TITAN HAS 'SEA LEVEL' LIKE THE EARTH
NASA
Saturn's moon Titan is nearly a thousand million miles away from the Earth,
but a recently published paper based on data from NASA's Cassini spacecraft
reveals a new way in which Titan and our own planet are similar. Just as the
surfaces of oceans on the Earth lie at an average elevation that we call
'sea level', Titan's seas also lie at an average elevation. That is the
latest finding that shows remarkable similarities between the Earth and
Titan, the only other object that we know of in our Solar System that has
stable liquid on its surface. The twist at Titan is that its lakes and seas
are filled with hydrocarbons rather than with liquid water, and water ice
overlain by a layer of solid organic material serves as the bedrock
surrounding the lakes and seas. The new paper finds that Titan's seas
follow a constant elevation relative to Titan's gravitational pull -- just
like the Earth's oceans. Smaller lakes on Titan, it turns out, appear at
elevations several hundred feet, or metres, higher than Titan's sea level.
Lakes at high elevation are commonly found on Earth. The highest lake
navigable by large ships, Lake Titicaca, has a water volume of nearly 900
cubic kilometres and a surface that is over 3,800 metres above sea level.
The new study of Titan suggests that elevation is important, because Titan's
liquid bodies appear to be connected under the surface in something akin to
a terrestrial aquifer system. Hydrocarbons appear to be flowing underneath
Titan's surface in the way that water flows through underground porous rock
or gravel on Earth, so nearby lakes communicate with each other and share a
common liquid level.
SURFACE OF GIANT STAR IMAGED
Georgia State University
Astronomers have produced the first detailed images of the surface of a
giant star, revealing a nearly spherical, dust-free atmosphere with complex
areas of moving material, known as convection cells or granules. The giant
star, named pi1 Gruis, is one of the stars in the southern constellation
Grus. An evolved star in the last major phase of its life, pi1 Gruis is
350 times the size of the Sun and resembles what our Sun will become at the
end of its life in five billion years. Studying that star gives
scientists insight about the future activity, characteristics and appearance
of the Sun. Convection, the transfer of heat by the bulk movement of gases
and liquids, plays a major role in astrophysical processes, such as energy
transport, pulsation and winds. The Sun has about two million convective
cells that are typically 2,000 km across, but theorists believe that giant
and supergiant stars should have only a few large convective cells because
of their low surface gravities. Determining the convection properties of
most evolved and supergiant stars, such as the sizes of granules, has been
challenging because their surfaces are frequently obscured by dust. In the
study summarized here, the researchers discovered that the surface of the
giant star pi1 Gruis had a complex convective pattern, and that a typical
granule measured 1.2 x 10^11 metres horizontally or 27% of the diameter of
the star.
This is the first time that astronomers have unambiguously imaged such a
giant star with that level of detail. The reason is that there is a limit
to the details that can be seen, related to the size of the telescope used
for the observations. The team used an interferometer, in which the light
from several telescopes is combined, achieving a resolution equivalent to
that of a much larger telescope. The star pi1 Gruis was observed with the
PIONIER instrument, which has four combined telescopes, in Chile in 2014
September. That study was also the first to confirm theories about the
characteristics of granules on giant stars. The images are important,
because the size and number of granules on the surface actually fitted very
well with models that predict what astronomers should be seeing. The
detailed images also showed different colours on the star's surface, which
correspond to varying temperatures. A star does not have the same surface
temperature throughout, and its surface provides our only clues to under-
stand its interior. As temperatures rise and fall, the hotter, more fluid
areas become brighter colours (whiter) and the cooler, denser areas become
darker (redder).
FIRST LIGHT FOR PLANET HUNTER
ESO
The newest addition to the La Silla observatory in northern Chile,
Exoplanets in Transits and their Atmospheres (ExTrA), has made its first
successful observations. ExTrA is designed to search for planets around
nearby red dwarf stars and study their properties. It is a French project
funded by the European Research Council and the French Agence National de
la Recherche. The telescopes will be operated remotely from Grenoble.
