Red Planet's watery past is prevalent across its surface
in the form of vast dried-out river-valley networks and gigantic outflow
channels clearly imaged by orbiting spacecraft.  Orbiters, together with
landers and rovers exploring the Martian surface, also discovered minerals
that can only form in the presence of liquid water.  But the climate has
changed significantly over the course of the planet's 4600-million-year
history, and liquid water cannot exist on the surface today, so scientists
are looking underground.  Early results from the 15-year-old Mars Express
spacecraft already found that water-ice exists at the planet's poles and is
also buried in layers interspersed with dust.  The presence of liquid water
at the base of the polar ice caps has long been suspected; after all, from
studies on Earth, it is well known that the melting point of water decreases
under the pressure of an overlying glacier.  Moreover, the presence of salts
on Mars could further reduce the melting point of water and keep the water
liquid even at below-freezing temperatures.  But until now evidence from the
'Mars Advanced Radar for Subsurface and Ionosphere Sounding' instrument,
MARSIS, the first radar sounder ever to orbit another planet, remained
inconclusive.  It has taken the persistence of scientists working with that
subsurface-probing instrument to develop new techniques in order to collect
as many high-resolution data as possible to confirm their exciting
conclusion.  Ground-penetrating radar uses the method of sending radar
pulses towards the surface and timing how long it takes for them to be
reflected back to the spacecraft, and with what strength.  The properties
of the material that lies under the ground influences the returned signal,
which can be used to map the sub-surface topography.

The radar investigation shows that south polar region of Mars is made of
many layers of ice and dust down to a depth of about 1.5 km in the 200-km-
wide area analysed in this study.  A particularly bright radar reflection
underneath the layered deposits is identified within a 20-km-wide zone.
Analysing the properties of the reflected radar signals and considering the
composition of the layered deposits and expected temperature profile below
the surface, the scientists interpret the bright feature as an interface
between the ice and a stable body of liquid water, which could be laden with
salty, saturated sediments.  For such a patch of water to be detectable by
MARSIS, it would need to be at least several tens of centimetres thick.
This sub-surface anomaly on Mars has radar properties matching water or
water-rich sediments.  The finding is somewhat reminiscent of Lake Vostok,
discovered some 4 km below the ice in Antarctica on Earth.  Some forms of
microbial life are known to thrive in the Earth's sub-glacial environments,
but underground pockets of salty, sediment-rich liquid water on Mars might
also provide a suitable habitat, either now or in the past.  Whether life
has ever existed on Mars remains an open question, and is one that Mars
missions, including the current European--Russian ExoMars orbiter and future
rover, will continue to explore.  Mars Express was launched 2003 June 2 and
is due to celebrate its 15 years in Mars orbit on December 25 this year.

Massachusetts Institute of Technology.

For nearly a century, astronomers have puzzled over the curious variability
of young stars residing in the Taurus-Auriga region some 450 light-years
away.  One star in particular has drawn astronomers' attention.  Every few
decades, the star's light has faded briefly before brightening again.  In
recent years, astronomers have observed the star dimming more frequently,
and for longer periods, raising the question: what is repeatedly obscuring
the star?  The answer, astronomers believe, could shed light on some of the
chaotic processes that take place early in a star's development.  Now
physicists have observed the star, named RW Aur A, using NASA's Chandra
X-Ray Observatory.  They have found evidence for what may have caused its
most recent dimming event: a collision of two infant planetary bodies, which
produced in its aftermath a dense cloud of gas and dust.  As those planetary
debris fell into the star, they generated a thick veil, temporarily obscuring
the star's light.  Computer simulations have long predicted that planets can
fall into a young star, but astronomers have never before observed such an
event.  If their interpretation of the data is correct, this would be the
first time that we have directly observed a young star devouring a planet
or planets.  The star's previous dimming events may have been caused by
similar smash-ups, of either two planetary bodies or large remnants of past
collisions that met head-on and broke apart again.  Scientists who study the
early development of stars often look to the Taurus-Auriga Dark Clouds, a
gathering of molecular clouds, which host stellar nurseries containing
thousands of infant stars.  Young stars form from the gravitational
collapse of gas and dust within those clouds.  Very young stars, unlike our
comparatively mature Sun, are still surrounded by rotating discs of debris,
including gas, dust, and clumps of material ranging in size from small dust
grains to pebbles, and possibly to fledgling planets.

