POSSIBLE WATER PLUMES ON EUROPA
NASA
Astronomers using the Hubble space telescope have imaged what may be
water-vapour plumes erupting from the surface of Jupiter's moon
Europa. That finding bolsters other Hubble observations suggesting
that the icy moon erupts with high-altitude water-vapour plumes. The
observation increases the possibility that missions to Europa may be
able to sample Europa's ocean without having to drill through miles of
ice. The plumes are estimated to rise about 200 km before,
presumably, raining material back down onto the surface. Europa
has a huge global ocean containing twice as much water as the Earth's
oceans, but it is protected by a layer of extremely cold and hard ice
of unknown thickness. The original goal of the team's observing
proposal was to determine whether Europa has a thin, extended
atmosphere, or exosphere. Using the same observing method that
detects atmospheres around planets orbiting other stars, the team
realized that, if there were water vapour venting from Europa's
surface, such an observation would be an excellent way to see it.
The atmosphere of an extra-solar planet blocks some of any light that
comes from behind it. If there is a thin atmosphere around Europa,
then when Europa transits in front of Jupiter it could block some of
the light of Jupiter, and we could see it as a silhouette. So
astronomers were looking for absorption features around the limb of
Europa as it transited the smooth face of Jupiter. In 10 separate
occurrences spanning 15 months, the team observed Europa passing in
front of Jupiter. It saw what could be plumes erupting on three of
those occasions. In 2012, another team detected evidence of water
vapour erupting from the frigid south-polar region of Europa and
reaching more than 160 km into space. Although both teams used
Hubble's 'Space Telescope Imaging Spectrograph' instrument, they used
totally independent methods and arrived at the same conclusion. If
confirmed, Europa would be the second moon in the Solar System known
to have water-vapour plumes. In 2005, the Cassini orbiter detected
jets of water vapour and dust spewing off the surface of Saturn's moon
Enceladus. Scientists may use the infrared vision of the James Webb
space telescope, which is scheduled to be launched in 2018, to confirm
venting or plume activity on Europa.
FIVE NEW NEPTUNE TROJANS
Physics.org
A team of astronomers using the Pan-STARRS 1 (PS1) survey has detected
five new so-called 'Neptune trojans' -- minor bodies sharing the same
orbit as the planet Neptune. The PS1 survey, which utilizes the first
Pan-STARRS (Panoramic Survey Telescope and Rapid Response System)
telescope in Hawaii, offers one of the best ways to search for Neptune
trojans. The survey, lasting from 2010 May to 2014 May, has made a
strong contribution to knowledge of the Solar System's minor bodies
owing to its very wide survey area and its optimized cadence for
finding moving objects. The researchers found four new L4 trojans,
meaning that they orbit Neptune's L4 Lagrangian point 60 degrees ahead
of te planet; they also found one L5 trojan (orbiting the L5 region
60 degrees behind it). The newly detected objects have sizes ranging
from 100 to 200 km in diameter. The new L5 trojan is dynamically more
unstable than the other four, indicating that it is probably only
temporarily captured into the Neptune trojan cloud. Orbital
simulations show that it librates stably only for several million
years. That suggests that it must be of recent capture origin. On
the other hand, all four new L4 trojans have stably occupied the 1:1
resonance with Neptune for more than 1 billion years. They may,
therefore, be of primordial origin. The team also found that there
seem to be two groups of Neptune trojans: one group with orbital
inclinations less than 10°, and the other with inclinations more than
18°. There is none with intermediate inclinations -- the trojan
population has a bimodal inclination distribution, but the small
numbers involved do not make for high statistical certainty.
NEW LOW-MASS OBJECTS DISCOVERED
Carnegie Institution for Science
When a star is young, it is often still surrounded by a primordial
rotating disc of gas and dust, from which planets can form. Astrono-
mers like to find such discs because they might be able to observe the
star part-way through the planet-formation process, but it is highly
unusual to find such discs around brown dwarfs or stars with very low
masses. Canadian astronomers have discovered four new low-mass
objects surrounded by discs. Three of the four objects are quite
small, between only 13 and 18 times the mass of Jupiter. The fourth
has about 120 times Jupiter's mass. (For comparison, the Sun is just
over 1,000 times more massive than Jupiter.) In a planet-forming
disc, the dust grains collide and aggregate to form pebbles, which
grow into boulders, and so on, increasing in size through planetes-
imals, planetary embryos, and finally rocky terrestrial planets (some
of which then become the cores for gas-giant planets). Astronomers
are able to identify those types of planet-birthing discs, because the
star heats up the surrounding dust, which affects the way it looks in
infrared light. However, some discs indicate that planet formation is
not ongoing, but has already finished. Those ones are made up of the
debris left behind by all the collisions during planet formation and
by subsequent collisions of the newly formed planets. Eventually
those dusty remains are swept away. But until that happens, a cooler,
thinner ring of dust surrounds the star. Some discs even represent an
intermediate stage between the planet-forming and dusty-remnant
phases. It is important for astronomers to try to distinguish between
those different types of discs, because then they can chart better the
way that planetary systems, including our own Solar System, are born
and change over time. The research team was able to determine that
the discs surrounding their four newly discovered low-mass objects
were all likely to be in a phase of planet-forming; none was in the
dusty-aftermath phase. Even more interesting, two of the objects are
possibly between 42 and 45 million years old. That would make them
the oldest objects surrounded by active disc systems ever found.