To detect and study exo-planets, ExTrA uses three 0.6-m telescopes. They
regularly monitor the amount of light received from many red dwarf stars and
look for a slight dip in brightness that could be caused by a planet passing
(transiting) across a star's disc and obscuring some of its light. The
transit method involves comparing the brightness of the star under study
with other reference stars to observe tiny changes. However, from the
ground it is difficult to make sufficiently precise measurements in that way
to detect small, Earth-sized planets. By using a novel approach that also
incorporates information about the brightness of the stars in many different
colours, however, ExTrA overcomes some of the limitations. The three ExTra
telescopes collect light from the target star and four comparison stars, and
that light is then fed through optical fibres into a multi-object spectro-
graph. That approach of adding spectroscopic information to traditional
photometry helps to mitigate the disruptive effect of the Earth'satmosphere,
as well as effects introduced by instruments and detectors, increasing the
precision achievable. Because a transiting planet will block a greater
proportion of the light from a smaller star, ExTrA will focus on targeting
nearby M dwarfs. Such stars are expected to host many Earth-sized planets,
making them prime targets for astronomers hoping to discover and study
distant planets that could be amenable to life. The nearest star to the
Sun, Proxima Centauri, is an M dwarf and has been found to have an orbiting
Earth-mass planet. Finding previously undetectable Earth-like planets is
only one of two key objectives for ExTrA. The telescope will also study the
planets it finds in some detail, assessing their properties and deducing
their composition to determine how similar they may be to the Earth.
BLACK HOLE HIDING IN GIANT STAR CLUSTER
ESO
Astronomers using ESO's MUSE instrument on the Very Large Telescope in Chile
have discovered a star in the globular cluster NGC 3201 that is behaving
very strangely. It appears to be orbiting an invisible black hole with
about four times the mass of the Sun -- the first such inactive stellar-mass
black hole found in a globular cluster and the first found by detecting its
gravitational pull. This important discovery impacts on our understanding
of the formation of such star clusters, black holes, and the origins of
gravitational-wave events. Globular star clusters are huge spheres of tens
of thousands of stars that orbit most galaxies. They are among the oldest
known stellar systems in the Universe and date back to near the beginning of
galaxy growth and evolution. More than 150 are currently known to belong to
the Milky Way. One particular cluster, called NGC 3201 and situated in the
southern constellation Vela, has now been studied with the MUSE instrument
on ESO's Very Large Telescope in Chile. An international team of astronomers
has found that one of the stars in NGC 3201 is behaving very oddly -- it is
being flung backwards and forwards at speeds of several hundred thousand
kilometres per hour, with the pattern repeating every 167 days. The
relationship between black holes and globular clusters is an important but
mysterious one. Because of their large masses and great ages, these
clusters are thought to have produced a large number of stellar-mass black
holes -- created as massive stars within them, and exploded and collapsed
over the long lifetime of the cluster.
The MUSE instrument can measure the motions of thousands of stars at the
same time. With the new finding, the team has for the first time been able
to detect an inactive black hole at the heart of a globular cluster -- one
that is not currently swallowing matter and is not surrounded by a glowing
disc of gas. They could estimate the black hole's mass through the
movements of the star caught up in its enormous gravitational pull. From its
observed properties the star was determined to be about 0.8 times the mass
of our Sun, and the mass of its mysterious counterpart was calculated at
around 4.36 times the Sun's mass -- almost certainly a black hole. Recent
detections of radio and X-ray sources in globular clusters, as well as the
2016 detection of gravitational-wave signals produced by the merging of two
stellar-mass black holes, suggest that relatively small black holes may be
more common in globular clusters than previously thought. Until recently,
it was assumed that almost all black holes would disappear from globular
clusters after a short time and that systems like this should not even
exist! But clearly that is not the case -- this discovery is the first
direct detection of the gravitational effects of a stellar-mass black hole
in a globular cluster. This finding helps in understanding the formation of
globular clusters and the evolution of black holes and binary systems.