In our Solar System, we have planets, not a massive disc around the Sun.
Such discs last for maybe 5 million to 10 million years, and in Taurus,
there are many stars that have already lost their discs, but a few still
have them.  If you want to know what happens in the end stages of disc
dispersal, Taurus is one of the places to look.  Astronomers focus on stars
that are young enough still to host discs.  They were particularly interest-
ed in RW Aur A, which is at the older end of the age range for young stars,
as it is estimated to be several million years old.  RW Aur A is part of a
binary ststem: it circles another young star, RW Aur B.  Both those stars
are about the same mass as the Sun.  Since 1937, astronomers have recorded
noticeable dips in the brightness of RW Aur A at intervals of decades.  Each
dimming event appeared to last for about a month.  In 2011, the star dimmed
again, this time for about half a year.  The star eventually brightened,
only to fade again in mid-2014.  In 2016 November, the star returned to its
full luminosity.  In 2017 January, RW Aur A dimmed again, and the team used
NASA's Chandra X-Ray Observatory to record X-ray emission from the star. 
In total, Chandra recorded almost 14 hours of X-ray data from the star.
After analyzing those data, the researchers reported several surprising
revelations.  The star's disc hosts a large amount of material; the star
is much hotter than expected; and the disc contains much more iron than
expected -- not as much iron as is found in the Earth, but more than in,
say, a typical moon in our Solar System.  (Our own moon, however, has far
more iron than the scientists estimated in the star's disc.)  This last
point was the most intriguing for the team.  Typically, an X-ray spectrum
of a star can show various elements, such as oxygen, iron, silicon, and
magnesium, and the amount of each element present depends on the temperature
within the star's disc.  Here, we see a lot more iron, at least a factor of
10 times more than before, which is very unusual, because typically stars
that are active and hot have less iron than others, whereas this one has
more.  Where has all that iron come from?  The researchers speculate that
the excess iron may have come from one of two possible sources.  The first
is a phenomenon known as a dust pressure trap, in which small grains or
particles such as iron can become trapped in 'dead zones' of a disc.  If the
disc's structure changes suddenly, such as when a partner star passes close
by, the resulting tidal forces can release the trapped particles, creating
an excess of iron that can fall into the star.  The second theory is the
more compelling one.  In that scenario, excess iron is created when two
planetesimals, or infant planetary bodies, collide, releasing a thick cloud
of particles.  If one or both planets are made partly of iron, their
smash-up could release a large amount of iron into the star's disc and
temporarily obscure its light as the material falls into the star.  There
are many processes that happen in young stars, but these two scenarios could
possibly make something that looks like what the team observed.  More
observations of the star will be needed in the future, to see whether the
amount of iron surrounding the star has changed -- a measure that could help
researchers determine the size of the iron's source.  For instance, if the
same amount of iron appears in, say, a year, that may signal that the iron
comes from a relatively massive source, such as a large planetary collision,
versus if there is very little iron left in the disc.  Much effort currently
goes into learning about exoplanets and how they form, so it is obviously
very important to see how young planets could be destroyed in interactions
with their host stars and other young planets, and what factors determine
whether they survive.

University of Michigan

Scientists have deduced that the Andromeda galaxy, our closest large
galactic neighbour, shredded and cannibalized a massive galaxy two
billion years ago.  Even though it was mostly shredded, that massive galaxy
left behind a rich trail of evidence: an almost invisible halo of stars
larger than the Andromeda galaxy itself, an elusive stream of stars and a
separate enigmatic compact galaxy, M32.  Discovering and studying that
decimated galaxy will help astronomers understand how disc galaxies like the
Milky Way evolve and survive large mergers.  The disrupted galaxy, named
M32p, was the third-largest member of the Local Group of galaxies, after the
Milky Way and Andromeda galaxies.  Using computer models, the astronomers
were able to piece together the evidence, revealing this long-lost sibling
of the Milky Way.  Scientists have long known that the nearly invisible
large haloes of stars surrounding galaxies contain the remnants of smaller
cannibalized galaxies.  A galaxy like Andromeda was expected to have
consumed hundreds of its smaller companions.  Researchers thought that would
make it difficult to learn about any single one of them.  Using new computer
simulations, the scientists were able to understand that even though many
companion galaxies were consumed by Andromeda, most of the stars in the
Andromeda's outer faint halo were mostly contributed by shredding a single
large galaxy.  They realized they could use that information of Andromeda's
outer stellar halo to infer the properties of the largest of the shredded
galaxies.  Astronomers have been studying the Local Group -- the Milky Way,
Andromeda and their companions -- for so long that it was rather shocking to
realize that the Milky Way had a large sibling, and we never knew about it.
The galaxy called M32p, which was shredded by the Andromeda galaxy, was at
least 20 times larger than any galaxy which merged with the Milky Way over
the course of its lifetime.  M32p would have been massive, making it the
third-largest galaxy in the Local Group after Andromeda and the Milky Way
galaxies.  The scientists say that this idea might also solve a long-
standing mystery -- the formation of Andromeda's enigmatic satellite galaxy
M32.  They suggest that the compact and dense M32 is the surviving centre of
the Milky Way's long-lost sibling, like the indestructible stone in a plum.
While M32 looks like a compact example of an old, elliptical galaxy, it
actually has lots of young stars.  It is one of the most compact galaxies
in the Universe.  The researchers say that their study may alter the
traditional understanding of how galaxies evolve.  They realized that the
Andromeda's disc survived an impact with a massive galaxy, which would
question the common wisdom that such large interactions would destroy discs
and form an elliptical galaxy.  The timing of the merger may also explain
the thickening of the disc of the Andromeda galaxy as well as a burst of
star formation two billion years ago, a finding which was independently
reached by French researchers earlier this year.  The Andromeda Galaxy, with
a spectacular burst of star formation, would have looked very different two
billion years ago.

