ASTEROID TX68 WILL NOT IMPACT THE EARTH THIS CENTURY
NASA
Last month, the news about a small asteroid coming close to the Earth
caused controversy among the astronomical community. NASA reported
that on March 5 the asteroid 2013 TX68, that two years ago passed by
the Earth, will fly by our planet at a comfortable distance between
17000 and 14 million kilometres, without posing any risk. However,
astronomers at NASA's Center for Near-Earth Object Studies (CNEOS)
warned about a remote chance (no more than 1 in 250-million) that that
30-metre asteroid could impact the Earth on 2017 September 28. Now,
even the slight chance has gone. By obtaining additional observations
of asteroid 2013-TX68, astronomers refined its orbital trajectory and
distance predictions, moving the date of the asteroid's Earth
encounter from March 5 to March 8 and confirming that that small rock
poses no threat. The new prediction for 2013 TX68 is that it will
pass about 5 million kilometres from our planet. There is a still a
chance that it could come closer but no less than 24,000 kilometres
above the surface of the Earth. The calculations delimit the path of
the rock in future years, and there is no risk of impact over the next
century. Prospects for observing the asteroid, which were not very
good to begin with, are now even worse because the asteroid is likely
to be further away and therefore dimmer than previously believed. The
asteroid 2013 TX68 was discovered by the Catalina Sky Survey on 2013
October 6 and has a diameter of 30 metres. For comparison, the
asteroid that entered over Russian skies near Chelyabinsk in 2013
February was approximately 20 metres. If an asteroid of the size of
2013 TX68 were to enter the Earth's atmosphere, it would be likely to
produce an explosion in the air with twice the energy of the
Chelyabinsk event.
DESTRUCTION OF ASTEROIDS CLOSE TO SUN
University of Helsinki
For two decades it was thought that most near-Earth objects (NEOs) --
asteroids and comets that may pose a hazard to life on Earth -- end
their existence in a dramatic final plunge into the Sun. A new study
finds instead that most of those objects are destroyed much further
from the Sun than previously thought. That explains several puzzling
observations that have been reported in recent years. An inter-
national team originally set out to construct a state-of-the-art model
of the NEO population that is needed for planning future asteroid
surveys and spacecraft missions. The model describes the NEOs' orbit
distribution and estimates the number of NEOs of different sizes. The
vast majority of NEOs originate in the doughnut-shaped main asteroid
belt between the orbits of Mars and Jupiter. The orbit of a main-belt
asteroid slowly changes as it is pushed by the uneven release of
excess solar heat from the asteroid's surface. The asteroid's orbit
eventually interacts with the orbital motions of Jupiter and Saturn,
changing the trajectory and possibly bringing the asteroid close to
the Earth. An asteroid is classified as an NEO when its smallest
distance from the Sun during an orbit is less than 1.3 times the
average Earth-Sun distance.
The team used the properties of almost 9,000 NEOs detected in about
100,000 images acquired over about 8 years by the Catalina Sky Survey
(CSS) near Tucson, Arizona, to construct the new population model.
One of the most challenging problems facing the team was computing
which asteroids they could actually detect. An asteroid appears as a
moving point of light against a background of fixed stars, but
detecting it in an image depends on two factors -- how bright it is
and how fast it is seen to be moving. If the telescope is not looking
in the right location at the right time when an asteroid is bright
enough and slow enough to be detected, we simply may never find it.
Accounting for such observational selection effects requires a
detailed understanding of the operations of the telescope and detector
systems and a tremendous amount of computing time even with fast
mathematical techniques. The team produced the best-ever model of the
NEO population by combining information about CSS's selection effects
with the CSS data and theoretical models of the orbit distributions of
NEOs that originate in different parts of the main asteroid belt. But
they noticed that their model had a problem -- it predicted that there
should be almost 10 times more objects in orbits that approach the Sun
to within 10 solar diameters. The team then spent a year verifying
their calculations before they came to the conclusion that the problem
was not in their analysis but in their assumptions of how the Solar
System works.