NEUTRON-STAR MERGER PUZZLE
McGill University
The afterglow from the distant neutron-star merger detected last August has
continued to brighten -- much to the surprise of astrophysicists studying
the aftermath of the massive collision that took place about 138 million
light-years away and sent gravitational waves rippling through the Universe.
New observations from the orbiting Chandra X-ray Observatory indicate that
the gamma-ray burst unleashed by the collision is more complex than
scientists initially imagined. Usually when we see a short gamma-ray burst,
the jet emission generated gets bright for a short time as it smashes into
the surrounding medium -- then fades as the system stops injecting energy
into the outflow. The new data could be explained by more complicated
models for the remnants of the neutron-star merger. One possibility is that
the merger launched a jet that shock-heated the surrounding gaseous debris,
creating a hot 'cocoon' around the jet that has glowed in X-rays and radio
light for many months. The X-ray observations chime with radio-wave data
reported last month by another team of scientists, which found that those
emissions from the collision also continued to brighten over time. While
radio telescopes were able to monitor the afterglow throughout the autumn,
X-ray and optical observatories were unable to watch it for around three
months, because the Sun was too close to that point in the sky during that
period. When the source emerged from that blind spot in the sky in early
December, the Chandra team jumped at the chance to see what was going on.
Sure enough, the afterglow turned out to be brighter in X-ray wavelengths,
just as it was in the radio.
That unexpected pattern has set off a scramble among astronomers to
understand what physics is driving the emission. This neutron-star merger
is unlike anything we have seen before. For astrophysicists, it is a gift
that seems to keep on giving. The neutron-star merger was first detected
on August 17 by the U.S.-based Laser Interferometer Gravitational-Wave
Observatory (LIGO). The European Virgo detector and some 70 ground- and
space-based observatories helped to confirm the discovery. The discovery
marks the first time that scientists have been able to observe a cosmic
event with both light waves and gravitational waves, the ripples in space-
time predicted a century ago by Einstein's general theory of relativity.
Mergers of neutron stars, among the densest objects in the universe, are
thought to be responsible for producing heavy elements such as gold and
platinum.
ORIGIN OF MOLECULES IN BLACK-HOLE WINDS
Northwestern University
The existence of large numbers of molecules in winds powered by supermassive
black holes at the centres of galaxies has puzzled astronomers since they
were discovered more than a decade ago. Molecules trace the coldest parts
of space, and black holes are the most energetic phenomena in the Universe,
so finding molecules in black-hole winds was like discovering ice in a
furnace. Astronomers questioned how anything could survive the heat of the
energetic outflows, but a new theory predicts that the molecules are not
survivors at all, but brand-new molecules, born in the winds with unique
properties that enable them to adapt to and thrive in the hostile environ-
ment. When a black-hole wind sweeps up gas from its host galaxy, the gas
is heated to high temperatures, which destroy any existing molecules. By
modelling the molecular chemistry in computer simulations of black-hole
winds, astronomers found that the swept-up gas can subsequently cool and
form new molecules.
That theory answers questions raised by previous observations made with
several cutting-edge astronomical observatories including the Herschel
Space Observatory and the Atacama Large Millimetre Array, a powerful radio
telescope located in Chile. In 2015, astronomers confirmed the existence
of energetic outflows from supermassive black holes found at the centres of
most galaxies. Those outflows sweep away everything in their path, expell-
ing the molecules that fuel star formation. The winds are also presumed to
be responsible for the existence of 'red and dead' elliptical galaxies, in
which no new stars can form. Then, in 2017, astronomers observed rapidly
moving new stars forming in the winds -- a phenomenon they thought would be
impossible given the extreme conditions in black-hole-powered outflows.
