Topic: Late October Astronomy Bulletin

[Clive]

AVALANCHES CAUSE COMET NUCLEI OUTBURSTS
Planetary Science Institute

New research shows that outbursts of comet nuclei are probably caused
by surface avalanches rather than geyser-like eruptions from within.
Rapid asymmetric brightening events of comets have been observed for
decades and have long been thought to be the result of some sort of
eruption of materials from deep within the interior of a comet.
High-resolution images from Rosetta observations of 67P/Churyumov-
Gerasimenko show outbursts that resemble plumes of material from
geysers on Earth.  However, there is a major problem with that model.
There is no internal heat source on comets to power geyser-like
eruptions.  Instead, outburst plumes are to be seen as the natural
result of avalanches.  The surfaces of comets have regions at the
bases of slopes and cliffs that are rich in icy materials, and are
actively sublimating, with ice turning directly into gas.  As the gas
leaves the surface of the comet, it produced a weak breeze.  When
granular materials on comets slide downslope or over a cliff, they
enter this sublimation breeze and are blown into a tightly collimated
plume of material that leaves the surface of the nucleus.  This model
is consistent with data collected by the Rosetta spacecraft, and
provides a physical mechanism that allows the outbursts to be studied
to determine where on the surface they come from and how much material
avalanched downslope.


DIONE HARBOURS SUB-SURFACE OCEAN
Royal Observatory of Belgium

New data from the Cassini mission to Saturn reveal that a sub-surface
ocean lies deep within Saturn's moon Dione.  Two other moons of
Saturn, Titan and Enceladus, are already known to hide global oceans
beneath their icy crusts.  In the new study, researchers of the Royal
Observatory of Belgium show that gravity data from recent Cassini
fly-bys can be explained if Dione's crust floats on an ocean 100 km
below the surface.  The ocean is several tens of kilometres deep and
surrounds a large rocky core.  Seen from within, Dione is very similar
to its smaller but more famous neighbour Enceladus, whose south-polar
region spurts huge jets of water vapour into space.  Dione seems to be
quiet now, but its broken surface bears witness to a tumultuous past.
According to the new study, Enceladus' ocean is much closer to the
surface, especially near the south pole where geysers erupt through a
few kilometres of crust.  These findings agree well with the discovery
last year by Cassini that Enceladus undergoes large oscillations,
called libration, during its orbit.  Enceladus' libration would be
much smaller if its crust were thicker.  As for Dione, the new study
finds that it harbours a deep ocean between its crust and core.
Dione's ocean has probably survived for the whole history of the moon.


POSSIBLE ADDITIONAL MOONS AROUND URANUS'S RINGS
University of Idaho

After re-examining data acquired by the Voyager 2 spacecraft,
astronomers have detected wavy patterns in two of Uranus's rings --
patterns that may be indicative of two undiscovered moons.  Like the
other gas giants in the Solar System, Uranus has a ring system, though
it is not nearly as spectacular as the one around Saturn.  And like
the other gas giants, Uranus has a lot of natural satellites -- 27 are
now known.  New research suggests that that number might have to be
revised; data collected by Voyager 2 during its historic 1986 fly-by
hint at two undiscovered moons lurking near a pair of Uranus rings.
The suspected new moons reside in Alpha and Beta -- the 5th and 6th
rings.  Voyager 2 found 10 moons when it visited the planet in 1986,
tripling the number of moons known to orbit the gas giant.  The two
rings exhibit a series of wavy patterns consistent with the presence
of two tiny moons.  The patterns may be wakes from small moonlets
orbiting outside those rings.  Importantly, the observations are
consistent with how Uranus's other moons, such as Cordelia and
Ophelia, are exerting gravitational pressure on the dust, rocks, and
ice within the rings, herding the particles along a narrow formation.
If the now-postulated moons exist, they are quite dark and small,
measuring only 4 to 14 km across.  That would make them smaller than
any other known moons orbiting the planet, which explains why Voyager
2 did not detect them directly.  The researchers are now planning to
inspect Uranus with the Hubble telescope.


