Topic: Early July Astronomy Bulletin

[Clive]

'LAKES' ON SATURN'S MOON TITAN
NASA

Saturn's moon Titan is home to seas and lakes filled with liquid
hydrocarbons, but what forms the depressions on the surface?  A new
study of data from the Cassini mission suggests that the moon's
surface dissolves in a process similar to that responsible for the
creation of sinkholes on the Earth.  Apart from the Earth, Titan is
the only body in the Solar System known to possess surface lakes and
seas, which have been observed by the Cassini spacecraft.  But at
Titan's frigid surface temperatures -- roughly minus 180C -- liquid
methane and ethane dominate Titan's hydrocarbon equivalent of the
Earth's water.  Cassini has identified two forms of methane- and
ethane-filled depressions that create distinctive features near
Titan's poles.  There are vast seas several hundred kilometres across
and up to several hundred metres deep, fed by branching, river-like
channels.  There also are numerous smaller, shallower lakes, with
rounded edges and steep walls that are generally found in flat areas.
Cassini also has observed many empty depressions.  The lakes are
generally not associated with rivers, and are thought to fill up by
rainfall and liquids feeding them from underground.  Some of the lakes
fill and dry out again during the 30-year seasonal cycle on Saturn and
Titan.  But how the depressions holding the lakes came to be there in
the first place is not understood.

Recently, scientists noticed that Titan's lakes are reminiscent of
what are known as karst landforms on the Earth, which result from the
erosion of dissolvable rocks, such as limestone and gypsum, by
groundwater and percolating rainfall.  Over time, that leads to
features like sinkholes and caves in humid climates, and salt pans
where the climate is more arid.  The rate of erosion creating such
features depends on factors such as the chemistry of the rocks, the
rainfall rate and the surface temperature.  While all of those aspects
clearly differ between Titan and the Earth, the researchers think that
the underlying processes may be similar.  They calculated how long it
would take for patches of Titan's surface to dissolve to create the
features.  They assumed that the surface is covered in solid organic
material, and that the main dissolving agent is liquid hydrocarbons,
and took into account present-day models of Titan's climate.  They
found that it would take around 50 million years to create a 100-metre
depression at Titan's relatively rainy polar regions, consistent with
the youthful age of the moon's surface.  They compared the erosion
rates of organics in liquid hydrocarbons on Titan with those of
carbonate and evaporite minerals in liquid water on the Earth and
found that the solution process occurs on Titan some 30 times more
slowly than on the Earth owing to the longer length of Titan's year
and the fact that it rains only during Titan's summer.  Nonetheless,
they believe that solution is a major cause of landscape evolution
on Titan and could be the origin of its lakes.  In addition, the
scientists calculated how long it would take to form lake depressions
at lower latitudes, where the rainfall is less.  The much longer
time-scale of 375 million years is consistent with the relative
absence of depressions at those latitudes.  Of course, there are
uncertainties: the composition of Titan's surface is not very well
constrained, and neither are the long-term precipitation patterns, but
the calculations are still consistent with the features seen today on
Titan's relatively youthful billion-year-old surface.  By comparing
Titan's surface features with examples on the Earth and doing a few
simple calculations, scientists have found that similar land-shaping
processes could be operating under very different climatic and
chemical regimes.


IMMENSE HYDROGEN CLOUD DISPERSING FROM EXOPLANET
Space Telescope Science Institute (STScI)

Astronomers using the Hubble telescope have discovered an immense
cloud of hydrogen dubbed 'The Behemoth' bleeding off a planet orbiting
a nearby star.  The enormous, comet-like feature is about 50 times the
size of the parent star.  The hydrogen is escaping from a warm,
Neptune-sized planet, because of the extreme radiation from the star.
Such a large phenomenon has not been seen previously around any
exo-planet.  Given that planet's small size, it may offer clues to how
hot super-Earths -- massive, rocky, hot versions of the Earth -- are
born around other stars through the evaporation of their outer layers
of hydrogen.  The cloud is very spectacular.  Although the escape rate
does not threaten the planet right now, we know that in the past, the
star, which is a faint red dwarf, was more active.  That means that
the planet evaporated faster during its first billion years of
existence.  Overall, astronomers estimate that it may have lost up to
10 per cent of its atmosphere.  The planet, named GJ 436b, is
considered to be a 'warm Neptune', because of its size and because it
is much closer to its star than Neptune is to the Sun.  Although it is
in no danger of having its atmosphere completely evaporated and being
stripped down to a rocky core, the planet could explain the existence
of so-called hot super-Earths that are very close to their stars.
Those hot, rocky worlds were discovered by the Convection Rotation and
Planetary Transits (CoRoT) spacecraft.  Hot super-Earths could be the
remnants of more massive planets that completely lost their thick,
gaseous atmospheres by the same type of evaporation.

