UNUSUAL COMET
Lowell Observatory
Lowell astronomers have measured abundances of five molecular
species in the comae of 150 comets and discovered that one of them,
96P/Machholz 1, has an unusual chemistry. Although Machholz 1 was
discovered in 1986, compositional measurements took place only during
the comet's 2007 apparition. Its anomaly is that the molecule
cyanogen (CN) is depleted by a factor of about 72 from the average of
other comets. The cause of that chemical anomaly is unknown, but each
of three possible explanations points to important (but differing and
mutually inconsistent) constraints on the evolution of comets.
One possible explanation is that Machholz 1 did not originate
in our Solar System, but escaped from another star. In that case,
the other star's proto-planetary disc might have had a lower abundance
of carbon, resulting in all carbon-bearing compounds having lower
abundances. It has been proposed that a large fraction of comets in
our own Solar System has escaped into interstellar space, so we might
expect that many comets formed around other stars would also have
escaped. Some of them could have crossed paths with the Sun, and
Machholz 1 might be an interstellar interloper.
Another possible explanation for Machholz 1's anomalous composition is
that it formed even farther from the Sun, in a colder or more extreme
environment, than other comets studied.
The third possibility is that Machholz 1 originated as a carbon-chain-
depleted comet but extreme heat altered its chemistry. While no other
comet has exhibited chemical changes due to heating by the Sun,
Machholz 1 has an orbit that takes it to well inside Mercury's orbit
every 5 years. We might therefore suspect that repeated high-
temperature cooking is the cause of its unusual composition, but the
only other comet known to show CN depletion has not been subjected to
such high temperatures.
UNDERGROUND WATER THE OBVIOUS CAUSE OF ENCELADUS GEYSERS
University of Central Florida
Cassini has shown that Saturn's satellite Enceladus has geysers
emitting plumes of water vapour and ice particles. Scientists have
come to the unremarkable conclusion that a water reservoir deep below
the icy crust of Enceladus is the source of the plumes. The ice
grains would condense from vapour escaping from the water source and
stream through the cracks in the ice crust on their way into space.
Another idea, that the plumes of gas and dust are caused by
evaporation of volatile ice freshly exposed to space when Saturn's
tidal forces open vents near the south pole, is being discounted,
because observations do not agree with the predicted timing of such
faults opening and closing as a result of tidal tension and
compression.
STUDENTS DISCOVER EXTRASOLAR PLANET
ESO
Three undergraduates from Leiden University in the Netherlands have
discovered an extra-solar planet. The planet, which turned up during
their research project, is about five times as massive as Jupiter.
It was discovered from its brightness variations, among nearly 16,000
stars which had been observed by the OGLE survey once or twice per
night for about four years between 1997 and 2000. The data had been
made public and offered a good test case for the students' algorithm,
which indicated that one of stars showed variations that could be due
to transits of a planet in front of it. The students then used the
2.2-m telescope at ESO to find out more about the star and the
possible planet. To make sure that it was a planet and not a brown
dwarf or a small star that was causing the brightness variations, they
needed to resort to spectroscopy, and were permitted to use the Very
Large Telescope at Paranal.
The planet has an orbital period of about 2.5 days. It is only 3
million miles from its star, making it very hot and much larger than
normal planets. The spectroscopy also showed that the star is of
earlier type than the Sun, with a surface temperature of almost 7000
degrees. It is the hottest star that has been found to have a planet,
and is rotating very fast. The radial-velocity method that has
discovered most extra-solar planets is less efficient on fast-rotating
stars.
BROWN DWARFS FORM LIKE STARS
Harvard-Smithsonian Center for Astrophysics
Astronomers have found evidence that brown dwarfs form like stars.
Observing in the far infra-red, they detected molecules of carbon
monoxide shooting outward from an object known as ISO-Oph 102. Such
molecular outflows are typically seen coming from young stars or
proto-stars. However, the ISO object has an estimated mass of 60
Jupiters, so it is thought to be a brown dwarf rather than a star.
Brown dwarfs have masses between 15 and 75 Jupiters, intermediate
between planets and stars; the theoretical minimum mass for a star to
sustain nuclear fusion is 75 Jupiter masses. It is not clear whether
they form like stars, from the gravitational collapse of gas clouds,
or if they form like planets, agglomerating rocky material until they
grow massive enough to draw in nearby gas.
A star forms when a cloud of interstellar gas draws itself together
through gravity, growing denser and hotter until fusion ignites. If
the initial gas cloud is rotating, that rotation will speed up as it
collapses, by the law of conservation of angular momentum. In order
to gather mass, the proto-star must shed its surplus angular momentum.
It somehow manages to do so by ejecting material in opposite
directions as a bipolar outflow. Brown dwarfs are less massive than
stars, so there is less gravity available to pull them together, and
astronomers have debated whether they could form in the same way.
Previous observations have hinted that they could, and the discovery
of the bipolar molecular outflow from ISO-Oph 102, albeit with much
smaller mass and speed than typical outflows from proto-stars, offers
support to the idea; so whether an object ends up as a brown dwarf or
a star may depends only on the amount of material available -- the
process is the same.
THE LEAP SECOND -- A SHORT PAUSE AT THE TURN OF THE YEAR
U.S. Naval Observatory, Washington
On 2008 December 31, a leap second will be interpolated into
Coordinated Universal Time (UTC, the successor to GMT). There will be
seven pips instead of six; the sequence of seconds markers will be
2008 December 31d 23h 59m 59s, 23h 59m 60s, 2009 January 1d 0h 0m 0s.
It will be the 24th leap second to be added to UTC, a uniform
time-scale maintained by atomic clocks since 1972. Historically, time
was based on the mean rotation of the Earth with respect to celestial
bodies. The invention of atomic clocks permitted the definition of a
much more precise time scale that is independent of the Earth's
rotation. In 1970 an international agreement established two time
scales -- one based on the Earth's rotation and another based on
atomic time. The problem is that the Earth's rotation is gradually
slowing down; that necessitates the occasional insertion of a 'leap
second' into the atomic time scale in order to keep the two within 0.9
second of one another. The International Earth Rotation and Reference
Systems Service (IERS) monitors the difference between the two time
scales and calls for leap seconds to be inserted when necessary
(always at the end of either June or December). Since 1972, leap
seconds have needed to be added at intervals that have ranged from 6
months to 7 years; the last one was on 2005 December 31.
Topic: December Astronomy News (Read 1122 times)