Topic: Late June Astronomy Bulletin

[Clive]

SURPRISING DISCOVERY ABOUT COMET'S ATMOSPHERE
Southwest Research Institute

A close-up of Comet 67P/Churyumov-Gerasimenko by NASA's ultraviolet
instrument surprised scientists by revealing that electrons close to
the comet's surface -- not photons from the Sun as had been believed
-- cause the rapid breakup of water and carbon dioxide molecules
spewing from the surface.  Since last August, ESA's Rosetta spacecraft
has been orbiting within a hundred miles of the comet.  The
spectrograph onboard, named Alice, observes in the far-ultraviolet
wavelength band, with a view to understanding the chemistry of the
comet's atmosphere, or coma.  The chemical composition of gases is
revealed by their spectra.  Alice is the first far-ultraviolet
spectrograph to operate at a comet.  Analysis of the relative
intensities of the observed atomic emissions shows that we are
directly observing the 'parent' molecules that are being broken up by
electrons in the immediate vicinity, within about 1 kilometre of the
comet's surface from which the parent molecules are vaporizing.  Much
of the water and carbon dioxide originates from 'plumes' erupting from
the comet's surface, similar to those that the Hubble telescope
discovered on Jupiter's moon Europa.  According to the new Alice data,
the explanation for the breakup of those molecules is similar to that
for the plumes on Europa, except that the electrons at the comet are
produced by solar radiation, while the electrons at Europa come from
Jupiter's magnetosphere.

By looking at the emission from hydrogen and oxygen atoms broken from
the water molecules, we also can actually trace the location and
structure of water plumes from the surface of the comet.  The far-
ultraviolet region of the spectrum allows scientists to detect the
most abundant elements in the Universe: hydrogen, oxygen, carbon and
nitrogen.  However, such measurements can be made only from outside
the Earth's atmosphere, either from orbiting observatories such as
Hubble or from planetary missions such as Rosetta.  From the Earth's
orbit, those atomic constituents can be seen only after their 'parent'
molecules, such as water and carbon dioxide, have been broken up by
sunlight, hundreds to thousands of kilometres away from the nucleus of
the comet.  The Alice spectrograph aboard Rosetta has also studied the
surface of Comet 67P/Churyumov-Gerasimenko and will be used in
further studies of its atmosphere as the comet approaches the Sun and
its plumes become more active on account of solar heating.  Rosetta-Alice
is one of two ultraviolet spectrographs named Alice currently flying in
space.  The other is New-Horizons-Alice, now approaching Pluto.


TWO CHAOTICALLY TUMBLING PLUTO MOONS
Space Telescope Science Institute (STScI)

A comprehensive analysis of all available Hubble data shows that two
of Pluto's moons, Nix and Hydra, are wobbling unpredictably.
Scientists believe that the other two small moons, Kerberos and Styx,
are probably in a similar situation.  Such chaos may arise because
the moons are embedded in a dynamically shifting gravitational field
caused by the system's two central bodies, Pluto and Charon, orbiting
one another.  The variable gravitational field induces torques that
send the smaller moons tumbling in unpredictable ways.  The torques
are increased by the fact that the moons are ellipsoidal rather than
spherical.  Hubble's monitoring of Pluto's four outer moons has also
shown that three of them, Nix, Styx, and Hydra, are locked together
in a resonance with a precise ratio among their orbital periods. 
That ties together their motions in a way similar to that of three of
Jupiter's large moons.  Hubble provides observational evidence that
the satellites are also orbiting chaotically.  However, that does not
necessarily mean that the system is on the brink of flying apart.
Astronomers would need to know a lot more about the system before they
could determine its long-term fate.  Surprisingly, Hubble also found
that the moon Kerberos is as dark as charcoal, while the other
satellites are as bright as white sand.  It was predicted that
pollution by dust blasted off the satellites by meteorite impacts
should coat all the moons, giving their surfaces a homogeneous look.
The New Horizons probe, which will fly by the Pluto-Charon system in
July, may help to settle the question of the asphalt-black moon as
well as the other oddities uncovered by Hubble.  The new discoveries
are being used in the planning of the New Horizons observations.  The
chaos in the Pluto-Charon system may offer insights into how planets
orbiting a double star might behave.  We are learning that chaos may
be a common trait of binary systems, and the Kepler space observatory
has found several planetary systems orbiting double stars.