GIANT PLASMA BALLS EJECTED FROM DYING STAR
NASA/Jet Propulsion Laboratory

The Hubble telescope has detected super-hot blobs of gas, each twice
as massive as the planet Mars, being ejected near a dying star.  The
plasma balls are moving so fast that it would take only half an hour
for them to travel the distance between the Earth and the Moon.
Astronomers estimate that such stellar 'cannon fire' has taken place
every 8.5 years for at least the past 400 years.  The fireballs
present a puzzle to astronomers, because the ejected material could
not have been shot out by the host star, called V Hydrae.  The star is
a bloated red giant, 1,200 light-years away, which has probably shed
at least half its mass into space during its death throes.  Red giants
are dying stars in the late stages of their existence, that are
exhausting the nuclear fuel that makes them shine.  They have expanded
in size and are shedding their outer layers into space.  The current
best explanation suggests that the plasma balls were launched by an
unseen companion star.  According to that theory, the companion would
have to be in an elliptical orbit that carries it close to the red
giant's puffed-up atmosphere every 8.5 years.  As the companion enters
the bloated star's outer atmosphere, it steals material, which then
settles into a disc around the companion and serves as the launching
pad for blobs of plasma, which travel at roughly half a million miles
per hour.  This star system could be the archetype to explain the
variety of glowing shapes called planetary nebulae, uncovered by
Hubble, that are seen around dying stars.  They are expanding shells
of glowing gas expelled by stars late in their evolution..

Hubble observations over the past two decades have revealed an
enormous complexity and diversity of structure in planetary nebulae.
The telescope has imaged knots of material in the glowing gas clouds
surrounding the dying stars.  Astronomers speculated that the knots
were actually jets ejected by discs of material around companion stars
that were not visible in the Hubble images (many stars in our Milky
Way galaxy are members of binary systems).  But the details of how the
jets were produced remained a mystery.  Astronomers want to identify
the process that causes the transformation from a puffed-up red giant
to a beautiful, glowing planetary nebula: dramatic changes occur over
roughly 200 to 1,000 years, which is the blink of an eye in cosmic
time.  The team used Hubble's 'Space Telescope Imaging Spectrograph'
(STIS) to conduct observations of V Hydrae and its surrounding region
over an 11-year period, first from 2002 to 2004, and then from 2011 to
2013.  The data showed a string of monstrous, super-hot blobs, each
with a temperature of more than 9,400°C -- almost twice as hot as the
surface of the Sun.  The researchers compiled a detailed map of the
blobs' locations, allowing them to trace the first clumps back to
1986.  The STIS data show blobs that have just been ejected, blobs
that have moved a little farther away, and blobs that are even farther
away.  STIS detected the giant structures as far away as 37000 million
miles away from V Hydrae, more than eight times further away than the
Kuiper Belt of icy debris at the edge of our Solar System is from the
Sun.  The blobs expand and cool as they move further away, and are
then not detectable in visible light.  But observations taken at
longer, sub-millimetre wavelengths in 2004, by the Sub-millimetre
Array in Hawaii, revealed fuzzy, knotty structures that may be blobs
launched 400 years ago.

A surprise from the STIS observation was that the disc does not fire
the monster clumps in exactly the same direction every 8.5 years.  The
direction flip-flops slightly, from side-to-side to back-and-forth,
owing to a possible wobble in the accretion disc.  Astronomers have
noted that V Hydrae is obscured every 17 years, as if something is
blocking its light.  Astronomers suggest that owing to the back-and-
forth wobble of the jet direction, the blobs alternate between passing
behind and in front of V Hydrae.  When a blob passes in front of the
star, it shields the red giant from view. The team hopes to use Hubble
to conduct further observations of the V Hydrae system, including the
most recent blob ejected in 2011.  The astronomers also plan to use
the Atacama array (ALMA) in Chile to study blobs launched over the
past few hundred years that are now too cool to be observed by Hubble.