Hiroshima University
Near the centre of the constellation of Cygnus is a star orbiting the first
black hole ever discovered.  Together, they form a binary system known as
Cygnus X-1.  The black hole is also one of the brightest sources of X-rays
in the sky.  However, the geometry of matter that gives rise to that light
was uncertain. The research team revealed that information from a new
technique called X-ray polarimetry.  Taking a picture of a black hole is not
easy.  For one thing, it is not yet possible to observe a black hole because
light cannot escape it.  Rather, instead of observing the black hole itself,
scientists can observe light coming from matter close to the black hole.  In
the case of Cygnus X-1, thst matter comes from the star that closely orbits
the black hole.  Most light that we see, such as from the Sun, vibrates in
many directions.  Polarization filters light so that it vibrates in one
direction.  That is how snow goggles with polarized lenses let skiers see
more easily where they are going down the mountain -- they work because the
filter cuts out light reflecting off the snow.  It's the same situation with
hard X-rays around a black hole.  However, hard X-rays and gamma rays coming
from near the black hole penetrate the filter.  There are no such 'goggles'
for those rays, so we need another special kind of treatment to direct and
measure that scattering of light.  The team needed to determine where the
light was coming from and where it was being scattered.  In order to make
both of those measurements, they launched an X-ray polarimeter on a balloon
called PoGO+.  From there, the team could piece together what fraction of
hard X-rays reflected off the accretion disc and identify the matter shape.

Two competing models describe how matter near a black hole can look in a
binary system such as Cygnus X-1: the lamp-post and the extended model.  In
the lamp-post model, the corona is compact and bound closely to the black
hole.  Photons bend toward the accretion disc, resulting in more reflected
light.  In the extended model, the corona is larger and spread around the
vicinity of the black hole.  In that case, the reflected light by the disc
is weaker.  Since light did not bend that much under the strong gravity of
the black hole, the team concluded that the black hole fitted the extended-
corona model.  With that information, the researchers can uncover more
characteristics about black holes.  One example is their spin.  The effects
of spin can modify the space-time surrounding the black hole.  Spin could
also provide clues to the evolution of the black hole.  It could be slowing
down in speed since the beginning of the Universe, or it could be accumu-
lating matter and spinning faster.

In 2013, ESA's Planck mission unveiled a new image of the cosmos: an all-
sky survey of the microwave radiation produced at the beginning of the
Universe.  That first light emitted by the Universe provides a wealth of
information about its content, its rate of expansion, and the primordial
fluctuations in density that were the precursors of the galaxies.  The
Planck consortium has now published the full and final version of those
data on the ESA website.  With its increased reliability and its data on
the polarisation of relic radiation, the Planck mission corroborates the
standard cosmological model with unrivalled precision for those parameters,
even if some anomalies still remain.  For that work the Planck consortium
called upon some three hundred researchers, in particular from CNRS, CNES
(the French national space agency), CEA (the French Alternative Energies and
Atomic Energy Commission) and several universities in France. 