Clues to the Pluto chaos first came when astronomers measured
variations in the brightness of the two moons Nix and Hydra, which
changed unpredictably.  The team analyzed Hubble images of Pluto taken
in 2005-2012 and compared the changes in the moons' reflectivity with
dynamical models of spinning bodies in complex gravitational fields.
Virtually all large moons, as well as small moons in close-in orbits,
keep one hemisphere facing their parent planet, as the Moon does to
the Earth, so the satellites' rotations are perfectly matched to their
orbital periods, as a consequence of gravitational tides between moon
and planet.  (Hyperion, which orbits Saturn, is the only other Solar-
System example of chaotic rotation; it is due to the combined gravita-
tional forces of the planet and it largest moon, Titan).  Pluto's
moons are hypothesized to have formed by a collision between Pluto and
another similar-sized body early in the history of the Solar System.
Pluto's large companion, Charon, which is almost half the size of
Pluto itself, was discovered in 1978.  Hubble discovered Nix and
Hydra in 2005, Kerberos in 2011, and Styx in 2012.  Those little
moons, measuring just tens of miles across, were found as part of a
search for potential hazards to the New Horizons spacecraft flyby.
Pluto and Charon are called a double planet because they orbit about a
common centre of gravity that is located in the space between the
bodies.  Some people regard the Earth--Moon system as a double planet,
too, although its centre of gravity falls within the Earth.  (Our Moon
has 1/80th of the Earth's mass, whereas Charon has 1/8th of Pluto's
mass.)  A combination of data from Hubble, New Horizons's brief
close-up look, and eventually the James Webb space telescope, should
lead to increased understanding of the Pluto-Charon system.  No
ground-based telescope has yet been able to detect the smallest moons.


AGEING STAR GIVES BIRTH TO PLANETARY NEBULA
ESO

Images made with the Very Large Telescope (VLT) have, for the first
time, revealed what appears to be an ageing star forming a butterfly-
like planetary nebula.  Observations of the red giant star L2 Puppis,
from the ZIMPOL mode of the newly installed SPHERE instrument, also
clearly showed a close companion.  The dying stages of stars continue
to pose questions, and the origin of such bipolar nebulae, with their
complex hourglass figures, doubly so.  At about 200 light-years away,
L2 Puppis is one of the closest red giants entering the final stages
of its evolution.  The new observations were made in visible light
with extreme adaptive optics, which correct images to a much higher
degree than standard adaptive optics, allowing faint objects and
structures close to bright sources of light to be seen in greater
detail.  They are the first published results from that mode and the
most detailed of such a star.  ZIMPOL can produce images that are
three times sharper than those from the Hubble telescope, and the new
observations show the dust that surrounds L2 Puppis in exquisite
detail.  They confirm earlier findings of the dust being arranged in a
disc, which we see almost exactly edge-on, but provide a much more
detailed view.  The polarization information from ZIMPOL allowed the
team to construct a three-dimensional model of the dust structures.

The astronomers found the dust disc to begin about 900 million
kilometres from the star -- slightly further than the distance from
the Sun to Jupiter -- and discovered that it flares outwards, creating
a symmetrical, funnel-like shape surrounding the star.  The team also
observed a second source of light about 300 million kilometres --
twice the distance from the Earth to the Sun -- from L2 Puppis. 
That very close companion star is likely to be another red giant of
slightly lower mass and less evolved.  The combination of a large
amount of dust surrounding a slowly dying star, along with the
presence of a companion star, is exactly the type of system expected
to create a bipolar planetary nebula.  Those three elements seem to be
necessary, but a considerable amount of good fortune is also still
required if they are to lead to the subsequent emergence of a
celestial butterfly from such a dusty chrysalis.  The origin of
bipolar planetary nebulae is one of the problems of modern
astrophysics, especially the question of how, exactly, stars return
the metallic elements that they synthesize back into space -- an
important process, because it is that material that will be used to
produce later generations of planetary systems.  In addition to
L2 Puppis's flared disc, the team found two cones of material, which
rise out perpendicularly from the disc. Importantly, within those
cones, they found two long, slowly curving plumes of material.  From
the origin points of the plumes, the team deduces that one is likely
to be the product of the interaction between the material from
L2 Puppis and the companion star's wind and radiation pressure, while
the other is likely to have arisen from a collision between the
stellar winds from the two stars, or be the result of an accretion
disc around the companion star.  Although much is not understood,
there are two leading theories of bipolar planetary nebulae, both
relying on the existence of a binary star system.  The new
observations suggest that both of the processes are in action around
L2 Puppis, making it appear very probable that the pair of stars will,
in time, give rise to a butterfly-like nebula.  With the companion
star orbiting L2 Puppis only every few years, we expect to see how the
companion star shapes the red giant's disc.  It will be possible to
follow the evolution of the dust features around the star in real time
-- a rare and exciting prospect.


