Topic: Late August Astronomy Bulletin

[Clive]

PLANET FOUND ORBITING PROXIMA CENTAURI
ESO

Astronomers have found clear evidence of a planet orbiting the closest
star to the Sun, Proxima Centauri.  The planet, designated Proxima b,
orbits its cool red parent star every 11 days and has a temperature
such that liquid water could exist on its surface.  The rocky object
is a little more massive than the Earth and is the closest exo-planet
to us, and it may also be the closest possible abode for life outside
the Solar System.  Proxima Centauri is too faint to be seen with the
unaided eye and lies near (and is gravitationally bound) to the much
brighter double star Alpha Centauri.  During the first half of 2016
Proxima Centauri was regularly observed with the HARPS spectrograph on
the ESO 3.6-metre telescope at La Silla in Chile and simultaneously
monitored by other southern telescopes.  That was the 'Pale Red Dot'
campaign, in which a team of astronomers was looking for the tiny
wobble of the star that would be caused by the gravitational pull of a
possible orbiting planet.  The Pale Red Dot data, when combined with
earlier observations made at ESO observatories and elsewhere, revealed
a clear signal of velocity variations of about +/- 5 kilometres per
hour --- about human walking pace.  The regular pattern of changing
radial velocities repeats with a period of 11.2 days.  The shifts
indicate the presence of a planet with a mass at least 1.3 times that
of the Earth, orbiting only about 7 million kilometres from Proxima.
Red dwarfs like Proxima Centauri are active stars and can vary in ways
that could mimic the presence of a planet.  To exclude that
possibility the team also monitored the changing brightness of the
star very carefully during the campaign.  Radial-velocity data taken
when the star was flaring were excluded from the final analysis.

Although Proxima b orbits much closer to its star than Mercury does to
the Sun, the star itself is far fainter than the Sun.  As a result,
Proxima b lies well within the 'habitable zone' around the star and
has an estimated surface temperature that would allow the presence of
liquid water.  Despite the temperate orbit of Proxima b, the
conditions on the surface may be strongly affected by the ultraviolet
and X-ray flares from the star --- far more intense than the Earth
experiences from the Sun.  Two separate papers discuss the habit-
ability of Proxima b, and its climate.  They find that the existence
of liquid water on the planet today cannot be ruled out and, in such
case, it may be present over the surface of the planet only in the
sunniest regions, either in an area in the hemisphere of the planet
facing the star or in a tropical belt.  Proxima b's rotation, the
strong radiation from its star and the formation history of the planet
makes its climate quite different from that of the Earth, and it is
unlikely that Proxima b has seasons.  This discovery will be the
beginning of extensive further observations, both with current
instruments and with the next generation of giant telescopes such as
the European Extremely Large Telescope.  Proxima b will be a prime
target for the hunt for evidence of life elsewhere in the Universe.
Indeed, the Alpha Centauri system is also the target of mankind's
first attempt to travel to another star system, the StarShot project.


BROWN DWARFS REVEAL EXOPLANET SECRETS
Carnegie Institution for Science

Brown dwarfs are smaller than stars, but more massive than giant
planets, so they provide a natural link between stellar and planetary
science.  However, they also show incredible variation when it comes
to size, temperature, and chemistry, which makes them difficult to
understand, too.  Astronomers have surveyed various properties of 152
suspected young brown dwarfs in order to categorize their diversity,
and found that atmospheric properties may be behind many of their
differences, a discovery that may apply to planets outside the Solar
System as well.  Scientists are very interested in brown dwarfs, which
hold promise for explaining not just planetary evolution, but also
stellar formation.  Such objects are more difficult to observe than
more-massive and brighter stars, but they vastly outnumber stars like
the Sun. They represent the smallest and lightest objects that can
form like stars do in the Galaxy, so they are an important 'book-end'
in astronomy.  For the moment, data on brown dwarfs can be used as
stand-ins for contemplating extra-solar worlds that we hope to study
with future instruments like the Webb Space Telescope.  Brown dwarfs
are far easier to study than planets, because they are not overwhelmed
by the brightness of a host star.  But the tremendous diversity we see
in the properties of the brown-dwarf population means that there is
still much about them that remains unknown or poorly understood.

