NEW CLUES TO BRIGHT SPOTS ON CERES
NASA
Ceres has more than 130 bright areas, most of which are associated
with impact craters. Researchers at the Max Planck Institute for
Solar-System Research in Gottingen, Germany, say that the bright
material may be a hydrated form of magnesium sulphate (the hexa-
hydrite). A different form of magnesium sulphate is familiar on Earth
as Epsom salt. The team, using images from Dawn's framing camera,
suggest that these salt-rich areas were left behind when water-ice
sublimated in the past. Impacts from asteroids would have unearthed
the mixture of ice and salt. The global nature of Ceres' bright spots
suggests that it has a subsurface layer that contains briny water-ice.
Ceres, whose average diameter is 940 km, has a surface that is
generally dark -- similar in brightness to fresh asphalt. The bright
patches that pepper the surface represent a large range of brightness,
with the brightest areas reflecting about 50% of the sunlight shining
on the area. But there has not been unambiguous detection of water
ice on Ceres; higher-resolution data are needed to settle that
question. The inner portion of a crater called Occator contains the
brightest material on Ceres. Occator itself is 90 km in diameter, and
its central pit, covered by the bright material, measures about 10 km
across and 500 m deep. Dark streaks, possibly fractures, traverse
the pit. Remnants of a central peak, which was up to 500 m high, can
also be seen. With its sharp rim and walls, and abundant terraces and
landslide deposits, Occator appears to be among the youngest features
on Ceres. Dawn mission scientists estimate its age to be about 78
million years.
Study authors write that some views of Occator appear to show a
diffuse haze covering the floor of the crater. That may be associated
with observations of water vapour that were reported in 2014 at Ceres
by the Herschel space observatory. The haze seems to be present in
views obtained about noon, local time, and is absent at dawn and dusk.
That suggests that the phenomenon resembles the activity at the
surface of a comet, with water vapour lifting tiny particles of dust
and residual ice. Future data and analysis may test that hypothesis
and offer clues about the process causing such activity. In a second
study, members of the Dawn scientific team examined the composition of
Ceres and found evidence for ammonia-rich clays. They used data from
the visible and infrared mapping spectrometer, a device that looks at
the spectrum of light reflected by the surface, allowing minerals to
be identified. Ammonia ice by itself would evaporate on Ceres today,
because the surface is too warm for it. However, ammonia molecules
could be stable if present in combination with other minerals. The
presence of ammoniated compounds raises the possibility that Ceres
did not originate in the main asteroid belt between Mars and Jupiter,
where it currently resides, but instead might have formed further out
in the Solar System.
Another idea is that Ceres formed close to its present position,
incorporating materials that drifted in from the outer Solar System -
near the orbit of Neptune, where nitrogen ices are thermally stable.
In comparing the spectrum of reflected light from Ceres to meteorites,
scientists found some similarities. They focused on the spectra of
carbonaceous chondrites, a type of carbon-rich meteorite possibly
analogues to Ceres. But the chondrites are not good matches at all
the wavelengths that the instrument sampled. In particular, there were
distinctive absorption bands, matching mixtures containing ammoniated
minerals, at wavelengths that can not be observed by Earth-based
telescopes. The scientists note that another difference is that
carbonaceous chondrites have bulk water contents of 15% to 20%, while
Ceres' content is as much as 30%. The study also shows that daytime
surface temperatures on Ceres range from -136° to -28°F. The maximum
temperatures were measured in the equatorial region. The temperatures
at and near the equator are generally too high to support ice at the
surface for a long time, the study authors say, but data from Dawn's
next orbit will reveal more details. Dawn has reached its final
orbital altitude over Ceres, about 385 km above the surface, and has
begun taking observations that include images at a resolution of 35
metres per pixel, infrared, gamma-ray and neutron spectra, and high-
resolution gravity data.
CURIOUS STAR CW LEO
RAS
New images of an intriguing red giant star, CW Leo, formerly known as
IRC 10216, have turned the usual astronomy narrative on its head, with
scrutiny focussed not only on the star but also on the astronomers who
study it. In just a couple of years, CW Leo, 400 light-years distant,
has changed its appearance completely, and a whole set of carefully
constructed models has had to be abandoned. Stars like the Sun become
red giants near the end of their lives, after most of their available
hydrogen fuel has been consumed. A red giant is a huge object --
dozens of times the size of the Sun -- that will eventually eject most
of its atmosphere into space and create a planetary nebula, leaving
behind a hot core that cools down over thousands of millions of years.
CW Leo is nearing the end of its red-giant stage and starting to eject
large amounts of matter. Although it is invisible to our eyes, to
astronomers CW Leo is one of the most famous stars in the sky. If we
could see infrared light, it would be the brightest star in the sky.
However the real excitement here is the extreme physics -- it is a
swollen luminous giant poised at the most self-destructive phase of
its existence. It is literally tearing itself apart, hurling dense
clouds of dust and gas out into the galaxy, dying amidst its own final
fireworks display.
The team of astronomers used images from the Keck and VLT telescopes,
and the Cassini space probe, to study CW Leo over more than a decade.
