Topic: Mid January Astronomy Bulletin

[Clive]

CASSINI OBSERVES RIVER ON TITAN
NASA

Cassini mission scientists have observed on Saturn's moon Titan what
appears to be a river valley that stretches more than 400 km from its
source to a large sea.  The scientists suppose that the river, which
is in Titan's north-polar region, carries liquid hydrocarbons, because
it appears dark along its entire length in the high-resolution radar
image, indicating a smooth surface.  Titan is the only planetary body
that we know of (other than our own) to have stable liquid on its
surface.  While the Earth's hydrological cycle operates with water,
Titan's equivalent one involves hydrocarbons such as ethane and
methane.  In 2008 Cassini's visual/infrared mapping spectrometer
confirmed liquid ethane at a lake known as Ontario Lacus in Titan's
southern hemisphere.  In Titan's equatorial regions, images taken in
late 2010 by Cassini's visible-light cameras revealed regions that
darkened after 'rain'fall.


TAU CETI'S PLANETS
BBC News

Astronomers using data from three planet-hunting missions (HARPS,
AAPS, and HIRES) believe that one of the nearest single Sun-like stars
has five planets.  Tau Ceti, 12 light-years away, now joins Alpha
Centauri B as a 'nearby' star known to have planets.  In both cases,
the planets were found by measuring the reflex motions of the parent
stars, as the planets moved in their orbits, by the radial-velocity
method initially developed about 50 years ago by the 'moderator' of
these Bulletins.  The velocity changes caused by planets are very
small and can be confused with small variations arising from other
causes.  Scientists have tried to refined their 'noise modelling' in
order to subtract the confusing signals and thereby scrape the barrel
for signals of planetary origin; the abundance of data available on
Tau Ceti gives that process a better chance of being reliable in the
case of that star than for others.

The technique used was to put in 'fake planets' -- to add false
planet-like periodic signals to the messy observed data -- and then
try to find ways to reduce the noise until the fake signals became
more and more visible in the data.  The scientists developed a
noise-modelling strategy which allowed them to recover the fake
signals that were initially buried in the noise -- but in the process
of doing that, they uncovered what seemed likely to be signals of five
actual planets as well.  They include planets between two and six
times the Earth's mass, with periods ranging from 14 to 640 days.  One
of them, dubbed Tau Ceti e, lies about half as far from Tau Ceti as
the Earth is from the Sun -- and because Tau Ceti is slightly smaller
and dimmer than the Sun, that puts the planet in the so-called
habitable zone.  (That means only that the temperature may be such that
liquid water could exist on it -- there is no implication that the
planet is actually inhabited.  People on Tau Ceti, if they existed,
would see Venus and Mars as being within the Sun's 'habitable zone',
but there is more to habitability than just being a certain distance
from a star!)


HYPERGIANT STAR'S HISTORY OVER 30 YEARS
Royal Observatory of Belgium

A team of scientists has investigated the hypergiant star HR 8752 for
30 years while it traversed the 'Yellow Evolutionary Void'. The Void
is a short stage in the lives of the most massive stars, when they
become very unstable. The team finds that the surface temperature of
HR 8752 rose surprisingly quickly from 5000 to 8000 degrees in less
than 30 years.  The discovery is a step towards resolving the enigma
of the hypergiants, the most luminous and massive stars in the Galaxy.
Hypergiants can shine millions of times more brightly than the Sun,
and they often have diameters several hundred times greater.  HR 8752
is a quarter of a million times more luminous than the Sun.  The star
is therefore visible just with binoculars, and even to the naked eye
from a dark-sky site, despite its great distance (of the order of a
kiloparsec -- three or four thousand light-years), in the
constellation Cassiopeia.  There are only about a dozen hypergiants
known in our Galaxy.  The 'Yellow Evolutionary Void' is a short stage
in the life of a very massive star when it can become unstable and its
temperature and luminosity can change quickly.  The team has
discovered that hypergiant atmospheres are very unstable inside the
Void because outwardly directed forces can be equal to, or sometimes
even stronger than, the force of gravity.  Owing to their unstable
atmospheres, hypergiants in that 'forbidden zone' can lose tremendous
amounts of mass, sometimes amounting to the mass of the Sun in a year.
When a hypergiant enters the 'Evolutionary Void' it suffers
disturbances to its equilibrium that conspire to cause it to leave it
again as quickly as possible, which is why almost all hypergiants are
found outside the Void.

