ASTRONOMERS FIND SUN'S SIBLING
University of Texas at Austin
A team of researchers has identified the first 'sibling' of the Sun
-- a star that was almost certainly born from the same cloud of gas
and dust as our star. It is called HD 162826, a star 15% more massive
than the Sun, located 110 light-years away in the constellation
Hercules. The star is not visible to the unaided eye, but can easily
be seen with low-power binoculars, not far from Vega. The team
identified HD 162826 as the Sun's sibling from among 30 candidates
found by several groups around the world looking for solar siblings.
It studied 23 of those stars by high-resolution spectroscopy with the
107-inch telescope at McDonald Observatory, and the remaining stars
(visible only from the southern hemisphere) with the Clay Magellan
Telescope at Las Campanas Observatory in Chile. In addition to
chemical analysis, the team also included information about the stars'
orbits -- their paths around the centre of the Milky Way galaxy.
Combining information on both chemical make-up and dynamics of the
candidates narrowed the field down to one: HD 162826. By coincidence,
that star has been studied by the McDonald Observatory Planet Search
team for more than 15 years. The studies have ruled out any 'hot
Jupiters' -- massive planets orbiting close to the star -- and
indicate that it is unlikely that a Jupiter analogue orbits the star,
either, but they do not rule out the presence of smaller terrestrial
planets.
While the finding of a single solar sibling is intriguing, the project
is also a preparatory exercise in how to identify solar siblings, in
preparation for the flood of data expected soon from surveys like
Gaia. The idea is that the Sun was born in a cluster with a thousand
or a hundred thousand stars, which formed more than 4500 million years
ago and has since broken up. The member stars have dispersed into
their own orbits around the Galactic Centre, taking them to different
parts of the Milky Way today. A few, like HD 162826, are still
nearby. The data coming soon from Gaia are not going to be limited to
the solar neighbourhood since Gaia will provide accurate distances and
proper motions for a (US) billion (10*9) stars, allowing astronomers
to search for solar siblings all the way to the centre of our Galaxy.
The number of stars that we can study will increase by a factor of
10,000. Astronomers can concentrate on certain key chemical elements,
ones whose abundances vary greatly among stars which otherwise have
very similar chemical compositions and depend on where in the Galaxy
the star formed. The team has identified the elements barium and
yttrium as particularly useful. Once many more solar siblings have
been identified, astronomers will be one step closer to knowing where
and how the Sun formed. To reach that goal, the dynamics specialists
will try to make models that run the orbits of solar siblings backward
in time, to find where they intersect: their birthplace.
JUPITER'S GREAT RED SPOT SHRINKS
Space Telescope Science Institute (STScI)
Jupiter's Great Red Spot -- a swirling anticyclonic storm feature
larger than the Earth -- has shrunk to the smallest size ever
measured. Astronomers have followed its shrinkage since the 1930s.
Historic observations as far back as the late 1800s gauged the GRS to
be as much as 25,500 miles on its long axis. The Voyager 1 and
Voyager 2 flybys of Jupiter in 1979 measured it at 14,500 miles
across. Starting in 2012, amateur observations revealed a noticeable
acceleration in the spot's shrinkage rate. The GRS's 'waistline' is
getting smaller by 580 miles per year and is now 10,250 miles. The
shape of the GRS has changed from an oval to a circle. The cause has
yet to be explained. In new observations it is apparent that very
small eddies are feeding into the storm, and astronomers hypothesize
that they may be responsible for the sudden change by altering the
internal dynamics and energy of the Great Red Spot. Researchers plan
to study the motions of the small eddies and also the internal
dynamics of the GRS to determine if the eddies can feed or sap
momentum entering the upwelling vortex.
LENGTH OF EXOPLANET DAY MEASURED
ESO
Observations from the Very Large Telescope (VLT) have, for the first
time, determined the rotation rate of an exoplanet, from the
rotational broadening of its spectral lines. The planet orbits the
naked-eye star Beta Pictoris, which lies about 63 light-years from the
Earth in the southern constellation Pictor. It was discovered nearly
six years ago and was one of the first exoplanets to be directly
imaged. It orbits its host star at a distance of eight times the
Earth-Sun distance, making it the closest exoplanet to its star to be
directly imaged. Its equator is moving at almost 100 000 km/h. For
comparison, Jupiter's equator has a speed of about 47 000 km/h, while
the Earth's is only 1700 km/h. Beta Pictoris b is more than 16 times
larger and 3000 times more massive than the Earth, yet a day on the
planet lasts only 8 hours.