As might be expected from such a roiling cauldron of heat and dust,
the star's appearance evolves, but in the past its evolution has been
quite a stately affair. However the new images catch something more
dramatic -- in the last couple of years, it has completely shed its
familiar identity and adopted an entirely new appearance. Such
behaviour is a serious problem for the astronomers who have spent
decades studying the system. It is pretty clear that what the new
images tell us is that CW Leo has just been ejecting clumps and plumes
of hot dust at random all this time. It is like a celestial version
of the famous 'Rorschach Ink Blot Test' in psychology. In trying to
find underlying structure to the clumps and blobs, we have seen little
more than our own preconceptions reflected back at us.
JUPITER-LIKE STORM ON SMALL STAR
NASA/Jet Propulsion Laboratory
Astronomers using data from the Spitzer and Kepler space telescopes
have discovered what appears to be a tiny star with a giant, cloudy
storm. The dark storm is akin to Jupiter's Great Red Spot, a
persistent storm that is larger than the Earth. While planets have
been known to have cloudy storms, this is the best evidence yet for a
star that has one. The star, in the constellation Lyra and referred
to as W1906+40, belongs to a thermally cool class of objects called
L-dwarfs. Some L-dwarfs are considered stars because they fuse atoms
and generate light, as our Sun does, while others, called brown
dwarfs, are known as 'failed stars' for their lack of atomic fusion.
W1906+40 is thought to be a star on the basis of estimates of its age
(the older the L-dwarf, the more likely it is to be a star). Its
temperature is about 2,200°K, which is cool enough for clouds to form
in its atmosphere. Spitzer has observed other cloudy brown dwarfs
before, finding evidence for short-lived storms lasting hours and
perhaps days. In the new study, the astronomers were able to study
changes in the atmosphere of W1906+40 for two years. The L-dwarf had
initially been discovered by the 'Wide-field Infrared Survey Explorer'
in 2011. The object happened to be located in the area of the sky
that the Kepler mission had been staring at for years to hunt for
planets.
Kepler identifies planets by looking for dips in starlight as planets
pass in front of their stars. In this case, astronomers knew that the
observed dips in the starlight were not coming from planets, but they
thought they might be looking at star spots -- which, like the Sun's
sunspots, are a result of concentrated magnetic fields. Star spots
would also cause dips in starlight as the star rotates. Follow-up
observations with Spitzer, which observes infrared light, revealed
that the dark patch was not a magnetic star spot but a cloudy storm
with a diameter three times that of the Earth. The storm is carried
round with the rotation of the star about every 9 hours. Spitzer's
photometry at two infrared wavelengths probed different layers of the
atmosphere and, together with the Kepler visible-light data, helped
to reveal the presence of the storm. The storm looks different when
viewed at different wavelengths, but astronomers say it would look
like a dark mark near the pole of the star. The researchers plan to
use Spitzer and Kepler to look for other stormy stars and brown dwarfs
in the future.
MAGNETIC FIELDS AT MILKY WAY'S CENTRAL BLACK HOLE
Harvard-Smithsonian Center for Astrophysics
Most people think of black holes as sucking in everything that gets
too close. But the supermassive black holes at the centres of
galaxies are more like cosmic engines, converting energy from
infalling matter into intense radiation that can outshine the combined
light from all the surrounding stars. If the black hole is spinning,
it can generate strong jets that blast across thousands of light-years
and shape entire galaxies. Such black-hole engines are thought to be
powered by magnetic fields. For the first time, astronomers have
detected magnetic fields just outside the event horizon of the black
hole at the centre of our Milky Way galaxy. That was achieved with
the Event-Horizon Telescope (EHT) -- a global network of radio
telescopes that link together to function as a single telescope the
size of the Earth. Since larger telescopes offer greater resolution,
the EHT will ultimately resolve features as small as 15 micro-
arcseconds, the angular equivalent of seeing a golf ball on the Moon.
Such resolution is needed because a black hole is a very compact
object. The Milky Way's central black hole, Sgr A*, has a mass about
4 million times as much as the Sun, yet its event horizon spans only
8 million miles -- smaller than the orbit of Mercury. And since it is
located 25,000 light-years away, that size corresponds to an angle of
only 10 micro-arcseconds. Fortunately, the intense gravity of the
black hole warps light and magnifies the event horizon so that it
appears larger on the sky -- about 50 micro-arcseconds, an angle that
the EHT can resolve.
The Event-Horizon Telescope made observations at a wavelength of
1.3 mm. The team found that that light is linearly polarized. On
Earth, sunlight becomes linearly polarized by reflection, which is
why sunglasses use polaroid to block light and reduce glare. In the
case of Sgr A*, polarized light is emitted by electrons spiralling
around magnetic-field lines. As a result, the light directly traces
the structure of the magnetic field. Sgr A* is surrounded by an
accretion disc of material orbiting the black hole. The team found
that magnetic fields in some regions near the black hole are
disorderly, with jumbled loops and whorls resembling intertwined
spaghetti. In contrast, other regions showed a much more organized
pattern, possibly in the region where jets would be generated. They
also found that the magnetic fields fluctuated on short time-scales of
only 15 minutes or so. Once again, the Galactic Centre is proving to
be a more dynamic place than we might have expected. As the EHT adds
more radio dishes around the world and gathers more data, it will
achieve greater resolution, with the goal of imaging a black hole's
event horizon directly for the first time.
