Topic: Late October Astronomy Bulletin

[Clive]

COMET ISON APPEARS INTACT
NASA

A new image of the incoming Comet ISON suggests that the comet is
intact, despite some suggestions that the supposedly fragile icy
nucleus might disintegrate as the Sun warms it.  The warmth that it
has received so far, however, is nothing to what it will get when it
passes closest to the Sun on November 28.  In a Hubble image taken on
October 9, the comet's solid nucleus is unresolved.  The coma
surrounding it is symmetrical and smooth, which it probably wouldn't
be if the nucleus were in a number of separate pieces.  A jet of dust
that was seen in April is no longer visible.  The comet will be
closest to the Earth on December 26, at a distance of 40 million
miles.


'PEBBLE' THOUGHT TO BE COMET FRAGMENT
Witwatersrand University

A team of South African scientists and international collaborators
believes that it has identified a small piece of rock as being
formerly part of a comet that entered the atmosphere above Egypt about
28 million years ago.  It exploded, melting some of the sand beneath
it and forming a huge amount of yellow silica glass which lies
scattered over a 6,000-square-km area of the Sahara.  A magnificent
specimen of the glass, polished by ancient jewellers, exists in
Tutankhamun's brooch with its striking yellow-brown scarab.  The
impact of part of the comet also produced microscopic diamonds.
Diamonds are produced from carbon-bearing material by high pressures,
such as occur deep in the Earth but can also be generated by shock.
Part of the comet reached the ground, and the shock of the impact
produced the diamonds.

The team's attention was attracted to a black pebble found years ago
by an Egyptian geologist in the area of the silica glass.  Chemical
analysis of the pebble led them to the conclusion that it is the first
recognized hand specimen of a comet nucleus, rather than simply an
unusual type of meteorite.  Comet fragments have not been found on the
Earth before, except as microscopic dust particles in the upper
atmosphere and in Antarctic ice.  Space agencies have spent large sums
to secure very small amounts of pristine comet matter.


CURIOSITY CONFIRMS ORIGINS OF MARTIAN METEORITES
American Geophysical Union

A new measurement of Mars' atmosphere by the Curiosity rover provides
the best evidence yet that certain meteorites that have been believed
to have come from Mars really did originate there, while at the same
time it provides a way to rule out Martian origins of other
meteorites.  The rover measured the isotopic composition of argon.
Isotopes of argon with masses of 36 and 38 exist naturally throughout
the Solar System, but on Mars their relative abundance is altered
because a lot of the planet's original atmosphere was lost into space,
with the lighter form being lost more readily because it requires less
energy to escape.  Past analyses by Earth-bound scientists of gas
bubbles trapped inside supposed Martian meteorites had already
narrowed the Martian argon ratio to between 3.6 and 4.5 (that is 3.6
to 4.5 atoms of argon-36 to every one argon-38), giving a supposed
Martian atmospheric value near four.  Measurements by the Viking
landers in the 1970s put the Martian ratio in the range four to seven;
the new measurement gives 4.2.

One of the reasons that scientists have been so interested in the
argon ratio in Martian meteorites is that it was -- before Curiosity
-- the best measure of how much atmosphere Mars has lost since the
wetter, warmer days billions of years ago.  Had Mars held onto its
entire atmosphere and its original argon, the isotopic ratio would be
the same as that of the Sun and Jupiter, whose gravities are too high
to allow argon to escape, so their argon ratio (5.5) represents that
of the primordial Solar System.  While argon comprises only a tiny
fraction of the gases lost to space from Mars, it is special because
it is a 'noble' gas, i.e. inert, not reacting with other elements or
compounds, and therefore a straightforward tracer of the history of
the Martian atmosphere.  Other isotopes measured by Curiosity also
support the idea of loss of atmosphere, but none so directly as argon.


WATER DISCOVERED IN REMNANTS OF EXTRASOLAR PLANET
University of Warwick

Astrophysicists have found the first evidence of a water-rich rocky
planetary body outside our Solar System in its shattered remains
orbiting the white-dwarf star GD 61, 170 light years away.  Using
observations obtained with the Hubble telescope and the Keck telescope
in Hawaii, they found an excess of oxygen -- a chemical signature that
they interpreted as implying that the debris had once been part of a
bigger body originally consisting of 26% water by mass.  By contrast,
only approximately 0.023% of the Earth's mass is water.  Evidence for
water outside the Solar System has previously been found in the
atmospheres of gas giants and in radio-astronomical observations of
gaseous material.  The new study marks the first time that it has been
attributed to a rocky body outside the Solar System, but that is not
surprising, because we have not known even of the existence of such
bodies until quite recently.  Ice is widespread in the Solar System:
the dwarf planet Ceres, and certain satellites of the major planets,
contain ice buried beneath an outer crust, and analogous discoveries
have recently been reported in such unlikely bodies as the Moon and
Mercury.  Some scientists believe that bodies like Ceres were the
source of the bulk of terrestrial water.  The researchers suggest that
the water detected around the white dwarf GD 61 may have come from a
planet that once orbited that star before it became a white dwarf.
Like Ceres, the water was most likely in the form of ice below the
planet's surface.  From the amount of rock and water detected in the
outer envelope of the white dwarf, the researchers estimate that the
disrupted planetary body had a diameter of at least 90 km.