It is not known why planets spin at different rates, but this first
measurement of an exoplanet's rotation is consonant with the trend
seen in the Solar System, where the more massive planets spin faster.
Beta Pictoris b is a very young planet, only about 20 million years
old (compared to 4.5 billion years for the Earth). Over time, the
exoplanet is expected to cool and shrink, which will make it spin even
faster. On the other hand, other processes might be at play that
change its spin. For instance, the spin of the Earth is slowing down
owing to tidal interaction with the Moon.
NEAREST 'HYPER-VELOCITY STAR' FOUND
University of Utah
Astronomers have discovered a 'hyper-velocity star' that is the
closest, second-brightest and among the largest of 20 found so far.
Hypervelocity stars appear to be former components of binary stars
that once orbited each other but got too close to the supermassive
black hole at the Galaxy's centre. The gravity of the black hole --
which has the mass of 4 million Suns -- captures one star so it orbits
the hole closely, and ejects the other on a trajectory headed beyond
the Galaxy. The new hypervelocity star was discovered with the 'Large
Sky Area Multi-Object Fibre Spectroscopic Telescope, or LAMOST,
located at the Xinglong Observing Station of the National Astronomical
Observatories of China, about 110 miles northeast of Beijing. LAMOST
has a 7-m aperture and has 4,000 optical fibres, which capture
spectra of as many as 4,000 stars at once. The star -- named
LAMOST-HVS1 -- stood out because its speed is about 600 km/s relative
to the Solar System (500 km/s with respect to the centre of the Milky
Way). Despite being the closest hypervelocity star, it is nonetheless
13 kpc (42,000 light years) from the Earth. It has a magnitude of
about 13, and is nine times the mass of the Sun, rather less than
another hypervelocity star, HD 271791, which was discovered in 2008
and is 11 times the mass of the Sun. As seen from the Earth, among
the hypervelocity stars only HD 271791 is brighter than LAMOST-HVS1.
A cluster of known hypervelocity stars, including the new one, is
located above the disc of our Milky Way galaxy, and their distribution
in the sky suggests that they originated near the Galaxy's centre.
MAGNETAR FORMATION SOLVED?
ESO
Magnetars are the bizarre super-dense remnants of supernova
explosions. They are the strongest magnets known in the Universe --
millions of times more powerful than the strongest magnets on Earth.
A team of astronomers using the Very Large Telescope (VLT) now
believes it has found the partner star of a magnetar for the first
time. The discovery helps to explain how magnetars form -- a
conundrum dating back 35 years -- and why that particular star did not
collapse into a black hole as astronomers might expect. When a
massive star collapses under its own gravity during a supernova
explosion it forms either a neutron star or black hole. Magnetars are
an unusual form of neutron star. They are tiny and extraordinarily
dense -- a teaspoonful of neutron-star material would have a mass of
about a billion tons -- but they also have extremely powerful
magnetic fields. Magnetar surfaces release vast quantities of gamma
rays when they undergo a sudden adjustment known as a starquake as a
result of the huge stresses in their crusts.
The Westerlund 1 star cluster, 16 000 light-years away in the southern
constellation of Ara, hosts one of the two dozen magnetars known in
the Milky Way. It is called CXOU J164710.2-455216 and it has greatly
puzzled astronomers. Earlier work showed that it must have been born
in the explosive death of a star about 40 times as massive as the Sun.
But that presents its own problem, since stars so massive are expected
to collapse to form black holes after their deaths, not neutron stars.
Researchers did not understand how it could have become a magnetar.
A possible solution was that the magnetar formed through the
interactions of two very massive stars orbiting one another in a
binary system so compact that it would fit within the orbit of the
Earth around the Sun. But, up to now, no companion star was detected
at the location of the magnetar in Westerlund 1, so astronomers used
the VLT to search for it in other parts of the cluster. They hunted
for runaway stars -- objects escaping the cluster at high velocities
-- that might have been ejected by the supernova explosion that formed
the magnetar. One star, known as Westerlund 1-5, was found to be
doing just that. Not only does it have the high velocity expected if
it is recoiling from a supernova explosion, but the combination of its
low mass, high luminosity and carbon-rich composition appear
impossible to replicate in a single star -- a 'smoking gun' that
suggests that it must have originally formed with a binary companion.