Brown dwarfs are too small to sustain the hydrogen-fusion process that
fuels stars, so after formation they slowly cool and contract over
time and their surface gravity increases.  That means that their
temperatures can range from nearly as hot as a star to as cool as a
planet, which is thought to influence their atmospheric conditions,
too.  Their masses also range between those of stars and of giant
planets, and they demonstrate great diversity in age and chemical
composition.  By quantifying the observable properties of so many
young brown-dwarf candidates, researchers were able to show that those
objects have vast ranges of colour and spectral features.  Identifying
the cause of those ranges was at the heart of the team's work.  By
locating the birth-places of many of the brown dwarfs, it was able to
eliminate age and chemical composition differences as the underlying
reasons for the great variation.  That left atmospheric conditions --
meaning weather phenomena or differences in cloud composition and
structure -- as the primary suspect for what drives the extreme
differences between objects of similar origin.  All of the brown-dwarf
birth-places identified in this work are regions that also host exo-
planets, so these same findings hold for giant planets orbiting nearby
stars.  The team considers the young brown dwarfs to be siblings of
giant exo-planets.  As close family members, they can probably be used
to investigate how the planetary ageing process works.


YOUNG MASSIVE STAR IDENTIFIED IN MILKY WAY
RAS
 
Astronomers have identified a young star, located almost 11,000 light-
years away, which could help us understand how the most massive stars
in the Universe are formed.  The young star, already more than 30
times the mass of our Sun, is still in the process of gathering
material from its parent molecular cloud, and will be even more
massive when it finally reaches adulthood.  The researchers, led by a
team at the University of Cambridge, have identified a key stage in
the birth of a very massive star, and found that such stars form in a
similar way to much smaller stars like our Sun -- from a rotating disc
of gas and dust.  In our Galaxy, massive young stars -- those with
masses at least eight times that of the Sun -- are much more difficult
to study than smaller stars.  That is because they live fast and die
young, making them rare among the 100,000 million stars in the Milky
Way, and on average, they are much further away.  An average star like
our Sun is formed over a few million years, whereas massive stars are
formed orders of magnitude more quickly -- in around 100,000 years.
Massive stars also burn through their fuel much more quickly, so they
have shorter overall life-spans, making them harder to catch when they
are infants.  The proto-star now identified resides in an infrared
dark cloud -- a very cold and dense region which makes an ideal
stellar nursery.  However, that rich star-forming region is difficult
to observe with conventional telescopes, since the young stars are
surrounded by a thick, opaque cloud of gas and dust.  But by using the
Submillimeter Array (SMA) in Hawaii and the Very Large Array (VLA) in
New Mexico, both of which use relatively long wavelengths of light,
the researchers were able to 'see' through the cloud and into the
stellar nursery itself.
 
By measuring the amount of radiation emitted by cold dust near the
star, and from the spectra of various different molecules in the gas,
the researchers were able to determine the presence of a Keplerian
disc -- one which rotates more quickly at its centre than at its edge.
It is exciting to find such a disc around a massive young star,
because it suggests that massive stars form in a similar way to lower-
mass stars, like our Sun.  From their observations, the team found the
mass of the proto-star to be over 30 times that of the Sun.  In
addition, the disc surrounding the young star was also calculated to
be relatively massive, between two and three times the mass of the
Sun.  Calculations suggest that the disc could in fact be hiding even
more mass under layers of gas and dust.  The disc may even be so
massive that it can break up under its own gravity, forming a series
of less-massive companion proto-stars.  The next step for the
researchers will be to observe the region with the Atacama Large
Millimetre Array (ALMA), located in Chile.  That powerful instrument
may allow any potential companions to be seen, and shed further light
on this intriguing young heavyweight in our Galaxy.
 