ALMA UNCOVERS SECRETS OF GIANT SPACE BLOB
ESO
An international team using ALMA, along with the Very Large Telescope
and other telescopes, has discovered the true nature of a rare object
in the distant Universe called a Lyman-alpha Blob. Up till now
astronomers did not understand what made those huge clouds of gas
shine so brightly, but ALMA has now seen two galaxies at the heart of
one of those objects and they are undergoing a frenzy of star
formation that is lighting up their surroundings. Those large
galaxies are in turn at the centre of a swarm of smaller ones in what
appears to be an early phase in the formation of a massive cluster of
galaxies. The two ALMA sources are expected to evolve into a single
giant elliptical galaxy. Lyman-alpha Blobs (LABs) are gigantic clouds
of hydrogen gas that can span hundreds of thousands of light-years and
are found at very large cosmic distances. The name reflects the
characteristic wavelength of ultraviolet light that they emit, known
as Lyman-alpha radiation. Since their discovery, the processes that
give rise to LABs have been an astronomical puzzle. But new
observations with ALMA may now have now cleared up the mystery. One
of the largest Lyman-alpha Blobs known, and the most thoroughly
studied, is SSA22-Lyman-alpha Blob 1, or LAB-1. Embedded in the core
of a huge cluster of galaxies in the early stages of formation, it was
the very first such object to be discovered, in 2000, and is located
so far away that its light has taken about 11.5 billion years to
reach us.
A team of astronomers has now used the Atacama Large Millimetre/
Sub-millimetre Array (ALMA) to observe light from cool dust clouds in
distant galaxies to look deeply into LAB-1. That allowed it to
pinpoint and resolve several sources of sub-millimetre emission. It
then combined the ALMA images with observations from the Multi-Unit
Spectroscopic Explorer (MUSE) instrument mounted on ESO's Very Large
Telescope (VLT), which map the Lyman-alpha light. That showed that
the ALMA sources are located in the very heart of the Lyman-alpha
Blob, where they are forming stars at a rate over 100 times that of
the Milky Way. Deep imaging with the Hubble space telescope and
spectroscopy at the Keck Observatory showed in addition that the ALMA
sources are surrounded by numerous faint companion galaxies that could
be bombarding the central ALMA sources with material, helping to drive
their high star-formation rates. The team then turned to a
sophisticated simulation of galaxy formation to demonstrate that the
giant glowing cloud of Lyman-alpha emission can be explained if
ultraviolet light produced by star formation in the ALMA sources
scatters off the surrounding hydrogen gas. That would give rise to
the Lyman-alpha Blob that we see. Understanding how galaxies form and
evolve is a massive challenge. Astronomers think Lyman-alpha Blobs
are important because they seem to be the places where the most
massive galaxies in the Universe form. In particular, the extended
Lyman-alpha glow provides information on what is happening in the
primordial gas clouds surrounding young galaxies, regions that are
very difficult to study, but critical to our understanding.
STELLAR COCOON WITH UNUSUAL CHEMISTRY
ESO
Japanese researchers have used ALMA to observe a massive star known as
ST11 in our neighbouring dwarf galaxy, the Large Magellanic Cloud
(LMC). Emission from a number of molecular gases was detected.
It indicated that the team had discovered a concentrated region of
comparatively hot and dense molecular gas around the newly ignited
star ST11. That was evidence that they had found something never
before seen outside the Milky Way -- a hot molecular core. The ALMA
observations revealed that the newly discovered core in the LMC has a
composition very different from that of otherwise similar objects in
the Milky Way. The most prominent chemical signatures in the LMC core
include familiar molecules such as sulphur dioxide, nitric oxide, and
formaldehyde, alongside the ubiquitous dust. But several organic
compounds, including methanol (the simplest alcohol molecule), had
remarkably low abundances in the newly detected hot molecular core.
In contrast, cores in the Milky Way have been observed to contain a
wide assortment of complex organic molecules, including methanol and
ethanol. The LMC has a low abundance of elements other than hydrogen
and helium. The research team suggests that that very different
galactic environment has affected the molecule-forming processes
taking place surrounding the newborn star ST11. That could account
for the observed differences in chemical compositions. It is not yet
clear if the large, complex molecules detected in the Milky Way exist
in hot molecular cores in other galaxies. Complex organic molecules
are of very special interest because some are connected to prebiotic
molecules formed in space. The newly discovered object in one of our
nearest galactic neighbours is an excellent object to help astronomers
address that issue. It might also raise a hypothetical question: how
could the chemical diversity of galaxies affect the development of
extragalactic life?