Astronomers hypothesized that their model would better match the
observations if NEOs are destroyed close to the Sun but long before an
actual collision. The team tested that idea and found an excellent
agreement between the model and the observed population of NEOs when
they eliminated asteroids that spend too much time within about 10
solar diameters of the Sun. The team's discovery helps to explain
several other discrepancies between observations and predictions of
the distribution of small objects in our Solar System. Meteors,
commonly known as shooting stars, are tiny bits of dust and rock that
are dislodged from the surfaces of asteroids and comets and burn up as
they enter our atmosphere. Meteors often travel in 'streams' that
follow the paths of their parent objects, but astronomers have been
unable to match most of the meteor streams in orbits closely
approaching the Sun with known parent objects. The new study suggests
that the parent objects were completely destroyed when they came too
close to the Sun -- leaving behind streams of meteors but no parent
NEOs. They also found that darker asteroids are destroyed farther
from the Sun than brighter ones, explaining an earlier discovery that
NEOs that approach closer to the Sun are brighter than those that keep
their distance from the Sun. The fact that dark objects are more
easily destroyed implies that dark and bright asteroids have different
internal compositions and structure. The most intriguing outcome of
the study is that it is now possible to test models of asteroid
interiors simply by keeping track of their orbits and sizes.
SEARCH FOR PLANET 9
CNRS
Using observations from the Cassini spacecraft, a team of French
astronomers has been able to specify the possible positions of a ninth
planet in the Solar System. The Kuiper-Belt objects, small bodies
similar to Pluto beyond Neptune, have a particular distribution that
is difficult to explain by pure chance. That is what led astronomers
at Caltech to propose the existence of a ninth planet of 10 Earth
masses, whose perturbations on Kuiper objects could have led to their
current distribution. Using numerical simulations, scientists
determined the possible orbit of the planet. To reproduce the
observed distribution of Kuiper-Belt Objects, that orbit, with a
semi-major axis of 700 AU, must be very eccentric (e ~ 0.6) and
inclined (i ~ 30°), but no constraint on the current position of the
planet is proposed in the Caltech study. That does not facilitate the
task of observers who need to search in all possible directions in
longitude to try to discover the planet. Since 2003, French
astronomers have been developing planetary ephemerides which calculate
the motion of planets in the Solar System with the highest accuracy.
In particular, using data from the Cassini spacecraft, the distance
between the Earth and Saturn is known with an uncertainty of about 100
metres. The researchers had the idea to use their model to test the
possibility of adding a ninth planet in the Solar System.
The French team's study shows that, depending on the position of the
planet from its perihelion (denoted 'true anomaly'), the ninth planet
induces perturbations in the orbit of Saturn that can be detected by
analyzing the radio data from the Cassini spacecraft, orbiting Saturn
since 2004. The researchers were able to compute the effect induced
by the ninth planet and to compare the perturbed orbit to the Cassini
data. For an angle from perihelion of less than 85° or greater than
-65°, the perturbations induced by the ninth planet are inconsistent
with the observed Cassini distances. The result is the same for the
sector from -130° to -100°. That result allows to exclude half of the
directions in longitude, in which the planet cannot be found. On the
other hand, it appears that for some directions, the addition of the
ninth planet reduces the discrepancies between the model calculated by
the astronomers and the observed data, by comparison with a model that
does not include such a ninth planet. That makes plausible the
presence of a ninth planet for an angle from perihelion between 104°
and 134°, with a maximum probability for 117°. The existence of a
ninth planet can be confirmed only by direct observation, but by
restricting the possible directions to look, the French research team
has made a useful contribution to that quest.
LONGEST-LASTING STELLAR ECLIPSE
Vanderbilt University
Imagine living on a world where, every 69 years, the Sun disappears in
a near-total eclipse that lasts for three and a half years! That is
just what happens in a binary-star system nearly 10,000 light-years
from us. The newly discovered system, known as TYC 2505-672-1, sets a
new record for both the longest-duration stellar eclipse and the
longest period between eclipses in a binary system. The previous
record-holder is Epsilon Aurigae, a giant star that is eclipsed by its
companion every 27 years for intervals ranging from 640 to 730 days.
