PRIMITIVE DUST SAMPLES ANALYSED
RAS
Whereas the planets in the inner Solar System, such as the Earth and
Mars, once experienced harsh conditions and have changed substantially
over the past 4.5 billion years, comets are believed to store the
original material of the early Solar System. Cometary dust is
therefore of interest as possibly being, or including, samples of
primitive interstellar material. Dust particles collected from the
upper atmosphere by NASA aircraft in 2003 April during the Earth's
passage through the dust stream left behind by Comet 26P/Grigg-
Skjellerup appear to include interplanetary material. The particles,
which are only a few microns in diameter, were analysed by scientists
from the UK, the US and Germany. Two grains have compositions thought
to match the nebula from which the Solar System formed. One dust
particle contained four pre-solar silicate grains with an unusual
chemical composition that matches predictions for silicates formed
from gas cooling after a supernova explosion. One of the grains, a
fragment of olivine, was found next to a hollow globule of carbon,
most likely of interstellar origin. Carbon-containing coatings are
suspected to have enabled some of the fragile silicate grains to
survive the space environment. Comparison with 'Deep Impact'
observations of Comet 9P/Tempel 1 and analyses of samples collected by
'Stardust' from Comet 81P/Wild 2 demonstrated differences between the
comets, which are all short-period comets with orbits constrained by
Jupiter's gravitational field. Comet 81P/Wild 2 appears to have much
higher levels of material formed in the inner Solar System; all the
comets, however, contained materials such as carbonates that commonly
indicate the presence of water.
LEAST-MASSIVE EXOPLANET SO FAR DISCOVERED
ESO
An international team using the 3.6-m telescope at La Silla in Chile
has discovered what may be the least massive planet so far detected
outside our Solar System. Orbiting Gliese 581, an M-type star in the
constellation Libra, the planet has a minimum mass only about twice as
great as the Earth's, whereas most other exoplanets identified have
been far more massive. (The actual mass of the planet cannot be
determined, as it depends upon the unknown orbital inclination.)
The planet joins three others previously detected around the same
star, and takes the designation Gliese 581 e. As with the previous
discoveries, its presence was detected by the radial-velocity
technique. The observations are right at the limits of current
technology. The recent announcement of the existence of Gliese 581 e
also includes a correction to the orbital period of one of the other
planets, Gliese 581 d, from 82 to 67 days.
DEBRIS AROUND DEAD STARS
RAS
White-dwarf stars are the compact, hot remnants, comparable in size
with the Earth, that are left behind when stars like our Sun reach the
end of their lives. In theory their atmospheres should consist
entirely of hydrogen and helium but are sometimes found to be
contaminated with heavier elements like calcium and magnesium.
Observations made with the Spitzer space telescope suggest that at
least 1%--3% of white-dwarf stars are contaminated in that way through
a rain of closely orbiting dust that may have originated from rocky
bodies like asteroids and emits the infrared radiation observed by
Spitzer. The dust is completely contained within the Roche limit of
the star -- close enough that any object larger than a few metres
would be torn apart by gravitational tides (the same phenomenon which
led to the creation of Saturn's rings). That is the basis of the
hypothesis that the dust discs around white dwarfs are produced by
tidally disrupted asteroids. In order to pass so close to the white
dwarf, an asteroid must be perturbed from its regular orbit further
out, perhaps by a close encounter with as-yet-unseen planets.
LOCAL STAR'S COOL COMPANION
RAS
'Brown dwarfs' are star-like objects whose masses are too small to
have created, in their cores,temperatures and pressures high enough to
ignite the nuclear reactions upon which stellar energy depends. They
have masses lower than those of stars but larger than those of
gas-giant planets like Jupiter. Owing to their low temperatures they
are very faint in visible light, and are best detected at infrared
wavelengths. Astronomers have now discovered a particularly cool
brown dwarf, orbiting the red dwarf star Wolf 940, some 40 light-years
from the Earth. It appears to be about 440 times as far from its star
as the Earth is from the Sun. It is roughly the same size as Jupiter,
despite being between 20 and 30 times as massive, and the resemblance
between the infrared spectra of the two objects is striking. Models
of brown-dwarf spectra, which are dominated by absorptions due to
water and methane, are sensitive to assumptions about their age and
chemical make-up. In most cases the ages and compositions of brown
dwarfs are unknown, and that can make it hard to tell where the models
are right and where they are going wrong. In the case of Wolf 940,
however, we can use what we know about the primary star to infer the
properties of the brown dwarf, and that makes the pair a useful find;
it can be thought of as a Rosetta Stone for decrypting what the light
from such cool objects is telling us.
