Topic: Mid August Astronomy Bulletin

[Clive]

CASSINI FINDS FLOODED CANYONS ON TITAN 
NASA

The Cassini spacecraft has found, on Saturn's large moon Titan, deep,
steep-sided canyons that are flooded with liquid hydrocarbons.  The
finding represents the first direct evidence of the presence of liquid-
filled channels on Titan, as well as the first observation of canyons
hundreds of metres deep.  The Cassini observations reveal that the
channels -- in particular, a network of them named Vid Flumina -- are
narrow canyons, generally a bit less than a kilometre wide, with slopes
steeper than 40 degrees.  The canyons also are quite deep -- those
measured are 240 to 570 metres from top to bottom.  The branching
channels appear dark in radar images, much like Titan's methane-rich
seas.  That suggested to scientists that the channels might also be
filled with liquid, but a direct detection had not been made until now.
Previously it was not clear if the dark material was liquid or merely
saturated sediment -- which at Titan's frigid temperatures would be made
of ice, not rock.  Cassini's radar is often used as an imager, providing
a window to see through the dense haze that surrounds Titan to reveal the
surface below.  But during a recent pass, the radar was used as an
altimeter, sending pings of radio waves to the moon's surface to measure
the height of features there.  The researchers combined the altimetry
data with previous radar images of the region to make their discovery.

The key to understanding the nature of the channels was the way Cassini's
radar signal reflected off the bottoms of the features.  The radar
instrument observed a glint, indicating an extremely smooth surface like
that observed from Titan's hydrocarbon seas.  The timing of the radar
echoes, as they bounced off the canyons' edges and floors, provided
direct measures of their depths.  The presence of such deep cuts in the
landscape indicates that whatever process created them was active for a
long time or eroded down much faster than in other areas of Titan's
surface.  The researchers' proposed scenarios include uplift of the
'terrain' and changes in sea level, probably both.  It is likely that a
combination of those processes led to the formation of the deep canyons,
but it is not clear to what degree each was involved.  What is clear is
that any description of Titan's evolution needs to be able to explain how
the canyons got there.  Terrestrial examples of both of those types of
canyon-carving processes are found along the Colorado River in Arizona.
An example of uplift powering erosion is the Grand Canyon, where the
terrain's rising altitude caused the river to cut deeply downward into
the landscape over the course of several million years.  For canyon
formation driven by variations in water level, look to Lake Powell.  When
the water level in the reservoir drops, it increases the river's rate of
erosion.  While the altimeter data also showed that the liquid in some of
the canyons around Ligeia Mare is at sea level -- the same altitude as
the liquid in the sea itself -- in others it sits tens of metres higher
in elevation.  The researchers interpret the latter to be tributaries
that drain into the main channels below.  Future work will extend the
methods used in this study to all the other channels that Cassini's radar
altimeter has observed on Titan.  The researchers expect their continued
work to produce a more comprehensive understanding of forces that have
shaped Titan's landscape.


NEW DISTANT DWARF PLANET BEYOND NEPTUNE
University of British Columbia

Astronomers have discovered a new dwarf planet orbiting in the disc of
small icy bodies beyond Neptune.  The new object is about 700 km in
diameter and has one of the largest orbits for a dwarf planet.
Designated 2015 RR245 by the International Astronomical Union's Minor
Planet Center, it was found with the Canada-France-Hawaii Telescope on
Mauna Kea, Hawaii, as part of the ongoing 'Outer Solar System Origins
Survey' (OSSOS).  The OSSOS project uses computers to hunt for the
images, and the team was presented with a bright object moving at such a
slow rate that it was clearly at least 120 times further from the Sun
than the Earth.  The size of RR245 is not yet exactly known, as its
surface properties need further measurement.  The vast majority of dwarf
planets like RR245 were destroyed or thrown from the Solar System as the
giant planets moved out to their present positions.  RR245 is one of the
few that survived to the present day, along with Pluto and Eris, the
largest known dwarf planets.  RR245 now circles the Sun among the remnant
population of tens of thousands of much smaller trans-Neptunian bodies,
most of which orbit unseen.  RR245 has been on its highly eccentric orbit
for at least the last 100 million years.  After hundreds of years further
than 80 astronomical units (AU) from the Sun, RR245 is travelling towards
its closest approach at five billion km (34 AU), which it will reach
around 2096.  As RR245 has been observed for only one of the 700 years it
takes to orbit the Sun, where it came from and how its orbit will slowly
evolve in the far future is unknown.  Its precise orbit will be refined
over the coming years, after which RR245 will be given a name.  As
discoverers, the OSSOS team can submit their preferred name for RR245 to
the International Astronomical Union for consideration.  RR245 is the
largest discovery and the only dwarf planet found by OSSOS, which has
discovered more than five hundred new trans-Neptunian objects.