Epsilon Aurigae is much closer -- about 2,200 light-years away -- and
brighter, which has allowed astronomers to study it extensively. The
leading explanation is that Epsilon Aurigae consists of a yellow giant
star orbited by a normal star slightly bigger than the Sun embedded in
a thick disc of dust and gas oriented nearly edge-on to us. One of
the great challenges in astronomy is that some of the most important
phenomena occur on astronomical time-scales, yet astronomers are
generally limited to much shorter human time-scales. Two astronomical
resources made the discovery possible: observations by the American
Association of Variable Star Observers (AAVSO) network and the Digital
Access to a Sky Century @ Harvard (DASCH) programme. AAVSO provided a
few hundred observations of TYC 2505-672-1's most recent eclipse. The
DASCH survey is based on thousands of photographic plates taken by
Harvard astronomers between 1890 and 1989 as part of a regular survey
of the northern sky. In recent years the university has begun
digitizing those plates. In the process TYC 2505-672-1 caught the eye
of researchers who found about 9,000 further images of the obscure
system, taken in the last eight years, that they could combine with
the 1,432 images taken over the last century at Harvard. The AAVSO
network obtained several hundred more observations of the system's
most recent eclipse to help fill in the picture.
The resulting analysis revealed a system similar to that of Epsilon
Aurigae, with some important differences. It appears to consist of a
pair of red giant stars, one of which has been stripped down to a
relatively small core and surrounded by an extremely large disc of
material that produces the extended eclipse. About the only way to
get such really long eclipse times is with an extended disc of opaque
material. Nothing else is big enough to block out a star for months
at a time. TYC-2505-672-1 is so distant that the amount of data that
the astronomers could extract from the images was limited. However,
they were able to estimate the surface temperature of the companion
star and found that it is about 2,000 degrees Celsius hotter than the
surface of the Sun. Combined with the observation that it appears to
be less than half the diameter of the Sun has led them to propose that
it is a red giant that has had its outer layers stripped away and that
the stripped material may account for the obscuring disc. However,
they do not know that for certain. In order to produce the 69-year
interval between eclipses, the astronomers calculate that they must be
orbiting at an extremely large distance, about 20 astronomical units,
which is approximately the distance between the Sun and Uranus. At
present even our most powerful telescopes can not resolve the two
objects, but perhaps technological advances will make that possible by
2080 when the next eclipse will occur.
NEW SOURCE OF INTENSE GAMMA RADIATION
RAS
Strong stellar winds are generated in binary systems consisting of
highly luminous and hot Wolf-Rayet stars and massive (several tens of
solar masses) OB companions. Wind collision may produce strong photon
emission with photon energies exceeding a hundred mega-electron-volts
(MeV). That phenomenon was considered as a possible source of gamma-
radiation for a long while. Though such radiation was detected only
once, with the famous Eta Carinae, which has been observed for more
than four centuries (particularly intensively after 1834, when one of
its stars underwent an explosion and for some time was the most
luminous star in the sky). Eta Carinae is comparatively close to the
Earth -- around 8,000 light-years. The stars in that system have
masses of 120 and (30-80) solar masses respectively, and shine
brighter than millions of Suns. If they were 30 light-years away from
the Earth, they would be as bright as the Moon, while the Sun would be
invisible at their present distance. Naturally, Eta Carinae was one
of the first candidates to consider as a possible source of gamma-
rays,and seven years ago high-energy radiation from that system was
indeed detected. However, one example was not enough to confirm the
model of binary stars emitting high-energy radiation, and the search
for similar sources was continued, which turned out to be a tricky
task. Recent calculations prove such star types as Eta Carinae to be
incredibly rare -- probably, one per galaxy like we inhabit, or less.
In 2013 an American-Austrian research team composed a list of seven
stellar systems containing Wolf-Rayet stars, where gamma radiation
could most probably appear. The research was based on two years of
observations and lacked data, so it was only possible to set an upper
limit on the high-energy radiation.