Because the Earth's atmosphere blocks most ultraviolet light,
astronomers needed a space telescope with ultraviolet capability to
find the 'Behemoth'.  Because the planet's orbit is tilted nearly
edge-on to our view, the planet can be seen passing in front of its
star.  Astronomers also saw the star eclipsed by the 'Behemoth'
hydrogen cloud around the planet.  Researchers think that such a huge
cloud of gas can exist around the planet because the cloud is not
rapidly heated and swept away by the radiation pressure from the
relatively cool red dwarf star.  Evaporation may have happened in the
earlier stages of our own Solar System, when the Earth had a hydrogen-
rich atmosphere that dissipated over 100 million to 500 million years.
If so, the Earth may then have sported a comet-like tail.  It is also
possible that it could happen to our atmosphere at the end of the
Earth's 'life', when the Sun swells up to become a red giant and boils
off our remaining atmosphere, before engulfing our planet completely.
GJ 436b resides very close to its star -- less than 3 million miles --
and whips around it in just 2.6 Earth days.  The exo-planet is at
least 6 billion years old, and may even be twice that age.  It has a
mass of around 23 Earths.  At just 30 light-years away, it is one of
the closest known extra-solar planets.  The ultraviolet technique used
in the study might also be able to detect the signature of oceans
evaporating on smaller, more Earth-like planets.  It will be extremely
challenging for astronomers to see water vapour directly on such
planets, because it is too low in the atmosphere and shielded from
telescopes.  However, when water molecules are broken by the stellar
radiation into hydrogen and oxygen, the relatively light hydrogen
atoms can escape the planet.  If scientists could spot such hydrogen
evaporating from a planet that is a bit more temperate and little less
massive than GJ 436b, that could be construed as a sign of an ocean on
the surface.


PUZZLE OF UNUSUALLY HOT BLUE STARS
Australian National University

Astronomers have at last understood small, unusually hot blue stars,
10 times hotter than the Sun, that are found in the middles of dense
star clusters.  The international team found that the so-called 'blue
hook' stars throw off their cool outer layers late in their 'lives'
because they are rotating so rapidly, making them more luminous than
usual.  The stars are only half the mass of the Sun, and until now we
could not explain how they became so luminous.  As the star was
forming billions of years ago from a disc of gas in the congested
centre of the star cluster, another star or stars must have collided
with the disc and destroyed it.  The research gives new insights into
star formation in the early Universe in the crowded centres of
clusters.  Star clusters are rare environments in the Universe, in
which many stars are born at the same time.  The team studied the
globular cluster Omega Centauri, the only such cluster easily visible
to the naked eye, which contains around 10 million stars in relatively
close proximity to one another.  The model shows that the stars in
clusters do not all form at once.  The blue stars must form in a
second generation of star formation.  Usually the large disc of
ionised gas around a newly-forming star locks its rotation through
magnetic effects.  For the progenitors of blue hook stars, however,
the early destruction of their discs allows the stars to spin up as
the gas comes together to form a star.  Because its high rotation rate
partially balances the inward force of gravity, the star consumes its
hydrogen fuel more slowly and evolves differently throughout its life.
The blue hook phase occurs after more than 10 billion years, when the
star has consumed nearly all its hydrogen and begins burning the
hotter fuel helium.  The different evolution processes leave it with a
heavier core which burns brighter than typical helium-burning stars.


GIANT GALAXY IS STILL GROWING
ESO

New observations with the Very Large Telescope have revealed that the
giant elliptical galaxy Messier 87 has swallowed an entire medium-
sized galaxy over the last billion years.  For the first time a team
of astronomers has been able to track the motions of 300 planetary
nebulae to find clear evidence of that event and also found evidence
of excess light coming from the remains of the totally disrupted
victim.  Astronomers expect that galaxies grow by swallowing smaller
galaxies.  But the evidence is usually not easy to see -- just as the
remains of the water thrown from a glass into a pond will quickly
merge with the pond water, so the stars in the infalling galaxy merge
in with the very similar stars of the bigger galaxy, leaving no trace.
But now a team of astronomers has applied a clever observational trick
that shows that M 87 merged with a smaller spiral galaxy in the last
billion years.  That result shows directly that large, luminous
structures in the Universe are still growing in a substantial way.
A large sector of M 87's outer halo now appears twice as bright as it
would if the collision had not taken place.  M 87 lies at the centre
of the Virgo Cluster of galaxies.  It is a vast ball of stars with a
total mass more than a million million times that of the Sun, lying
about 50 million light-years away.

