VERTICAL STRUCTURES IN SATURN'S RINGS
SSI
In images that are possible only because Saturn is near its equinox,
the Cassini spacecraft has observed in the planet's otherwise flat
rings vertical structures that are attributable to the gravitational
effects of a small nearby moon. The search for ring material
extending well above and below the ring plane has been a major goal of
the imaging team during Cassini's 'Equinox Mission', the 2-year period
around the equinox -- the moment when the Sun is directly over the
planet's equator. That geometry, which occurs every half-Saturn-year,
or about 15 Earth years, lowers the Sun's angle to the ring plane and
makes out-of-plane structures easy to detect by the long shadows that
they cast across the rings' broad expanse. In recent weeks, Cassini's
cameras have spotted not only the predictable shadows of some of
Saturn's moons, but also the shadows of newly revealed vertical
structures in the rings themselves. The observations have lent
support to an analysis that indicates how small moons in very narrow
gaps can have considerable and complex effects on the edges of their
gaps, and that such moons may be smaller than previously believed.
The 8-km moon Daphnis orbits within the 42-km-wide Keeler Gap in
Saturn's outer A ring, and its gravitational pull perturbs the orbits
of the particles forming the gap's edges. The slight eccentricity
(the elliptical deviation from a circular path) of Daphnis' orbit can
bring it close to the gap edges. There, its gravity causes larger
effects on ring particles than when it is farther away. Previous
Cassini images have shown that, as a consequence, the moon's effects
can be time-variable and lead to waves caused by Daphnis to change in
shape with time and with distance from the moon. However, the new
analysis also illustrates that when such a moon has an orbit inclined
to the ring plane, as Daphnis' is, the time-variable edge waves also
have a vertical component to them. That result is supported by new
images that show the shadows of the vertical waves created by Daphnis
cast onto the nearby ring. The lengths of the shadows indicate
wave heights as large as 1.6 km above the ring plane, twice as high as
previously known vertical ring structures. The main rings are only
about 10 metres thick.
PLANET-FORMING DISC ORBITING TWIN STARS
AAS
A sequence of images collected by a radio telescope in Hawaii clearly
reveals the presence of a rotating molecular disc orbiting the young
binary-star system V4046 Sagittarii. It has been suggested that the
images provide a snapshot of the process of formation of giant
planets, comets, and Pluto-like bodies. The results also indicate
that such objects may form just as easily around double stars as
around single stars like our Sun. The disc orbiting V4046 Sagittarii
binary system extends from the approximate radius of Neptune's orbit
out to about ten times that radius. The observers believe that V4046
Sagittarii is one of the clearest examples yet discovered of a
Keplerian, planet-forming disc orbiting a young star system. The
particular system consists of a pair of roughly solar-mass stars that
are approximately 12 million years old and are separated by 5 solar
diameters. It seems that, at least for some stars, formation of
Jovian-mass planets may continue well after the few million years
which astronomers have deduced is characteristic of the formation time
for most such planets.
RARE MAGNETAR DISCOVERED
NASA
The Swift satellite has reported multiple blasts of radiation from a
rare object known as a soft gamma repeater, or SGR. The object,
designated SGR 0501+4516, was the first of its type discovered for a
decade and is only the fifth confirmed SGR. Astronomers think that
the eruptions of SGRs arise from very highly magnetized objects --
magnetars. Magnetars are neutron stars -- the crushed cores of
exploded stars -- that, for reasons not yet known, possess ultra-
strong magnetic fields, as much as 10*14 times stronger than the
Earth's. A magnetar half as far away as the Moon would wipe the
magnetic strips off every credit card on the planet. [Silly remark!
- of course, if a star were as close as that, magnetar or not,
substantial effects could be expected. If the Sun were at that
distance, for example, it would do more than just interfere with the
magnetic strips on credit cards. -- ED.] Magnetars allow us to study
extreme conditions that cannot be reproduced on Earth. Both SGRs and
a related group of high-energy neutron stars, called anomalous X-ray
pulsars, are thought to be magnetars. But, all told, only 15 examples
are known.
SGR 0501+4516, estimated to lie about 15,000 light-years away, was
discovered only because its outburst gave it away. Astronomers think
that an unstable configuration of the star's magnetic field triggers
the eruptions. Once the magnetic field resumes a more stable
configuration, the activity ceases and the star returns to quiet and
dim emission. Twelve hours after Swift pinpointed SGR 0501+4516,
XMM-Newton began the most detailed study of a fading magnetar outburst
ever attempted. The object underwent hundreds of small bursts over a
period of more than four months. Only five days after the initial
eruption, INTEGRAL detected X-rays that were beyond the energy range
XMM-Newton can see. That was the first time that such transient
high-energy X-ray emission has been detected during an SGR's outburst
phase; it disappeared again within ten days of the outburst.
FIRST EXO-PLANET FOUND IN ANOTHER GALAXY?
Discovery Space
Only 20 years ago, astronomers were arguing over whether a colleague
had discovered the first exo-planet. Researchers have now found more
than 300 exo-planets in our Milky Way galaxy, and the new topic of
discussion is whether a group of astronomers has detected the first
extragalactic planet, in the Andromeda galaxy. A new study to be
published in the Monthly Notices of the Royal Astronomical Society
explains that it should be possible to identify extragalactic planets
with the technique of gravitational micro-lensing, in which a distant
source star is briefly magnified by the gravity of an object passing
in front of it. That technique has already found several planets in
our galaxy, out to distances of thousands of light-years. Extending
the method from thousands to millions of light-years should be
possible. In fact, it may have already been accomplished.
A team of astronomers first conducted a simulation of micro-lensing
events in the Andromeda galaxy, which is more than 2 million
light-years away. Once they had determined the clues that a planet in
Andromeda would show, they looked at a survey, completed in 2004, that
showed an unusual light-curve. That event, the group says, matches
its theory and can be attributed to a mass about six times that of
Jupiter. That suggests either a planet, or a small companion star
such as a brown dwarf. Unfortunately, the astronomers cannot go back
to check the result from 2004, because micro-lensing events are
unpredictable and brief, lasting a few days at most. But the
researchers hope to look for stronger evidence of other extragalactic
planets. Rather than looking for the transient brightening of distant
stars, they will have to look for transient brightenings of individual
pixels in the images. A 4-m telescope, continually observing
Andromeda for nine months, might pick out one or two exo-planets,
simply by analyzing the pixels in the galactic image.
Topic: Late June Astronomy Bulletin (Read 910 times)