Astronomers utilized data from seven years of Fermi-LAT observations
and discovered that Gamma Velorum is a source of gamma-radiation.
That system contains two stars with masses of 30 and 10 solar masses.
Their orbital parameters are well studied and they are separated by
about the same distance as the Earth and the Sun. The luminosity of
that binary system is about 200,000 times that of the Sun, and strong
stellar winds imply a very high mass-loss rate -- up to a hundred-
thousandth of a solar mass every year. Though that figure may seem
small, actually that amount is huge, particularly in comparison with
the solar wind, which only amounts only to 10 to the power -14 solar
mass per annum. As the stellar winds in the Gamma Velorum system
collide at a speed exceeding 1000 km/s, particles are accelerated in
the shock. Though the mechanism of the acceleration is still unknown,
it definitely leads to the high-energy photon radiation that was
detected by Fermi LAT. Searching for similar sources in the Galactic
plane is much more complicated, since it is a powerful gamma-ray
source itself, and detecting small photon excess coming from colliding
stellar winds becomes much more difficult with that background. But
the Gamma Velorum system lies above the plane and it is comparatively
close to us. The discovery would probably not have happened if it
were much further away or closer to the plane.
LIGO's TWIN BLACK HOLES BORN INSIDE SINGLE STAR
Harvard-Smithsonian Center for Astrophysics
On 2015 September 14, the Laser Interferometer Gravitational-wave
Observatory (LIGO) detected gravitational waves from the merger of two
black holes 29 and 36 times the mass of the Sun. Such an event is
expected to be dark, but the Fermi space telescope detected a gamma-
ray burst just a fraction of a second after LIGO's signal. New
research suggests that the two black holes might have resided inside a
single, massive star whose 'death' generated the gamma-ray burst.
Normally, when a massive star reaches the end of its 'life', its core
collapses into a single black hole. But if the star was spinning very
rapidly, its core might stretch into a dumbbell shape and fragment
into two clumps, each forming its own black hole. A very massive
star, as is needed here, often forms out of the merger of two smaller
stars. And since the stars would have revolved around each other
faster and faster as they spiralled together, the resulting merged
star would be expected to spin very quickly. After the black hole
pair formed, the star's outer envelope collapsed inwards toward them.
In order to power both the gravitational-wave event and the gamma-ray
burst, the twin black holes must have been born close together, with
an initial separation of order the size of the Earth, and merged
within minutes. The newly formed single black hole then fed on the
infalling matter, consuming up to a Sun's worth of material every
second and powering jets of matter that blasted outward to create the
burst.
Fermi detected the burst just 0.4 seconds after LIGO detected
gravitational waves, and from the same general area of the sky.
However, the European INTEGRAL gamma-ray satellite did not confirm the
signal. Even if the Fermi detection is a false alarm, future LIGO
events should be monitored for accompanying light irrespective of
whether they originate from black-hole mergers. If more gamma-ray
bursts are detected from gravitational-wave events, they will offer a
promising new method of measuring cosmic distances and the expansion
of the universe. By observing the afterglow of a gamma-ray burst and
measuring its redshift, then comparing it to the independent distance
measurement from LIGO, astronomers could constrain the cosmological
parameters. Astrophysical black holes are much simpler than other
distance indicators, such as supernovae, since they are fully defined
just by their mass and spin.
PROLONGED DEATH OF TYPE 1a SUPERNOVA
American Museum of Natural History
Three years after its explosion, a type-Ia supernova continues to
shine brighter than expected. Observations made with the Hubble space
telescope suggest that the powerful explosions produce an abundance of
a heavy form of cobalt that gives the heat from nuclear decay an extra
energy boost. The work could help researchers to pinpoint the parents
of type-Ia supernovae -- a type of stellar explosion that is frequently
used to measure distances to far-away galaxies -- and reveal the
mechanics behind such explosions. Type-Ia supernovae became very
important to physics, as a whole, a couple of decades ago when they
were used to show that the expansion of the universe is accelerating.