A 'GHOST FROM THE MILKY WAY'S PAST'
Plataforma SINC
When our Galaxy was born, around 13 billion years ago, a plethora of
clusters containing millions of stars emerged. But over time, they
have been disappearing. However, hidden behind younger stars that
were formed later, some old and dying star clusters remain, such as
the one called E 3. European astronomers have now studied that
testimony to the beginnings of our Galaxy. Globular clusters are
spherical-shaped stellar groupings which can contain millions of
stars. There are about 200 of them in the Milky Way, but few are as
intriguing to astronomers as the E 3 cluster. It is about 30,000
light-years away, in the southern constellation Chameleon. A team of
Spanish and Italian astronomers has named it 'a ghost from the Milky
Way's past'. That globular cluster, and a few similar ones -- such as
Palomar 5 and Palomar 14 -- are 'ghosts' because they appear to be in
the last stages of their existence, and we say 'from the past' because
they are very old. They were formed when our galaxy was virtually
new-born. E 3 is almost hidden behind younger and brighter objects
located between the cluster and the Earth, but it has been possible to
observe it with the Very Large Telescope at ESO. The data revealed
some surprises. Unlike typical globular clusters, which contain
hundreds of thousands and in some cases millions of stars, the object
studied has only a few tens of thousands of them. Additionally, it
does not have the typical circular symmetry, but a much distorted,
almost ghostly, rhomboidal shape, contorted by the galactic
gravitational waves.
According to another study on E 3 by Michigan State University
researchers, the cluster is chemically homogeneous, that is, it does
not have several star populations in its interior. That is character-
istic of an object that was created en bloc, in one single episode, as
is supposed to have happened when our Galaxy was born: very large star
clusters (containing millions of stars) were formed, but what remains
of them today are objects like E 3, ghosts from a distant past. The
study of those objects enables us to gain insight into the infancy of
the Milky Way. Despite the recently published new data on that
strange globular cluster, astronomers still have to clarify whether it
was really formed in our Galaxy or not. It is known that some of its
clusters are not native to the Milky Way, but were captured, even
though they can currently be seen in its interior. Long ago, our
galaxy cannibalised other smaller galaxies and kept their globular
clusters. The rest were formed in-situ. It is suggested that the
object analysed could be dynamically related to other clusters, such
as 47 Tucanae, one of the richest and largest of the Milky Way. They
could even share the same stream of stars. If that were the case, it
would support the hypothesis that E 3 was captured in the distant
past.
FAINTEST GALAXY FROM EARLY UNIVERSE
Space Telescope Science Institute (STScI)
Astronomers using the Hubble and Spitzer space telescopes have
observed the faintest object ever seen in the early Universe. It
existed about 400 million years after the 'Big Bang', 13.8 billion
years ago. The team has nicknamed the object Tayna, which means
'first-born' in Aymara, a language spoken in the Andes and Altiplano
regions of South America. Though Hubble and Spitzer have detected
other galaxies that are record-breakers for distance, this object
represents a smaller, fainter class of newly forming galaxies that
until now have largely evaded detection. Such very dim objects may be
more representative of the early Universe, and offer new insight into
the formation and evolution of the first galaxies. The remote object
is part of a discovery of 22 young galaxies at ancient times located
nearly at the observable horizon of the Universe, representing a
substantial increase in the number of known very distant galaxies.
The new object is comparable in size to the Large Magellanic Cloud
(LMC; well known to be a satellite galaxy of our Milky Way). It is
rapidly making stars at a rate ten times faster than the LMC. The
object might be the growing core of what may evolve into a full-sized
galaxy.
The small and faint galaxy was seen only because of a natural 'magni-
fying glass' in space. As part of its 'Frontier Fields' programme,
Hubble observed a very massive cluster of galaxies, MACS J0416.1-2403,
located about 4 billion light-years away and having a mass as much
as 10 to the power 15 Suns. The cluster acts as a powerful natural
lens by bending and concentrating the light of far-more-distant
objects behind it. The cluster's gravity makes the distant proto-
galaxy look 20 times brighter than it really is. Its distance was
estimated by building a colour profile from combined Hubble and
Spitzer observations. The expansion of the Universe causes the light
from distant galaxies to be increasingly reddened with increasing
distance. Though many of the galaxy's new stars are intrinsically
blue-white, their light has been shifted into infrared wavelengths
that are measurable by Hubble and Spitzer. Absorption by intervening,
cool, intergalactic hydrogen also makes the galaxies look redder.
The finding suggests that the very early Universe will prove to be
rich in galaxies for the upcoming James Webb Space Telescope to
observe. Astronomers expect that the Webb will allow us to see the
embryonic stages of galaxy birth shortly after the Big Bang.
Topic: Late December Astronomy Bulletin (Read 1398 times)