Launched in 2009, the Planck satellite mapped the cosmic microwave back-
ground, microwave radiation emitted 380,000 years after the Big Bang, when
the Universe was still a hot, almost completely homogeneous gas.  Tiny
variations in its temperature provide information about its content, its
rate of expansion and the properties of the primordial fluctuations that
gave rise to the galaxies.  An initial analysis of the data set was
published in 2015, in the form of eight all-sky maps that included the
polarisation of the cosmic microwave background, which determines how the
waves that make up light vibrate on tiny scales.  That key information
bears the imprint of the last interaction between light and matter in the
primordial Universe.  However, only a preliminary analysis had been carried
out on it.

The polarisation of relic radiation produces a signal 50 to 100 times weaker
than that of its temperature and 10 to 20 times weaker than that emitted by
the polarised emission of Galactic dust.  Thanks to its HFI (High Frequency
Instrument), the Planck satellite nonetheless obtained an extremely precise
map of primordial polarisation across the entire sky.  This was a world
first and provides us with a wealth of information.  Comprehensive,
definitive and more reliable, the data published on 2018 July 17 confirm
the preliminary findings, supporting a model which provides an excellent
description of the content of the Universe in terms of ordinary matter, cold
dark matter and dark energy (whose nature is unknown), with an inflation
phase at its very beginning.  That cosmological model can now be derived
using temperature or polarisation data independently, with comparable
accuracy.  It considerably reinforces the standard model of cosmology,
however surprising that may be.  The results are described in a set of a
dozen scientific papers, involving around 300 researchers.  However, some
anomalies and limitations remain.  In particular, the rate of expansion of
the Universe differs by a few per cent depending on whether the data from
the Hubble Space Telescope or from the Planck mission are used.  That
question is still an open one, and a lot of telescopes will be marshalled
in an attempt to resolve the issue.

Observations made with the Very Large Telescope have for the first time
revealed the effects predicted by Einstein's general relativity on the
motion of a star passing through the extreme gravitational field near the
super-massive black hole in the centre of the Milky Way.  That long-sought
result represents the climax of a 26-year-long observational campaign using
ESO's telescopes in Chile.  Obscured by thick clouds of absorbing dust, the
closest supermassive black hole to the Earth lies 26,000 light-years away at
the centre of the Milky Way.  That gravitational monster, which has a mass
four million times that of the Sun, is surrounded by a small group of stars
orbiting around it at high speed.  That extreme environment -- the strongest
gravitational field in our galaxy -- makes it the perfect place to explore
gravitational physics, and particularly to test Einstein?s general theory of
relativity.  New infrared observations from the GRAVITY, SINFONI and NACO
instruments on ESO's Very Large Telescope (VLT) have now allowed astronomers to follow one of the stars, called S2, as it passed very close to the
black hole during 2018 May.  At the closest point the star was at a distance
of less than 20 billion kilometres from the black hole and moving at a
speed of more than 25 million kilometres per hour -- almost three per cent
of the speed of light.  The team compared the position and velocity measure-
ments from GRAVITY and SINFONI respectively, along with previous observa-
tions of S2 using other instruments, with the predictions of Newtonian
gravity, general relativity and other theories of gravity.  The new results
are inconsistent with Newtonian predictions and in excellent agreement with
the predictions of general relativity.

The new measurements clearly reveal an effect called gravitational red-
shift.  Light from the star is stretched to longer wavelengths by the very
strong gravitational field of the black hole.  And the change in the
wavelength of light from S2 agrees precisely with that predicted by
Einstein's theory of general relativity.  This is the first time that such
a deviation from the predictions of the simpler Newtonian theory of gravity
has been observed in the motion of a star around a supermassive black hole.
The team used SINFONI to measure the velocity of S2 towards and away from
the Earth and the GRAVITY instrument in the VLT Interferometer (VLTI) to make precise measurements of the changing position of S2 in order to define the
shape of its orbit.  GRAVITY creates such sharp images that it can reveal the
motion of the star from night to night as it passes close to the black hole,
26 000 light-years from Earth.  The first observations of S2 with GRAVITY,
about two years ago, already showed that it would be the ideal black hole
laboratory.  During the close passage, scientists could even detect the
faint glow around the black hole on most of the images, which allowed them
to follow the star precisely in its orbit, ultimately leading to the
detection of the gravitational redshift in the spectrum of S2.  More than
one hundred years after he published his paper setting out the equations of
general relativity, Einstein has been proved right once more -- and in a
much more extreme laboratory than he could have possibly imagined!  Here in
the Solar System we can only test the laws of physics now and under certain
circumstances.  So it's very important in astronomy to also check that those
laws are still valid where the gravitational fields are very much stronger.
Continuing observations are expected to reveal another relativistic effect
very soon -- a small rotation of the star's orbit, known as Schwarzschild
precession -- as S2 moves away from the black hole.