PLANETS ORBITING TWO SUNS ARE SURVIVORS
York University

Planets that revolve around two suns may surprisingly survive the
violent late stages of the stars' lives, according to new research.
The finding is surprising, because planets orbiting close to a single
sun, like Mercury and Venus in our Solar System, would be destroyed
when the ageing star swells into a red giant.  The study found that
planets orbiting binary stars -- also referred to as circum-binary
planets or "Tatooine worlds" -- often escape destruction by moving out
to larger orbits.  That is very different from what will happen in our
own Solar System a few thousand million years from now, when the Sun
starts to evolve and expand to such a size that it will engulf Mercury
and Venus and possibly the Earth too, faster than they can migrate out
to larger orbits.  It seems that if we had a second star in the centre
of our Solar System, things might go differently.  Binary-star systems
are common, and if the two stars are close enough to one another, when
one starts evolving and expanding into a giant, they exchange material
and spiral towards each other, resulting in their sharing a common
atmosphere (also called a common envelope).  The binary-star system
ends up losing a large amount of mass, or might be destroyed in a
supernova explosion.  The team found that the planets will mostly
survive the common-envelope phase -- even those orbiting very close to
their stars.  In addition, the planets can migrate to larger orbits,
analogous to what it would be like if Venus moved out to where Uranus
orbits the Sun.  In some cases, planets can even reach more than twice
the distance to Pluto.  Interestingly, when there are multiple planets
orbiting a binary star, some can be ejected from the system, while
others can switch places or even collide with their stars.  The
reconfiguration can be quite dramatic when there are several planets.
Although all of the known circum-binary planets are gas giants, it is
possible that, somewhere, there is a terrestrial circum-binary planet
that migrates to an orbit that now makes the planet potentially
habitable for a time.


MILKY WAY'S ANCIENT HEART
ESO

Astronomers using the infrared VISTA telescope have for the first time
discovered ancient stars, of the type known as RR Lyrae, in the centre
of the Milky Way.  RR Lyrae stars typically reside in ancient stellar
populations over 10,000 million years old.  Their discovery suggests
that the bulging centre of the Milky Way probably grew through the
merging of primordial star clusters.  The stars may even be the
remains of the most massive and oldest surviving star cluster of the
entire Milky Way.  Our Milky Way has a densely populated centre -- a
feature common to many galaxies, but ours is the only one close enough
to study in detail.  The discovery of RR Lyrae stars provides evidence
that helps astronomers to decide between two main competing theories
for how nuclear bulges form.  RR Lyrae stars are typically found in
dense globular clusters.  They are variable stars, and the brightness
of each RR Lyrae star fluctuates regularly.  By observing the length
of each cycle of brightening and dimming in an RR Lyrae, and also
measuring the star's brightness, astronomers can calculate its
distance.  Unfortunately, these excellent distance-indicator stars are
frequently outshone by younger, brighter stars and in some regions
they are hidden by dust.  Therefore, locating RR Lyrae stars right in
the extremely crowded heart of the Milky Way was not possible until
the public VVV survey was carried out using infrared light.  Even so,
the team described the task of locating the RR Lyrae stars in amongst
the crowded throng of brighter stars as 'daunting'.  Its hard work was
rewarded, however, with the identification of a dozen RR Lyrae stars.
The discovery indicates that remnants of ancient globular clusters are
scattered within the centre of the Milky Way's bulge.  The theory that
galactic nuclear bulges form through the merging of globular clusters
is contested by the competing hypothesis that the bulges are actually
due to the rapid accretion of gas.