That discovery allowed the astronomers to reconstruct the stellar life
story that permitted the magnetar to form, in place of the expected
black hole. In the first stage of that process, the more massive star
of the pair begins to run out of fuel, transferring its outer layers
to its less massive companion (which is destined to become the
magnetar), causing it to rotate more and more quickly. Rapid rotation
appears to be the essential ingredient in the formation of the
magnetar's ultra-strong magnetic field. In the second stage, as a
result of the mass transfer, the companion itself becomes so massive
that it in turn sheds a large amount of its recently-gained mass. Much
of that mass is lost but some is passed back to the original star that
we still see shining today as Westerlund 1-5. It is that process of
swapping material that has imparted the unique chemical signature to
Westerlund 1-5 and allowed the mass of its companion to fall to low
enough levels that a magnetar was born instead of a black hole. It
seems that being a component of a double star may therefore be an
essential ingredient in the recipe for forming a magnetar. The rapid
rotation created by mass transfer between the two stars appears
necessary to generate the ultra-strong magnetic field and then a
second mass-transfer phase allows the magnetar-to-be to slim down
sufficiently so that it does not collapse into a black hole at the
moment of its death.
ENTIRE STAR CLUSTER THROWN OUT OF ITS GALAXY
Harvard-Smithsonian Center for Astrophysics
The galaxy M87 has thrown an entire star cluster towards us at more
than 1000 km/s. Astronomers have found runaway stars before, but this
is the first time they have found a runaway star cluster. The newly
discovered cluster has been named HVGC-1; the acronym stands for
'hypervelocity globular cluster'. Globular clusters are relics of the
early Universe. Such groupings usually contain thousands of stars
crammed into a ball a few dozen light-years across. The Milky Way
galaxy has about 150 globular clusters, but the giant elliptical
galaxy M87 has thousands. It took a stroke of luck to find HVGC-1.
The discovery team has been studying the space around M87. It first
sorted objects by colour to separate stars and galaxies from globular
clusters. Then it used the Hectospec instrument on the MMT in Arizona
to examine hundreds of globular clusters in detail. A computer
automatically analyzed the data and calculated the speed of every
cluster. Any oddities were examined by hand and most of them turned
out to be glitches, but HVGC-1 was different -- its surprisingly high
velocity was real.
Astronomers are not sure how HVGC-1 was ejected at such a high speed
but say that one scenario depends on M87 having a pair of supermassive
black holes at its core. The star cluster passed too close to those
black holes. Many of its outer stars were plucked off, but the dense
core of the cluster remained intact and was flung away at tremendous
speed. HVGC-1 is moving so fast that it will escape from M87
altogether. In fact, it may have already left the galaxy and be
sailing out into intergalactic space.
NEARBY GALAXY IS A 'FOSSIL' FROM THE EARLY UNIVERSE
Carnegie Institution
A team of scientists has analyzed the chemical elements in the faint
galaxy called Segue 1, and determined that it is effectively a fossil
galaxy left over from the early Universe. Astronomers hoping to learn
about the first stages of galaxy formation after the Big Bang use the
chemical compositions of stars to help them unravel the histories of
the Milky Way and other nearby galaxies, and were able to categorize
Segue 1's uniquely ancient composition. Stars form from gas clouds,
and their composition mirrors the chemical composition of the gas from
which they were born. Only a few million years after stars begin
burning, the most-massive stars explode in titanic blasts called
supernovae. Those explosions seed the nearby gas with heavy elements
produced by the stars during their lifetimes. The very oldest stars
consist almost entirely of the two lightest elements, hydrogen and
helium, because they were born before ancient supernova explosions
built up significant amounts of heavier elements. In most galaxies,
the process is cyclical, with each generation of stars contributing
more heavy elements to the raw material from which the next set of
stars will be born. But not in Segue 1 -- in contrast to all other
galaxies, the new analysis shows that Segue 1's star formation ended
at what would ordinarily be an early stage of a galaxy's development.