New stars form from molecular gas, so the new theory of molecule formation
helps to explain the formation of new stars in winds. It upholds previous
predictions that black-hole winds destroy molecules upon first collision but
also predicts that new molecules -- including hydrogen, carbon monoxide and
water -- can form in the winds themselves. This is the first time that the
molecule-formation process has been simulated in full detail, and is a very
compelling explanation for the observation that molecules are ubiquitous in
supermassive-black-hole winds, which has been one of the major outstanding
problems in the field. Astronomers predict that the new molecules formed in
the winds are warmer and brighter in infrared radiation compared to pre-
existing molecules. That theory will be put to the test when NASA launches
the James Webb Space Telescope in spring 2019. If the theory is correct,
the telescope will be able to map black-hole outflows in detail by means of
infrared radiation.
COMPLEX ORGANIC MOLECULES FOUND IN THE LMC
National Radio Astronomy Observatory
The nearby dwarf galaxy known as the Large Magellanic Cloud (LMC) is a
chemically primitive place. Unlike the Milky Way, that semi-spiral
collection of a few times 10 to the 10 stars lacks our Galaxy's rich
abundance of elements like carbon, oxygen, and nitrogen. Owing to the
dearth of such elements, astronomers predicted that the LMC should
contain a comparatively paltry amount of complex carbon-based molecules.
Previous observations of the LMC seemed to support that view. New
observations with the Atacama Large Millimetre/submillimetre Array (ALMA),
however, have uncovered the surprisingly clear chemical 'fingerprints' of
the complex organic molecules methanol, dimethyl ether, and methyl formate.
Though previous observations found hints of methanol in the LMC, the last
two are unprecedented findings and stand as the most complex molecules ever
conclusively detected outside our Galaxy. Astronomers discovered the
molecules' faint millimetre-wavelength 'glow' emanating from two dense
star-forming embryos in the LMC, regions known as 'hot cores'. The
observations may provide insights into the formation of similarly complex
organic molecules early in the history of the Universe. Even though the
Large Magellanic Cloud is one of our nearest galactic companions, we expect
that it should share some chemical similarity with distant, young galaxies
from the early Universe. Astronomers refer to the lack of heavy elements
as 'low metallicity'. It takes several generations of star birth and star
death to seed a galaxy liberally with heavy elements, which then get taken
up in the next generation of stars and become the building blocks of new
planets. Young, primordial galaxies simply did not have enough time to
become so chemically enriched. Dwarf galaxies like the LMC probably
retained the same youthful makeup because of their relatively low masses,
which severely throttle back the pace of star formation. Owing to its low
metallicity, the LMC offers a window into those early, adolescent galaxies.
Star-formation studies of the LMC provide a stepping stone to understand
star formation in the early Universe.
The astronomers focused their study on the N113 star-formation region in
the LMC, which is one of that galaxy's most massive and gas-rich regions.
Earlier observations of that area with the Spitzer Space Telescope and the
Herschel Space Observatory revealed a startling concentration of young
stellar objects -- proto-stars that have just begun to heat their stellar
nurseries, causing them to glow brightly in infrared light. At least a
portion of that star formation is due to a domino-like effect, where the
formation of massive stars triggers the formation of other stars in the
same general vicinity. Astronomers used ALMA to study several young
stellar objects in the region to understand better their chemistry and
dynamics. The ALMA data surprisingly revealed the telltale spectral
signatures of dimethyl ether and methyl formate, molecules that have never
been detected so far away. Complex organic molecules, those with six or
more atoms including carbon, are some of the basic building blocks of
molecules that are essential to life on Earth and -- presumably -- else-
where in the Universe. Though methanol is a relatively simple compound
compared to other organic molecules, it nonetheless is essential to the
formation of more complex organic molecules, like those that ALMA recently
observed, among others. If those complex molecules can readily form around
proto-stars, it is likely that they would endure and become part of the
proto-planetary disks of young star systems. Such molecules were probably
delivered to the primitive Earth by comets and meteorites, helping to jump-
start the development of life on our planet. The astronomers speculate
that, since complex organic molecules can form in chemically primitive
environments like the LMC, it is possible that the chemical framework for
life could have emerged relatively early in the history of the Universe.
Topic: Early February Astronomy Bulletin (Read 1600 times)