FERMI FINDS RECORD-BREAKING BINARY STAR
NASA
Scientists using the Fermi gamma-ray space telescope have found the
first gamma-ray binary in another galaxy and the most luminous one
ever seen. The dual-star system, dubbed LMC P3, contains a massive
star and a crushed stellar core that interact to produce a cyclic
flood of gamma rays, the highest-energy form of light. Fermi has
detected only five such systems in our own galaxy, so finding one so
luminous and distant is quite exciting. Gamma-ray binaries are prized
because the gamma-ray output changes significantly during each orbit
and sometimes over longer time-scales. The variation lets us study in
detail many of the emission processes common to other gamma-ray
sources. Those rare systems contain either a neutron star or a black
hole, and radiate most of their energy in the form of gamma rays.
Remarkably, LMC P3 is the most luminous such system known in gamma
rays, X-rays, radio waves and visible light, and it is only the second
one discovered with Fermi. LMC P3 lies within the expanding debris of
a supernova explosion located in the Large Magellanic Cloud (LMC). In
2012, scientists using the Chandra X-ray observatory found a strong
X-ray source within the supernova remnant and showed that it was
orbiting a hot, young star many times the Sun's mass. The researchers
concluded that the compact object was either a neutron star or a black
hole, and classified the system as a high-mass X-ray binary (HMXB).
In 2015, the team began looking for new gamma-ray binaries in Fermi
data by searching for the periodic changes characteristic of such
systems. The scientists discovered a 10.3-day cyclic change centred
near one of several gamma-ray point sources recently identified in the
LMC. One of them, called P3, was not linked to objects seen at any
other wavelengths but was located near the HMXB. The question arose,
were they the same object?
To find out, astronomers observed the binary in X-rays with the Swift
satellite, at radio wavelengths with the Australia Telescope Compact
Array near Narrabri, and in visible light with the 4.1-m Southern
Astrophysical Research Telescope in Chile and the 1.9-m telescope at
the SAAO in South Africa. The Swift observations clearly revealed the
same 10.3-day emission cycle seen in gamma rays by Fermi. They also
indicated that the brightest X-ray emission occurs opposite to the
gamma-ray peak, so when one reaches maximum the other is at minimum.
Radio data exhibit the same period and out-of-phase relationship with
the gamma-ray peak, confirming that LMC P3 is indeed the same system
investigated by Chandra. The optical observations show changes due to
binary orbital motion, but because scientists do not know how the
orbit is tilted with respect to our line of sight, they can only
estimate the individual masses. The star is between 25 and 40 times
the Sun's mass, and if we are viewing the system at an angle midway
between face-on and edge-on, which seems most likely, its companion is
a neutron star about twice the Sun's mass. If, however, we view the
binary nearly face-on, then the companion must be significantly more
massive and must be a black hole.
In gamma-ray binaries, the compact companion is thought to produce a
'wind' of its own, one consisting of electrons accelerated to near the
speed of light. The interacting outflows produce X-rays and radio
waves throughout the orbit, but those emissions are detected most
strongly when the compact companion travels along the part of its
orbit closest to the Earth. Through a different mechanism, the
electron wind also emits gamma rays. When light from the star
collides with high-energy electrons, it receives a boost to gamma-ray
levels. Called inverse-Compton scattering, that process produces more
gamma rays when the compact companion passes near the star on the far
side of its orbit as seen from our perspective. Before Fermi was
launch, gamma-ray binaries were expected to be more numerous than they
have turned out to be. Hundreds of HMXBs are catalogued, and those
systems are thought to have originated as gamma-ray binaries following
the supernova that formed the compact object. It is certainly a
surprise to detect a gamma-ray binary in another galaxy before we find
more of them in our own. One possibility is that the gamma-ray
binaries that Fermi has found are rare cases where a supernova formed
a neutron star with an exceptionally rapid spin, which would enhance
how it produces accelerated particles and gamma rays.
CHINESE SPACE LAB WILL FALL TO EARTH NEXT YEAR
Science Daily
China's first-ever space lab will burn up in the Earth's atmosphere
towards the end of next year. The 8.5-ton Tiangong-1 spacecraft is
currently intact and orbiting the Earth at an altitude of 370 km,
which is a bit lower than the International Space Station, which
usually stays about 400 km up. Tiangong-1 will probably fall back to
Earth in the second half of 2017, but its demise should not cause
problems here on the ground. China is monitoring Tiangong-1 (whose
name means 'Heavenly Palace' in Mandarin) closely, and will issue
appropriate warnings if the space lab threatens to hit a satellite.
China will release a forecast of Tiangong-1's fall to Earth 'if
necessary'. The update seems to confirm speculation that China is no
longer in control of the 10.3-m space lab which was launched in 2011
September to test docking technologies and other skills that China
will need to build its planned space station in the early 2020s.
After all, if operators were still controlling the space lab, they
could steer it to a guided re-entry over an empty stretch of ocean at
a specified time. Two astronauts are scheduled to arrive at
Tiangong-2 in mid- to late October for a 30-day stay. The Shenzhou-9
and Shenzhou-10 crews spent eight and 12 days aboard Tiangong-1,
respectively. China is not part of the multi-national consortium,
led by the United States and Russia, that operates the 400-ton
International Space Station. China aims to have its own 54-ton space
station in Earth orbit by 2022 or so.
Topic: Early October Astronomy Bulletin (Read 1329 times)