Segue 1 may have failed to progress further because of its unusually
small size.
Research suggests that Segue 1 is the least-chemically-evolved galaxy
known. After the initial few supernova explosions, it appears that
only a single generation of new stars formed, and then for the last 13
billion years the galaxy has not been creating stars. Because it has
stayed in the same state for so long, Segue 1 offers unique
information about the conditions in the Universe shortly after the Big
Bang. Other galaxies have undergone multiple supernova explosions
since their formation. The first supernovae to blow up, from the most
massive stars, produce elements like magnesium, silicon, and calcium.
Later explosions of smaller stars primarily make iron. Segue 1's
uniquely low iron abundance relative to other elements shows that its
star formation must have stopped before any of the iron-forming
supernovae occurred. Its truncated evolution means that the products
of the first explosions in Segue 1 have been preserved. Intriguingly,
very heavy elements like barium and strontium are nearly absent from
Segue 1's stars. The heaviest elements in that galaxy are at the
lowest levels ever found, and that gives us clues about what those
first supernovae looked like. Studying individual stars in dwarf
galaxies can be difficult, and Segue 1, which orbits our own Milky
Way, is particularly small, containing only about a thousand stars.
Just seven stars in the entire galaxy are in the red-giant phase of
their evolution, making them bright enough for modern telescopes to
detect the features astronomers use to measure the abundance of each
chemical element. Three of the seven red giants have heavy-element
abundances more than 3,000 times lower than that of the Sun,
highlighting the primitive nature of the galaxy. The team used one of
the 6.5-m Magellan telescopes in Chile to observe five of the Segue 1
stars, while one was studied with the 10-m Keck I telescope in Hawaii.
The final star was identified and measured by a competing team using
ESO's 8.2-m VLT.
NEW ELEMENT CONFIRMED
Science Daily
The periodic table has been extended, with the announcement of the
confirmation of the yet-to-be-named element 117. In 2010 a
US--Russian collaboration announced that it had produced atoms of an
element with 117 protons, filling a gap that appeared when 118 was
made four years earlier. However, the International Union of Pure and
Applied Chemistry (IUPAC) insists on corroboration by two independent
teams before it allows new elements to be added to the Periodic Table,
although a temporary name of Ununseptium is in use until confirmation
has been made. It has taken four years, but confirmation appears
finally to have arrived. The discovery was made by a team at the GSI
laboratory in Germany which fused calcium 48 and berkelium 249. That
is not easy, because berkelium 249 is both hard to produce in
substantial quantities and has a half life of only 320 days -- less
than half of any amount produced will still exist a year after it was
made. By watching the alpha particles emitted, the team concluded
that they were the product of two decay chains, both originating with
294117, that is an atom with 117 protons and 177 neutrons. One of the
chains included the isotopes 270Db and 266Lr, the latter adding four
neutrons to the previous highest isotope of lawrencium. In general
large atoms have shorter half lives (they decay more quickly through
radiation) as their masses become greater. However, what are known as
'islands of stability' exist, and the authors believe that the
one-hour half-life of 270Db marks an important step towards the
identification of even-more-long-lived nuclei of superheavy elements.
The manufacturing process was hardly efficient. More than 1000 atoms
of 48Ca, not a common isotope in its own right, were fired at the
berkelium to produce just four atoms of 117. Nevertheless it is likely
that element 117 will be accepted. Element 117 is the most recent of
six elements first announced by the Joint Institute for Nuclear
Research in Russia. Of them, 113, 115 and 118 remain unconfirmed,
although claims have been made for the first two. Such a small sample
does not allow us to learn much about the chemistry of element 117.
Ununseptium's position in the periodic table places it under the
halogen elemnts such as fluorine and chlorine, but the strong capacity
to capture electrons that makes those elements so reactive weakens as
one goes down the table, and in fact it is thought if one could ever
produce enough 117 to observe chemical interactions it would be more
likely to lose electrons than gain them. The next question is, how
can we create elements 119 and 120? To do that, however, a projectile
heavier than 48Ca will need to be found; researchers are working on
identifying the best candidate.
Topic: Late May Astronomy Bulletin (Read 1283 times)