GALAXY PAIR WANDERS IN FROM 'LOCAL VOID'
NASA/Goddard Space Flight Center

The Hubble Space Telescope has found two tiny dwarf galaxies that have
wandered from a vast cosmic wilderness into a nearby galaxy cluster.
After being quiescent for billions of years, they are ready to begin a
firestorm of star birth.  Studying those and other similar galaxies can
provide further clues to dwarf-galaxy formation and evolution.  The Hubble
observations suggest that the galaxies, called Pisces A and B, are late
bloomers because they have spent most of their existence in the Local
Void, a region of the Universe sparsely populated with galaxies.  The
Local Void is roughly 150 million light-years across.  Under the steady pull
of gravity from the galactic cluster, the dwarf galaxies have at last entered
a crowded region that is denser in intergalactic gas.  In that gas-rich
environment, star birth may have been triggered by gas raining down on
the galaxies as they plough through the denser region.  Another idea
is that the duo may have encountered a gaseous filament, which
compresses gas in the galaxies and stokes star birth.  The objects are
at the edge of a nearby filament of dense gas.  Each galaxy contains
only about 10 million stars.  Dwarf galaxies are the building blocks
from which larger galaxies were formed in the early Universe.
Inhabiting a sparse desert of largely empty space for most of the
Universe's history, the two galaxies avoided that busy construction
period.  They may have spent most of their history in the void.  If
that is so, the void environment would have slowed their evolution.
Evidence for the galaxies' void address is that their hydrogen content
is somewhat high relative to that of similar galaxies.  In the past,
galaxies contained higher concentrations of hydrogen.  But the two
galaxies of interest here seem to retain that more primitive
composition, rather than the enriched composition of contemporary
galaxies, owing to a less vigorous history of star formation.  The
galaxies also are quite compact relative to the typical star-forming
galaxies in our own neighborhood.

The dwarf galaxies are small and faint, so finding them is extremely
difficult.  Astronomers found them by using radio telescopes in a
unique survey to measure the hydrogen content in the Milky Way.  The
observations found thousands of small blobs of dense hydrogen gas.
Most of them are gas clouds within our own Galaxy, but astronomers
identified 30 to 50 of the blobs as possible external galaxies. The
researchers used the WIYN telescope in Arizona to study 15 of the most
promising candidates in visible light.  On the basis of those
observations, the research team selected the two that are the most
likely candidates to be nearby galaxies and analyzed them with
Hubble's 'Advanced Camera for Surveys'. Hubble confirmed that both of
them, Pisces A and B, are dwarf galaxies.  The Hubble telescope is
aptly suited to study nearby, dim dwarf galaxies because it can
resolve individual stars and help astronomers estimate the galaxies'
distances.  Distance is important for determining a galaxy's
brightness, and, in these Hubble observations, for calculating how far
away the galaxies are from nearby voids.  Pisces A is about 19 million
light-years from the Earth and Pisces B roughly 30 million light-years
away.  On the basis of the galaxies' locations, an analysis of the
stars' colours allowed the astronomers to trace the star-formation
history of both galaxies.  Each galaxy contains about 20 to 30 bright
blue stars, a sign that they are very young, less than 100 million
years old.  The team estimates that less than 100 million years ago,
the galaxies doubled their star-formation rate.  Eventually, the star
formation may slow down again if the galaxies become satellites of a
much larger galaxy.


FIRST U.S. ASTEROID SAMPLE-RETURN MISSION
NASA

NASA is preparing to launch its first mission to return a sample of an
asteroid to Earth.  The mission will help scientists investigate how
planets formed and how life began, as well as improve our under-
standing of asteroids that could collide with the Earth.  The Origins,
Spectral Interpretation, Resource Identification, Security-Regolith
Explorer (OSIRIS-REx) spacecraft will travel to the near-Earth
asteroid Bennu and bring a sample back for study.  The craft is
scheduled for launch on September 8 from Cape Canaveral in Florida and
should reach its asteroid target in 2018.  After a careful survey of
Bennu to characterize the asteroid and to locate the most promising
sample sites, OSIRIS-REx will collect between 60 and 2000 grams of
surface material with its robotic arm and return the sample to Earth
via a detachable capsule in 2023.