COMET McNAUGHT HAD UNUSUALLY LONG ION TAIL
RAS
British scientists have shown from Ulysses spacecraft data that Comet
McNaught, which in early 2007 became the brightest comet seen for 40
years, disturbed a region of space much larger than that occupied by
the visible tail. Analysis of magnetometer data suggests that the
comet was surrounded by a shock wave created where the fast-flowing
particles of the solar wind were slowed down abruptly when they
impinged on the ionized gas emitted from the comet's nucleus. It was
just by chance that Ulysses happened to pass through Comet McNaught's
tail; it encountered the tail of ionized gas at a distance downstream
of the comet's nucleus more than 1.5 times the distance between the
Earth and the Sun -- much further away than the visible dust tail
extended. Ulysses took 8 days to traverse the shocked solar wind
surrounding Comet McNaught, compared to 2.5 days in shocked wind
surrounding Comet Hyakutake in 1996. The Giotto spacecraft's
encounter with Comet Grigg-Skjellerup in 1992 took less than an hour
from one shock crossing to another; to cross the shocked region at
Comet Halley took a few hours. The comparisons show that Comet
McNaught was not only spectacular from the ground but was an unusually
large obstacle to the solar wind.
VENUS IS VOLCANICALLY ACTIVE
ESA
Venus Express has returned the clearest indication yet that Venus is
still active. Relatively young lava flows have been identified by
their emission of infrared radiation. The finding suggests that the
planet remains capable of volcanic eruptions. The sparseness of
craters on Venus suggests that something is wiping the planet's surface
clean. That something is thought to be volcanic activity, but the
question is whether it happens quickly or slowly -- whether there is
some sort of cataclysmic volcanic activity that resurfaces the entire
planet with lava, or a gradual sequence of smaller volcanic eruptions.
The latter are suggested by maps of the infrared brightness or
'emissivity'. Astronomers concentrated on three regions that are
analogous to Hawaii, well-known for its active vulcanism. Those
regions on Venus have higher emissivities than their surroundings,
indicating different compositions. On Earth, lava flows react rapidly
with oxygen and other elements in the atmosphere, changing their
composition. On Venus, the process should be similar, though more
vigorous because of the hotter, denser atmosphere. The researchers
interpret the areas of high emissivity as lava flows that have not
undergone as much weathering as their surroundings, implying that they
are relatively recent, possibly even still forming.
PLUTO SHOWS CHANGES
NASA
A comparison of Hubble Telescope images of Pluto obtained in 1994 and
2003 shows that the northern hemisphere has brightened while the
southern hemisphere has dimmed. Ground-based observations suggest
that Pluto's atmosphere doubled in mass during approximately the same
interval. Pluto gets so cold that its atmosphere can actually freeze
and fall to the ground. If the Earth's atmosphere did that, it would
make a layer 30 feet thick, but Pluto has far less atmosphere. When
it is on the ground, Pluto's entire blanket of air is no more than
a frosty film of nitrogen and methane. Until the mid-1980s, Pluto's
northern hemisphere had been tilted away from the Sun for over 100
years, accumulating a substantial amount of frost. Now the northern
hemisphere is coming into sunlight and appears, as shown in the Hubble
images, to have been growing brighter. The atmosphere might also be
changing in response to Pluto's highly eccentric orbit. During the
late 1980s, Pluto approached as close to the Sun as it ever gets and
was consequently warming. Surface frosts exposed to such 'warmth' may
be subliming -- that is, changing back into gas.
DUSTY DISCS IN PLANETARY SYSTEMS
RAS
Two stars observed in the infrared with the MIDI interferometer, which
combines the light from the 8-m units of the VLT in Chile, appear to
have discs of rocky and dusty material at distances comparable to that
from the Earth to the Sun. The stars concerned, both considerably
younger than the Sun, are HD 69830, of spectral type K0 V, in the
constellation Puppis and thought to have three planets with masses
comparable to Neptune, and Eta Corvi, type F2 V. Earlier observations
had indicated that both stars had discs; Eta Corvi is known to have
cold material around it at a distance of 150 Astronomical Units
(Earth--Sun distances; 1 AU is about 150 million km). With MIDI a
relatively small dusty disc around HD 69830 was clearly seen; it lies
between 7.5 and 360 million km from the star. A similar disc was
found close to Eta Corvi, lying between 24 to 450 million km out.
Those results represent the first resolution of dusty discs so close
to their parent stars.
COOLEST BROWN DWARF FOUND NEAR SUN
University of Hertfordshire
Brown dwarfs are bodies with masses in the range between those of
giant planets and the faintest stars. Some are isolated, while others
orbit normal stars or exist in star clusters. Astronomers have now
discovered a previously unknown brown dwarf just 2.9 parsecs (9 light-
years) away -- the seventh-closest star, and the first to be found
so close since Luyten 726-8 was discovered in 1948. The star, UGPS
J0722-05, has a temperature of 400-500 K and is far less luminous and
significantly cooler than previously known objects. The Jupiter-sized
object emits only 0.000026% as much energy as the Sun. Since 1995,
more than 100 methane brown dwarfs, or T dwarfs, have been found, with
spectra similar to that of the planet Jupiter and with effective
temperatures in the range 500-1300 K. The detection of even cooler
bodies will open a new arena for atmospheric physics and may help to
determine the formation rate of stars and brown dwarfs in our Galaxy
as a function of both mass and time.