YOUNGEST (AND PRESUMED LOWEST-MASS) BROWN DWARFS
RAS
Astronomers have found three brown dwarfs with estimated masses
less than 10 times that of Jupiter, making them among the youngest and
lowest-mass such objects so far detected. They were found in the
star-forming region IC 348, almost 1000 light-years away, in the
constellation Perseus. IC 348 is approximately 3 million years old --
extremely young astronomically -- which makes it a good place to
search for the lowest-mass brown dwarfs. The ones now identified are
isolated in space -- they are not orbiting any star, although they are
gravitationally bound to IC 348. Their atmospheres all show evidence
of methane absorption, which was used to select and identify the young
objects.
The team is said to have set out to find a population of brown dwarfs
in order to help theoreticians to develop more accurate models, needed
to test current star-formation theories, for the distribution of
masses in a newly formed population. (But a special effort to find a
particular constituent of the population, so far from helping to
achieve a more accurate model, seems like a deliberate effort to bias
the statistics!) The discovery of the dwarfs in IC 348 has allowed
new limits to be set on the lowest-mass objects. Brown dwarfs cool
with age, and current models estimate that their surfaces are at
approximately 600-700 degrees Celsius. That is extremely cool for
objects that have just formed, and implies that they have the lowest
masses of any such object that has been seen to date, although no
actual measurement of their masses has been possible.
COMPLEX MOLECULES DETECTED
RAS
Scientists from the Max-Planck Institute have been using the IRAM 30-m
telescope in Spain to observe the millimetre-wavelength emission from
molecules in the star-forming region Sagittarius B2, close to the
centre of our Galaxy. Large molecules of many different sorts have
been detected in that region in the past, including alcohols,
aldehydes, and acids. Two new molecules have now been discovered --
ethyl formate (C2H5OCHO) and n-propyl cyanide (C3H7CN); they are
the most complex of their kind yet detected in interstellar space.
DISTANT WATER
RAS
Astronomers have found the most distant signs of water to date. The
water vapour is thought to be contained in a jet ejected from a super-
massive black hole at the centre of a galaxy named MG J0414+0534.
The water emission is seen as a maser, where molecules in the gas
amplify and emit beams of microwave radiation in much the same way as
a laser emits beams of light. The faint signal is only detectable as
a result of serendipitous gravitational lensing, where the gravity of
a massive galaxy in the foreground acts as a cosmic telescope, bending
and concentrating light from the distant galaxy and making in this
case a clover-leaf pattern of four images of MG J0414+0534. The water
maser was detectable only in the two brightest images. The radiation
from the water maser was emitted when the Universe was only about 2.5
billion years old, a fifth of its current age.
SURVEY THROWS LIGHT ON FORMATION OF MASSIVE GALAXIES
RAS
First results from the 'GOODS' survey being made with the Hubble
telescope indicate how the most massive galaxies in the early
Universe assembled to form the most massive objects seen today.
The observations are part of the 'Great Observatories Origins Deep
Survey' (GOODS), a campaign that is using the Spitzer, Hubble and
Chandra space telescopes together with the XMM Newton X-ray
observatory to study distant parts of the Universe. Early results
show that the most massive galaxies, which have masses roughly 10
times larger than the Milky Way's, were involved in significant levels
of mergers and interactions when the Universe was just 2-3 billion
years old. The GOODS results show that the galaxies did not form in a
simple collapse in the early Universe, but that their formation was
gradual, taking about 5 billion years.
SPECTACULAR FLARING FROM EXTRAGALACTIC JET
STScI
Images taken in ultraviolet light have shown the brightening of a jet
of gas blasting from the core of the giant elliptical galaxy M87,
which is located 54 million light-years away in the Virgo Cluster.
The outburst is coming from a blob of gas called HST-1, embedded in
the jet, a powerful narrow beam of hot gas thought to be produced by a
super-massive black hole in the core of the galaxy. HST-1, which is
some 200 light-years from the galaxy's centre, was discovered and
named in 1999 by Hubble astronomers, who have been following the
activity for 7 years, obtaining a ultraviolet light-curve of the
event. Other telescopes have been monitoring HST-1 in different
wavelengths, including radio and X rays. The Chandra X-ray telescope
was the first to report the brightening in 2000. In 2003, HST-1 had
become brighter than M87's luminous core, and in 2005 it became 90
times brighter than it was in 1999. Then the flare began to fade, but
it intensified again in late 2006, though the second outburst was
fainter than the first one.
Topic: Late April Astronomy Bulletin (Read 911 times)