The team finds HR 8752 to be a very rare hypergiant which has partly
traversed the Void.  The changes of its atmosphere were closely
monitored with regular observations over 30 years.  Around 1980
HR 8752 was very similar to the eruptive hypergiant Rho Cas of spectral
type F, but then the temperature of HR 8752's atmosphere rapidly
increased by 3000 degrees and now shows the spectral properties of a
hotter A-type star.  Astronomers are baffled as to how such enormous
changes could occur in that period of time.  Between 1900 and 1980 the
atmospheric temperature of HR 8752 stayed almost constant around 5000
degrees, but it rose very rapidly to 8000 degrees between 1985 and
2005.  The team calculates that the stellar radius decreased from 750
to 400 times that of the Sun.  In 1985 the team embarked on a
long-term spectroscopic observing programme when it found that the
remarkable hypergiant was exactly at the border of the 'Yellow Void'
and starting to cross over.  HR 8752 had to struggle through the Void
which has changed the physical properties of its atmosphere.  The team
further demonstrates that the Void actually consists of two parts in
which the atmosphere of the hypergiant is unstable.  The fate of
HR 8752 is not clear, but there are hints that massive hypergiants
may perish in powerful supernova explosions, or else they quickly
traverse the Void and transform into a hotter type of erratic stars
known as the 'Luminous Blue Variables'.


RARE NEW TYPE OF GALAXY
ESO

A new class of galaxies has been identified in observations from the
VLT, Gemini South telescope and the CFHT.  The galaxies have a green
glow that arises from emission by oxygen that has been ionised by
radiation emitted from the surroundings of massive black holes.  Many
galaxies have at their centres massive black holes that cause the gas
around them to glow.  However, in the newly observed type the entire
galaxy is glowing, not just the centre; the glow is thought to be
powered by central black holes that were formerly very active but are
now switching off.

Astronomers at the Gemini Observatory came across the first such
object, which looked like a galaxy but was bright green.  The object
has been labelled J224024.1-092748, or J2240 for short.  It is in
Aquarius and its light has taken about 3.7 billion years to reach us.
After the discovery, the team searched for other such objects and
found 16 more with similar properties.  The new class of galaxies has
been nicknamed 'green bean galaxies' because of their colour.  In many
galaxies the material around the supermassive black hole at the centre
gives off intense radiation and ionises the surrounding gas, making it
glow strongly.  The glowing region in a typical active galaxy is
usually up to 10% of the diameter of the galaxy, but in the case of
J2240, and the others discovered since, it spans the entire object.
J2240, however, appeared to have a much less active black hole at its
centre than would be expected from the size and brightness of the
glowing region.  The team thinks that the hole must have been much
more active in the past, and that the glow will gradually dim as the
already-emitted ionising radiation passes through it and out of
the galaxy.


SEVEN PRIMITIVE GALAXIES AT THE DAWN OF TIME
STSI

Astronomers using the Hubble telescope have observed seven primitive
galaxies that formed more than 13 billion years ago, when the Universe
was less than 3% of its present age.  The deepest images from Hubble
to date yield the first statistically robust sample of galaxies that
tells how abundant they were close to the era when they first formed.
The results show a smooth decline in the number of galaxies with
increasing look-back time to about 450 million years after the big
bang.  The observations support the idea that galaxies assembled
continuously over time, and may also have provided enough radiation to
re-ionise the Universe a few hundred million years after the big bang.
One of the galaxies may be the furthest yet identified, observed as it
was 380 million years after the birth of the Universe, corresponding
to a redshift of 11.9.

The results are from a Hubble survey of an intensively studied patch
of sky known as the Ultra Deep Field (UDF).  In a new campaign called
UDF 2012, aimed at observing extremely red-shifted galaxies,
astronomers made specially long exposures with Hubble's camera in
near-infrared light.  A major goal of the new programme was to
determine how rapidly the number of galaxies increased over time in
the early Universe.  This measure provides evidence for how quickly
galaxies build up their constituent stars.  The team estimated the
galaxies' distances from their colours determined through five filters
at different near-infrared wavelengths.  For galaxies whose light has
been shifted a long way into infrared wavelengths, the intervening
hydrogen will have absorbed all the light that we would now see at
visible wavelengths and most of that at near-infrared wavelengths.
Such galaxies will therefore not be seen except in the longer-
wavelength infrared filters, which hold the key to discovering the
earliest galaxies.

Astronomers have long debated whether hot stars in such early galaxies
could have provided enough radiation to warm the cold hydrogen that
formed soon after the big bang.  That process, called re-ionisation,
is thought to have occurred, like the formation of the galaxies, in a
gradual process between 200 million and a billion years after the
Universe's birth.  The process made the Universe transparent to light,
allowing astronomers to look far back into time.  The galaxies in the
new study are seen in that early epoch.


NEW MARS ROVER TO LAUNCH IN 2020
NASA

Building on the success of Curiosity's landing on Mars, NASA has
announced plans for further efforts, including a new rover to be
launched in 2020, a mission that represents another step towards
sending astronauts there.  The programme also includes the 2013 launch
of an orbiter to study the Martian upper atmosphere, and the 'InSight'
mission, which is to try to understand the deep interior of Mars, as
well as participation in planned ESA missions.