Rather than try to look at all the stars in M 87 -- there are far too
many and they are too faint to be studied individually -- the team
looked at planetary nebulae, the glowing shells around ageing stars.
Because they are 'emission-line objects' that shine very brightly at a
particular wavelength in the green, they can be distinguished from the
surrounding stars.  Observation of the light from the nebulae with a
powerful spectrograph can also reveal their motions.  We are
witnessing a single recent accretion event where a medium-sized galaxy
fell through the centre of M 87, and as a consequence of the enormous
gravitational tidal forces, its stars are now scattered over a region
that is 100 times larger than the original galaxy.  The team also
looked very carefully at the light distribution in the outer parts of
M 87, and found evidence of extra light coming from the stars in the
galaxy that had been pulled in and disrupted. The observations have
also shown that the disrupted galaxy has added younger, bluer stars to
M 87, and so it was probably a star-forming spiral galaxy before its
merger.


854 ULTRA-DARK GALAXIES DISCOVERED IN COMA CLUSTER
Stony Brook University

Astronomers have discovered 854 'ultra-dark galaxies' in the Coma
Cluster by analyzing data from the 8.2-m Subaru Telescope.  The new
discovery far surpasses the 2014 discovery of 47 dark galaxies, and
suggests that galaxy clusters are the key environment for the
evolution of 'dark' galaxies.  The findings suggest that such galaxies
appear very diffuse and are very probably enveloped by something very
massive.  The ultra-dark galaxies are similar in size to the Milky
Way, but have only a thousandth of the number of stars.  The stellar
population within such fluffy extended galaxies is subject to rapid
disruption by a strong tidal force detected within the cluster.
Researchers believe that something invisible must be protecting the
fragile star systems of such galaxies, something with a high mass.
That 'something' is very likely an excessive amount of dark matter.
The component of visible matter, such as stars, is calculated to
contribute only one per cent or less to the total mass of each galaxy.
The rest -- dark matter -- accounts for more than 99 per cent.  The
Subaru telescope revealed that the dark galaxies contain old stellar
populations and show a spatial distribution similar to those of other
brighter galaxies in the Coma Cluster.  It suggests that there has
been a long-lived population of galaxies within the cluster and the
amount of visible matter that they contain is extremely low compared
to the average fraction within the Universe.

The galaxies are dark because they have lost gas needed to create new
stars during, or after, their largely unknown formation process
billions of years ago.  From their preferential presence within the
cluster, it seems likely that the cluster environment played a role in
the loss of gas, which affects star formation within the galaxy.
Several loss mechanisms are possible, including ram-pressure stripping
by intra-cluster gas, gravitational interactions with other galaxies
within the cluster, and gas outflows due to simultaneous supernova
explosions triggered by the ram pressure or gravitational encounters.
Dark matter is one of the unresolved problems in cosmology.  Studies
of such interplay between dark matter and stars and gas in galaxies
are increasingly attracting attention from researchers.


BRIGHTEST GALAXY YET FOUND IN EARLY UNIVERSE
ESO

Astronomers using ESO's Very Large Telescope have discovered by far
the brightest galaxy yet found in the early Universe, and found strong
evidence that examples of the first generation of stars remain within
it.  Those massive, brilliant, and previously purely theoretical
objects were the creators of the first heavy elements in history --
the elements necessary to forge the stars around us today, the planets
that orbit them, and life as we know it.  The newly found galaxy,
labelled CR7, is three times brighter than the brightest distant
galaxy known up to now.  Astronomers have long theorised the existence
of a first generation of stars, known as Population III stars, that
were born out of the primordial material from the Big Bang.  All the
heavier chemical elements, such as oxygen, nitrogen, carbon and iron,
which are essential to life, were forged in stars.  The first stars
must have formed out of the only elements to exist then: hydrogen,
helium and trace amounts of lithium.  Those 'Population III' stars
would have been enormous -- several hundred or even a thousand times
more massive than the Sun -- blazing hot, and transient -- exploding
as supernovae after only about two million years.  But until now the
search for physical proof of their existence has been inconclusive.

The X-shooter and SINFONI instruments on the VLT found strong ionised
helium emission in CR7 but -- crucially and surprisingly -- no sign of
any heavier elements in a bright pocket in the galaxy.  That meant
that the team had discovered the first good evidence for clusters of
Population III stars that had ionised gas within a galaxy in the early
Universe.  Within CR7, bluer and somewhat redder clusters of stars
were found, indicating that the formation of Population III stars had
occurred in waves, as had been predicted.  What the team directly
observed was the last wave of Population III stars, suggesting that
such stars should be easier to find than previously thought: they
reside among regular stars, in brighter galaxies, not just in the
earliest, smallest, and dimmest galaxies, which are so faint as to be
extremely difficult to study.  Further observations are planned to
confirm beyond doubt that what have been observed are Population III
stars, and to search for and identify further examples.