FAINTEST HISSES REVEAL FAMOUS STAR'S PAST LIFE
RAS

Astronomers have managed to see into the past of a nearby star millions
of years before its famous explosion, using a telescope in remote outback
Australia at a site free from FM radio interference.  Astronomers
observing the region at the lowest-ever radio frequencies have helped to
improve our understanding of stellar explosions.  The research paints a
picture of the star's life long before its death in what was the closest
and brightest supernova yet seen, now known as supernova remnant 1987A
(SN 1987A), which collapsed spectacularly almost 30 years ago in a
neighbouring galaxy, the Large Magellanic Cloud.  Much had been known
about the immediate past of that star through study of the remnants
resulting from the its collapse in 1987 However, it was the detection of
the very faint hiss through low-frequency radio astronomy that has
provided the latest insights.  Previously, out of the dead star's multi-
million-year life, only about 0.1%, or 20,000 years, had been observable.
Operating the Murchison Widefield Array in the West Australian desert,
the radio astronomers were able to 'see' right back to when the star was
in its long-lasting red-supergiant phase.
 
Previous studies focused on material that was ejected into space when the
star was in its final blue-supergiant phase.  Researchers found the red
supergiant lost its matter at a slower rate and generated winds that
pushed into its surrounding environment less quickly than was previously
assumed.  The new data improve our knowledge of the composition of space
in the region of SN 1987A; we can now go back to our simulations and
improve them, the better to reconstruct the physics of supernova explo-
sions.  The key to gaining the new insights was the quiet environment in
which the radio telescope is located.  Nobody knew what was happening at
low radio frequencies, because the signals from our own Earthbound FM
radio drown the faint signals from space.  Now, by studying the strength
of the radio signal, astronomers can for the first time calculate how
dense the surrounding gas is, and thus understand the environment of the
star before it died.
 

GIANT STELLAR VOID IN MILKY WAY
RAS
 
A major revision is required in our understanding of our Milky Way
Galaxy, according to a team of Japanese, South African and Italian
astronomers who find that there is a huge region around the centre of the
Galaxy which is devoid of young stars.  The Milky Way is a spiral galaxy
containing many thousand million stars, with our Sun about 26,000 light-
years from its centre.  Measuring the distribution of the stars is
crucial to our understanding of how our Galaxy formed and evolved.
Pulsating stars called Cepheids are ideal for that purpose.  They are
much younger (between 10 and 300 million years old) than our Sun (4,600
million years old) and they pulsate in brightness in regular cycles.  The
length of the cycle is related to the luminosity of the Cepheid, so if
astronomers monitor them they can establish how bright the star really
is, compare it with how bright it looks from here, and work out its
distance.  Finding Cepheids in the inner Milky Way is difficult, however,
as the Galaxy is full of interstellar dust which attenuates light and
hides many stars from view.  The team compensated for that, with an
analysis of near-infrared observations made with a Japanese--South-
African telescope located at Sutherland, South Africa.  To their surprise
they found hardly any Cepheids in a huge region stretching for thousands
of light-years from the core of the Galaxy.  Astronomers already found
some time ago that there are Cepheids in the central heart of our Milky
Way (in a region about 150 light-years in radius).  Now we find that,
outside that, there is a huge Cepheid desert extending out to 8000
light-years from the centre.  That suggests that a large part of the
Galaxy, called the Extreme Inner Disc, has no young stars -- a conclusion
that is contrary to other recent work, but in line with the work of radio
astronomers who see no new stars being born in that desert.  Cepheids
have more typically been used to measure the distances of objects in the
distant Universe, and the new work is an example instead of the same
technique revealing the structure of our own Milky Way.
 

LIGO GRAVITATIONAL WAVES MAY HAVE ORIGINATED FROM PRIMORDIAL BLACK HOLES
Kyoto University

Binary black holes recently discovered by the LIGO--Virgo collaboration
could be primordial entities that formed just after the Big Bang,
according to a report by Japanese astrophysicists.  If further data
support that conclusion, it could mark the first confirmed finding of a
primordial black hole, guiding theories about the beginnings of the
Universe.  In February, the Virgo collaboration announced the first
successful detection of gravitational waves.  The waves were created from
a merger of two black holes thirty times the mass of the Sun.  It is
extremely rare for such massive black holes to form in the present-day
Universe.  After that announcement, many astrophysicists started
considering how such massive black holes were created, and how such
black-hole binaries were formed.  As a starting point, the team
hypothesized that primordial black holes -- formed following the Big Bang
-- were distributed randomly in space.  The Universe was extremely hot
and dense when it was first born.  Primordial black holes came into being
when gravitational collapse happened in regions which were especially
dense.  They have a completely different origin from black holes that
form from celestial bodies.

The research team evaluated, on the basis of general relativity, how
often black holes merge in the present epoch.  They found that the
LIGO--Virgo team's observational data on merger frequencies would fall in
to place if the binaries were primordial, and if they constitute a
thousandth of all dark matter in the Universe.  Primordial black-hole
binaries were discussed extensively in the 1990s; however, interest in
them waned when observations implied that their number was limited.  To
date, no one has found any primordial black holes, possibly making the
LIGO--Virgo observations the first of their kind.  Theoretical models
about the beginnings of the Universe are still hotly contested.  Some
models necessarily predict the existence of primordial black holes, so
their discovery might offer important clues about the Universe's early
days.  When more observational data related to black-hole binaries have
accumulated, it may become possible to decide whether they are truly
primordial.