ROCKY PLANETS MAY BE COMMON IN THE MILKY WAY
RAS
Astronomers have found evidence that rocky planets are commonplace in
our Galaxy. A survey of white dwarfs, the compact remnants of stars
that were once like our Sun, found that many show signs of
contamination by heavier elements and possibly water. White dwarfs
are the endpoint of stellar evolution for the vast majority (>90%) of
all stars in the Milky Way. Because they ought to have almost pure
hydrogen or helium atmospheres, if heavier elements such as calcium,
magnesium and iron are found then they are interpreted as external
pollutants. For decades, it was believed that the interstellar
medium (the tenuous gas between the stars) was the source of the
metals in the polluted white dwarfs. The team used data from the Sloan
Digital Sky Survey (SDSS), a project that aims to survey the sky in
infrared light, imaging more than 100 million objects and following up
1 million of them by obtaining their spectra. By examining the
positions, motions and spectra of the white dwarfs identified in the
SDSS, the team shows that an interstellar origin for the metals is no
longer a satisfactory theory. Instead, rocky planetary debris is
probably the usual culprit. The new work indicates that at least 3%
and perhaps as much as 20% of all white dwarfs are contaminated in
this way, with the debris most likely in the form of rocky minor
planets with a total mass of about that of an asteroid 140 km in
diameter. That implies that a similar proportion of stars like our
Sun, as well as stars that are somewhat more massive, like Vega and
Fomalhaut, build terrestrial-type planetary systems. The scientists
also measured the composition of the pollutants through their
spectroscopic signatures, which stand out in the otherwise pure
atmosphere of the white dwarfs. It appears that a significant
fraction of the stars are polluted with material that contained water,
with implications for the frequency of habitable planets around
other stars.
BLACK HOLES AND GALAXY DEATH
RAS
Black holes are thought to reside at the centre of almost every
galaxy, with some growing to more than a billion times the mass of the
Sun. Now a team of UK astronomers has proposed that such super-
massive black holes are commonplace, release more than enough
energy to strip their host galaxies apart, and in the process shut
down star formation in their galaxies for good. For many years black
holes have fascinated scientists and the public alike, with their
peculiar ability to warp space and time and their sinister tendency to
devour everything they encounter. Before matter falls in, as it
swirls around the black hole it forms an 'accretion disc', where it
heats up and radiates energy. The super-massive black holes have such
strong gravitational fields that the infalling matter releases a vast
amount of energy, making each accretion disc far brighter than the
combined output of the billions of stars in the galaxy around it.
One of the consequences of such outpouring of energy is that it drives
away cool gas and dust, the raw ingredients of new stars. That
permanently shuts down star-formation in the surrounding galaxy, whose
remaining stars age, end their lives, and are never replaced. The new
study considered the role of super-massive black holes in the
development of galaxies. To search for them, the team used the Hubble
telescope and the Chandra X-ray observatory to observe in optical,
near-infrared and X-ray light. In particular, the astronomers looked
for galaxies which have a very high emission of X-rays, a probable
signature of black holes devouring gas and dust. From the space
telescopes' data astronomers find that at least 1/3 of all the massive
galaxies they observed not only contain super-massive black holes,
but that at some point in their histories the emission from the holes'
accretion discs far outshines the galaxies themselves. The energy
output of regions around the black holes is high enough to strip apart
every massive galaxy in the cosmos 25 times over, whilst the X-ray
emission from them turns out to dwarf that from every other source in
the Universe put together.
POSSIBLE MICROQUASAR IN STARBURST GALAXY M82
RAS
Radio astronomers at Jodrell Bank have discovered a strange new object
in M82, a galaxy that is 10 million light-years away and is forming
new stars at a prodigious rate, many of them massive stars that die
quickly, a supernova explosion occurring every 20 to 30 years. The
new object, which appeared last May, has perplexed astronomers, who
have never seen anything quite like it before. The object turned on
very rapidly within a few days and has shown no sign of decaying in
brightness over the first months of its existence. The new young
supernova explosions that astronomers expect to see in M82 brighten at
radio wavelengths over several weeks and then decay over several
months, so that explanation seems unlikely. The plausibility of a
supernova explanation was further undermined when very accurate
positional monitoring by the UK network of radio telescopes, MERLIN,
tentatively detected a change in position for the object over the
first 50 days. It was equivalent to an apparent motion of over four
times the speed of light. Such large apparent velocities are not seen
in supernova remnants and are usually only found with relativistic
jets ejected from accretion discs around massive black-hole systems.
The nucleus of M82 may contain a super-massive black hole. The new
detection lies at a position close to, but several arcseconds away
from, the dynamical centre of M82 -- far enough away that it would
seem unlikely that this object is associated with the central
collapsed core of the galaxy. The new source could be the first radio
detection of an extragalactic 'micro-quasar'. Examples of such
systems within the Milky Way are found as X-ray binaries with
relativistic jets ejected from an accretion disc around a collapsed
star fuelled with material dragged from a close binary companion.
However, this object would be brighter than any Galactic example yet
detected, has lasted months longer than any known X-ray binary, and
lies at a position in M82 where no variable X-ray source has been yet
been detected.
LOFAR OPENS UP LOW-FREQUENCY UNIVERSE
RAS
The Low Frequency Array (LOFAR), a new pan-European radio-astronomy
instrument, has started mapping the Universe at very long wavelengths,
a part of the electromagnetic spectrum that is relatively unexplored.
Astronomers hope LOFAR will allow them to study cosmic rays, pulsars,
and the magnetic field within our own and nearby galaxies. LOFAR will
also compile a census of radio-emitting galaxies from the very early
Universe, which may help us to understand how galaxies formed and
evolved over cosmic time.
Topic: Late April Astronomy Bulletin (Read 559 times)