SAPeople News

South Africa has unveiled a new super radio telescope that will study galaxy
formation, a first phase of what will be the world's largest telescope.  The
64-dish MeerKAT telescope in the Northern Cape region of South Africa will
be integrated into a multinational Square Kilometre Array (SKA).  When fully
operational, the SKA telescope will be 50 times more powerful than current
telescopes.  The telescope will be the largest of its own kind in the world,
with image-resolution quality exceeding the Hubble Space Telescope by a
factor of 50.  The SKA will comprise a forest of 3,000 dishes spread over
a total area of a square kilometre across remote terrain in several African
countries, as well as Australia, to allow astronomers to see deeper into
space with unparalleled detail.  The SKA, which is expected to be fully
operational by 2030, will explore exploding stars, black holes and traces of
the universe's origins some 14 billion years ago.  The telescope is being
built by an international consortium, including Australia, Britain, Canada,
China, India, Italy, New Zealand, Sweden and the Netherlands.  Other African
countries involved are Botswana, Ghana, Kenya, Madagascar, Mauritius,
Mozambique, Namibia and Zambia.  Last month, scientists linked a powerful
optical telescope, MeerLITCH, built 200 km south of Carnarvon, with the
MeerKAT to allow for simultaneous optical and radio study of cosmic events
as they occur.

Astronomers using ALMA and NOEMA have made the first definitive detection of a radioactive molecule in interstellar space.  The radioactive part of the
molecule is an isotope of aluminium.  The observations reveal that the
isotope was dispersed into space after the collision of two stars, that left
behind a remnant known as CK Vulpeculae.  This is the first time that a
direct observation has been made of that element from a known source.
Previous identifications of that isotope have come from the detection of
gamma rays, but their precise origin had been unknown.  The team used the
Atacama Large Millimeter/submillimeter Array (ALMA) and the NOrthern
Extended Millimeter Array (NOEMA) to detect a source of the radioactive
isotope aluminium-26.  The source, known as CK Vulpeculae, was first seen
in 1670 and at the time it appeared to observers as a bright, red 'new
star'.  Though initially visible with the naked eye, it quickly faded and
now requires powerful telescopes to see it, a dim central star surrounded
by a halo of glowing material flowing away from it.  348 years after the
initial event was observed, the remains of this explosive stellar merger
have led to the clear and convincing signature of a radioactive version of
aluminium, known as aluminium-26.  This is the first unstable radioactive
isotope definitively detected outside the Solar System.  Unstable isotopes
have an excess of nuclear energy and eventually decay into a stable form.
The team detected the unique spectral signature of molecules made up of
aluminium-26 and fluorine (26AlF) in the debris surrounding CK Vulpeculae,
which is about 2000 light-years away.  As these molecules spin and tumble
through space, they emit a distinctive fingerprint of millimetre-wavelength
light, a process known as rotational transition.  Astronomers consider that
to be the 'gold standard' for detections of molecules.  The observation of
that particular isotope provides fresh insights into the merger process that
created CK Vulpeculae.  It also demonstrates that the deep, dense, inner
layers of a star, where heavy elements and radioactive isotopes are forged,
can be churned up and cast out into space by stellar collisions.

The astronomers also determined that the two stars that merged were of
relatively low mass, one being a red giant star with a mass somewhere
between 0.8 and 2.5 times that of our Sun.  Being radioactive, aluminium-26
will decay to become more stable, and in that process one of the protons in
the nucleus decays into a neutron.  During the process, the excited nucleus
emits a photon with very high energy, which we observe as a gamma ray.
Previously, detections of gamma-ray emissions have shown that around two
solar masses of aluminium-26 are present across the Milky Way, but the
process that created the radioactive atoms was unknown.  Furthermore, owing
to the way that gamma rays are detected, their precise origin was also
largely unknown.  At the same time, however, the team has concluded that
the production of aluminium-26 by objects similar to CK Vulpeculae is
unlikely to be the major source of aluminium-26 in the Milky Way.  The mass
of aluminium-26 in CK Vulpeculae is roughly a quarter of the mass of Pluto,
and given that these events are so rare, it is highly unlikely that they are
the sole producers of the isotope in the Milky Way galaxy.  This leaves the
door open for further studies into such radioactive molecules.

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