However, because their observations can only detect what is being
accreted in recent history, the estimate of its mass is on the
conservative side.  It is likely that the object was as large as
Vesta, one of the largest minor planets.  Originally GD 61 was a star
somewhat bigger than the Sun, and host to a planetary system.  About
200 million years ago, GD 61 completed its evolution and became a
white dwarf, yet parts of its planetary system survived.  The
water-rich minor planet entered an orbit that took it very close to
the star, where it was disrupted by the star's gravity.  The
researchers believe that de-stabilising the orbit of the minor planet
requires a so-far-unseen, much larger planet in orbit around the white
dwarf.  At this stage, all that remains of the rocky body is simply
dust and debris in orbit around the white dwarf.  In the remnants lie
chemical clues which are said to point towards a previous existence as
a water-rich terrestrial body.


ONE OF THE LARGEST STARS KNOWN IS TEARING ITSELF APART
RAS

Stars with masses tens of times larger than that of the Sun have very
short and dramatic lives compared to those of less-massive ones.  Some
of the most massive stars have lifetimes of less than a few million
years before they exhaust their nuclear fuel and explode as
supernovae.  At the very ends of their lives they become unstable and
eject a lot of material from their outer envelopes.  That material has
been enriched with heavy elements by nuclear reactions in the
interior, and includes many of the elements that form rocky planets
like ours, such as silicon and magnesium.  How the material is ejected
and how the loss affects the evolution of the stars is however still
unknown.

Using the Very Large Telescope Survey Telescope (VST) at the Paranal
Observatory in Chile an international team of astronomers has been
surveying the Galaxy with a special filter to detect nebulae of
ionized hydrogen.  Meanwhile the VST Photometric H-Alpha Survey
(VPHAS) has been searching the Galaxy for ejected material from
evolved stars.  Both observed the star cluster Westerlund 1, which is
a massive cluster of several hundred thousand stars about five
kiloparsecs away in the southern constellation Ara, but our view of it
is so hampered by gas and dust that it appears comparatively dim in
visible light.  One of the stars, known as W26, in Westerlund 1 was
observed to be surrounded by a cloud of glowing hydrogen.  Such clouds
glow because they are ionized, meaning that the electrons have been
stripped away from the hydrogen atoms.  Clouds of that type are not
normally found around red supergiant stars such as W26; indeed, this
is the first ionized nebula ever discovered around such a star.
W26 itself is too cool to ionize the gas; the astronomers speculate
that the source of the ionizing radiation may be either hot blue stars
elsewhere in the cluster, or possibly a fainter, but much hotter,
companion star to W26.

W26 is one of the largest stars ever recognized, with a radius 1500
times that of the Sun, and is also one of the most luminous red
supergiants known.  Such large and luminous massive stars are highly
evolved, so W26 must 'soon' come to the end of its 'life' and explode
as a supernova.  The nebula observed around W26 is very similar to the
one surrounding SN 1987A, the remnant of a star that exploded as a
supernova in 1987.  SN 1987A was the closest observed supernova to the
Earth since 1604, and gave astronomers a chance to study the
properties of such explosions.  Studying objects like the nebula
around W26 may help astronomers to understand the mass-loss processes
that affect massive stars and lead up to their explosive demise.


MOST DISTANT GRAVITATIONAL LENS HELPS WEIGH GALAXIES
ESA

An international team of astronomers has found the most distant
gravitational lens yet -- a galaxy that, as predicted by Einstein's
general theory of relativity, deflects and intensifies the light of an
even more distant object.  The discovery provides an opportunity to
determine the mass of a distant galaxy.  Since the first find in 1979,
numerous such gravitational lenses have been discovered.  In addition
to providing tests of Einstein's theory, gravitational lenses have
proved to be valuable tools.  Notably, they enable us to determine the
mass of the matter that is bending the light -- including the mass of
the still-enigmatic 'dark matter'.  The lens also magnifies the
background light source, acting as a natural telescope that gives
astronomers a more detailed look at distant galaxies than is otherwise
possible.

Gravitational lensing involves two objects: one is further away and
supplies the light, and the other is the lensing mass or gravitational
lens, which sits between us and the distant light source, and whose
gravity deflects the light.  When the observer, the lens, and the
distant light source are precisely aligned, the observer sees an
Einstein ring -- a perfect circle of light that is the projected and
greatly magnified image of the distant light source.

The recent discovery was made completely by chance.  It looked like an
extremely young galaxy, but it seemed to be at a much larger distance
than expected.  The Hubble telescope showed it to be an almost perfect
Einstein ring, indicating a gravitational lens with very precise
alignment of the lens and the background light source.  The lensing
mass is so distant that the light, after deflection, has travelled 9.4
billion years to reach us.  Not only is this a new record, the object
also serves an important purpose: the amount of distortion caused by
the lensing galaxy allows a direct measurement of its mass.  That
provides an independent check on astronomers' usual method of
estimating distant galaxy masses -- which rely on extrapolation from
'nearby' ones.  Happily, the 'usual methods' pass the test.