A UNIVERSE OF TWO TRILLION GALAXIES
RAS

An international team of astronomers has found that the Universe
contains at least two trillion galaxies, ten times more than previously
thought.  Astronomers have long sought to determine how many galaxies
there are in the observable Universe, the part of the cosmos where
light from distant objects has had time to reach us.  Over the last 20
years scientists have used images from the Hubble telescope to
estimate that the Universe we can see contains around 100-200
billion galaxies.  Current astronomical technology allows us to study
just 10% of these galaxies, and the remaining 90% will be seen only
when bigger and better telescopes are developed (if then).  The team
began by performing the initial galaxy-counting analysis, work which
was crucial for establishing the feasibility of the larger-scale
study.  The team then converted pencil-beam images of deep space from
telescopes around the world, and especially from the Hubble telescope,
into 3-D maps.  Those allowed astronomers to calculate the density of
galaxies as well as the volume of one small region of space after
another.  That enabled the team to establish how many galaxies we have
missed -- much like an intergalactic archaeological dig.  The results
of the study are based on the measurements of the number of observed
galaxies at different epochs through the Universe's history.  When the
team examined how many galaxies there were at a given epoch it found
that there were significantly more at earlier times.

It appears that when the Universe was 'only' a few billion
years old there were ten times as many galaxies in a given volume of
space as there are within a similar volume today.  Most of those
galaxies were low-mass systems with masses similar to those of the
satellite galaxies surrounding the Milky Way.  That is very surprising
as we know that, over the 13.7 billion years of cosmic evolution
since the Big Bang, galaxies have been growing through star formation
and mergers with other galaxies.  Finding more galaxies in the past
implies that significant evolution must have occurred to reduce their
number through extensive merging of systems.  We are missing the vast
majority of galaxies because they are very faint and far away.  The
number of galaxies in the Universe is a fundamental question in
astronomy, and it is astonishing that over 90% of the galaxies in the
cosmos have yet to be studied.


MARS-BOUND ASTRONAUTS FACE CHRONIC DEMENTIA
University of California, Irvine

Will astronauts travelling to Mars remember much of it?  Scientists
have found that exposure to highly energetic charged particles -- much
like those found in the Galactic cosmic rays that will bombard astro-
nauts during extended space flights -- causes significant long-term
brain damage in test rodents, resulting in cognitive impairments and
dementia.  The study follows one last year showing somewhat shorter-
term brain effects of Galactic cosmic rays.  For the new study,
rodents were subjected to charged-particle irradiation (fully ionized
oxygen and titanium) at the NASA Space Radiation Laboratory at New
York's Brookhaven National Laboratory.  Six months after exposure, the
researchers still found significant levels of brain inflammation and
damage to neurons.  Imaging revealed that the brain's neural network
was impaired through the reduction of dendrites and spines on the
neurons, which disrupts the transmission of signals among brain cells.
Those deficiencies were parallel to poor performance on behavioural
tasks designed to test learning and memory.  In addition, the team
discovered that the radiation affected 'fear extinction', an active
process in which the brain suppresses prior unpleasant and stressful
associations, as when someone who nearly drowned learns to enjoy water
again.  Deficits in fear extinction could make one prone to anxiety,
which could become problematic over the course of a three-year trip to
and from Mars.

Similar types of more severe cognitive dysfunction are common in
brain-cancer patients who have received high-dose, photon-based
radiation treatments.  While dementia-like deficits in astronauts
would take months to manifest, the time required for a mission to Mars
is sufficient for such impairments to develop.  People working for
extended periods on the International Space Station, however, do not
face the same level of bombardment with Galactic cosmic rays because
they are still within the Earth's protective magnetosphere.  Partial
solutions are being explored.  Spacecraft could be designed to include
areas of increased shielding, such as those used for rest and sleep.
However, the highly energetic charged particles will traverse the ship
nonetheless and there is really no escaping them.  Preventive treat-
ments offer some hope.  Researchers are working on pharmacological
strategies involving compounds that scavenge free radicals and protect
neurotransmission.