EXILED STARS EXPLODE FAR FROM HOME
University of California - Berkeley

Images obtained by the Hubble space telescope confirm that three
supernovae discovered several years ago exploded in intergalactic
space, having been flung from their home galaxies millions or billions
of years earlier.  Most supernovae are found inside galaxies
containing hundreds of billions of stars, one of which might explode
per century per galaxy.  The lonely supernovae, however, were found
between galaxies in three large clusters of several thousand galaxies
each.  The stars' nearest neighbours were probably 300 light-years
away, nearly 100 times farther than our Sun's nearest stellar
neighbour, Proxima Centauri, 4.24 light years distant.  Such rare
solitary supernovae provide an important clue to what exists in the
vast empty spaces between galaxies, and can help astronomers
understand how galaxy clusters formed and evolved throughout the
history of the Universe.  The new study confirms the discovery between
2008 and 2010 of three apparently hostless supernovae by the Multi-
Epoch Nearby Cluster Survey from the Canada-France-Hawaii telescope on
Mauna Kea in Hawaii.  The CFHT was unable to rule out a faint galaxy
hosting the supernovae, but the sensitivity and resolution of images
from the Hubble telescope are 10 times better and clearly show that
the supernovae exploded in empty space, far from any galaxy.  They
thus belong to a population of solitary stars that exist in most if
not all clusters of galaxies.

While stars and supernovae typically reside in galaxies, galaxies
situated in massive clusters experience gravitational forces that
wrench away about 15% of the stars, according to a recent survey.  The
clusters have so much mass, though, that the displaced stars remain
gravitationally bound within the sparsely populated intra-cluster
regions.  Once dispersed, the lonely stars are too faint to be seen
individually unless they explode as supernovae.  The team is searching
for bright supernovae in intra-cluster space as tracers to determine
the population of unseen stars. Such information provides clues about
the formation and evolution of large-scale structures in the Universe.
The team also found that a fourth exploding star discovered by CFHT
appears to be inside a red, round region that could be a small galaxy
or a globular cluster.  If the supernova is in fact part of a globular
cluster, it marks the first time a supernova has been confirmed to
explode in one of those small, dense clusters of fewer than a million
stars.  All four supernovae were in galaxy clusters sitting about a
billion light-years from us.  Most theoretical models for Type Ia
supernovae involve a binary-star system, so the exploding stars would
have had companions throughout their lifetimes.  Each companion must
have been either a lower-mass white dwarf that eventually got too
close and was fragmented into a ring that was cannibalized by the
primary star, or a regular star from which the primary white-dwarf
star stole gas from its outer layers.  Either way, such transfer of
material caused the primary to become unstably massive and explode as
a Type Ia supernova.


LONELY GALAXY "LOST IN SPACE"
ESA/Hubble Information Centre

Although the Universe may seem spacious, most galaxies are clumped
together in groups or clusters and a neighbour is, in cosmological
terms, never far away.  But a galaxy known as NGC 6503 has found
itself in a lonely position, at the edge of a strangely empty patch of
space called the Local Void.  NGC 6503 is 'only' some 18 million
light-years away from us in the constellation of Draco, making it one
of the closest neighbours from our Local Group.  It spans some 30,000
light-years, about a third of the size of the Milky Way.  The Hubble
Legacy ExtraGalactic UV Survey (LEGUS) is exploring a sample of nearby
galaxies, including NGC 6503, to study their shape, internal
structure, and the properties and behaviour of their stars.  That
survey uses 154 orbits of observing time on Hubble; by contrast, a
typical Hubble observing programme lasts from a few to a few tens of
orbits.  The Local Void is a patch of space, thought to be about 150
million light-years across, that seems to be curiously devoid of
galaxies.  Astronomers using Hubble discovered that the emptiness of
that region has quite an effect on the space around us -- the Milky
Way is being strongly pulled away from it by the gentle but relentless
tug of other nearby galaxies.  NGC 6503 lies right on the edge of the
void.  It has an almost non-existent central bulge surrounded by a
massive halo of gas.  The galaxy's central region is a good example of
something known as a 'low-ionization nuclear emission region', or
LINER.  Those are less luminous than nuclei of some of the brightest
galaxies.  Emission from NGC 6503's heart is believed to be the result
of a starved black hole that is only just being kept active, receiving
a very small amount of infalling gas.


PHILAE COMET LANDER WAKES UP
BBC NEWS

The European Space Agency (ESA) says that its comet lander, Philae,
has woken up and re-started transmissions.  Philae, the first space-
craft to land on a comet, was dropped onto the surface of Comet 67P by
its mother ship, Rosetta, last November.  It worked for 60 hours
before its battery went flat.  The battery was supposed to be kept up
by a solar-powered charger.  Unfortunately, although Philae was
dropped onto a sunlit plain, the anchors that were supposed to keep it
where it landed did not work, and in the very small gravitational
field of the comet the craft made two big bounces and ended up in the
shade of a cliff, although it was still the right way up.  The comet
has since moved nearer to the Sun and Philae has enough power to work
again.  An account linked to the probe tweeted the message, "Hello
Earth!  Can you hear me?"  After 85 seconds, the transmission stopped
and scientists say they now waiting for the next contact.  Philae has
a minimum operating temperature of -35C and has 24 watts available.
It is supposedly holding large amounts of data that it had already
collected and that scientists still hope it will download once it is
in contact again.  The lander is designed to analyze the ice and rocky
fragments that make up the comet, and scientists now hope to be able
to carry out experiments to see whether comets could have been the
source of life on Earth.  Comets contain a lot of water and carbon,
substances useful to life.

When Philae first sent back images of its landing location, it was
evidently in a dark ditch.  The Sun was obscured by a high wall,
limiting the amount of light that could reach the solar panels.
Scientists knew they only had a limited amount of time - about 60
hours -- to receive data before the robot's battery ran flat.  But it
also appeared that Philae's mission might not be over for good when
the battery ran out.  The comet has been moving in towards the Sun,
and the hope was that Philae would at some stage get enough sunlight
to re-boot itself -- and so it has proved.  Scientists must now hope
that they can get enough power into Philae to carry out the full range
of planned experiments.  One ambition not fulfilled before the robot
went to sleep was to try to drill into the comet, to examine its
chemical make-up.  One attempt was made last year, and it failed. 
A second attempt will now become a priority. 

The Rosetta probe took 10 years to reach 67P, and the lander -- about
the size of a washing-machine -- bounced at least a kilometre when it
touched down.  Before it lost power, Philae sent back images of its
surroundings that showed it was in a dark location with high walls
blocking sunlight from reaching its solar panels.  Its exact location
on the duck-shaped comet remains unknown.  ESA had a good idea of
where it was likely to be, within a few tens of metres, but could not
bring Rosetta, the mother ship, close enough to the comet to obtain
conclusive pictures.  Continued radio contact should now allow precise
coordinates to be determined.  Comet 67P is currently 205 million km
from the Sun, and getting closer.  It is due in August to get as close
as 186 million km, before then sweeping back out into the outer Solar
System.  As it nears the Sun, the comet will warm up and some of its
ices will melt.  That process will throw out a huge shroud of gas and
dust, and if Philae can keep working it will provide an extraordinary
view of what is happening right at the surface of 67P.