Yet we still do not know exactly what type of star system explodes as
a type-Ia supernova or how the explosion takes place. A lot of
research has gone into those two questions, but the answers are still
elusive. Current research indicates that type-Ia supernova explosions
originate from binary-star systems in which at least one star is a
white dwarf, the dense remains of a star that was a few times more
massive than our Sun. The explosion is the result of a thermonuclear
chain reaction, which produces a large amount of heavy elements. The
light that researchers see when a type-Ia supernova explodes comes
from the radioactive decay of an isotope of nickel (56Ni) into an
isotope of cobalt (56Co) and then into a stable isotope of iron
(56Fe). Although peak brightness is reached relatively quickly, and
most researchers stop watching supernovae after about 100 days past
the beginning of the explosion, the light continues to radiate for
years.
Previous studies predicted that, about 500 days after an explosion,
researchers should see a sharp drop-off in the brightness of such
supernovae, an idea called the 'infrared catastrophe'. However, no
such drop-offs have been observed, and researchers suggested in 2009
that that must be due to the radioactive decay of 57Co. That is a
heavier isotope of cobalt with a longer half-life than 56Co, and it is
expected to provide an extra energy source that would kick in around
two to three years after the explosion. The researchers tested the
prediction directly by using the Hubble telescope to observe the
type-Ia supernova SN 2012cg more than three years after it exploded in
the galaxy NGC 4424, which is about 50 million light-years away -- not
far in cosmological terms. Astronomers saw the supernova's brightness
evolve just as predicted. Interestingly, though, they found that the
amount of 57Co needed to produce the observed brightness was about
twice the amount expected. Those two pieces of information provide
fresh constraints on progenitor and explosion models. Stated
differently, we now have a new piece in the puzzle that is type-Ia
supernovae, one of the most important tools in modern cosmology.
There is one caveat to the results: the excess brightness measured by
the researchers could be due to a phenomenon known as a 'light echo'
instead of 57Co. A light echo happens when light from an explosion
interacts with a large dust cloud, which scatters the light in all
directions. In that case, light from the explosion would reach the
Earth twice: once directly from the supernova and then some years
later as the result of the echo. To rule out the possibility of the
light coming from an echo, more observations will have to be made of
type-Ia supernovae that are closer to the Earth.
GALAXY TRAILED BY PLUME OF GAS
International Centre for Radio-Astronomy Research (ICRAR)
Astronomers have discovered a spectacular tail of gas more than
300,000 light-years across coming from a nearby galaxy. The plume is
made up of hydrogen and is five times longer than the galaxy itself.
The discovery was made by scientists at the Laboratoire d'Astro-
physique de Marseille in France. Scientists noticed long ago that the
galaxy NGC 4569 contained less gas than expected but they could not
see where it had gone. NGC 4569 is in the Virgo cluster, a group of
galaxies 55 million light-years from our own Milky Way. It is
travelling through the cluster at about 1200 km/s, and it is that
movement that is causing the gas to be stripped from the galaxy.
We know that big clusters of galaxies trap a lot of hot gas, so when
a galaxy enters the cluster it feels the pressure of all the gas,
and that pressure is able to strip matter away from the galaxy.
The discovery was made when the research team used a very sensitive
camera on the Canada-France-Hawaii telescope to observe NGC 4569 for
longer than ever before.
BLACK HOLES BANISH MATTER INTO COSMIC VOIDS
RAS
We live in a Universe dominated by unseen matter, and on the largest
scales, galaxies and everything that they contain are concentrated
into filaments that stretch around the edges of enormous voids. The
dark holes have been thought to be almost empty until now, but now a
group of astronomers thinks that they may possibly contain as much as
20% of the 'normal' matter in the cosmos, and that galaxies make up
only 1/500th of the volume of the Universe. Looking at cosmic
microwave radiation, modern satellite observatories like COBE, WMAP
and Planck have gradually refined our understanding of the composition
of the Universe, and the most recent measurements suggest it consists
of 4.9% 'normal' matter (i.e. the matter that makes up stars, planets,
gas and dust, or 'baryons'), whereas 26.8% is the mysterious and
unseen 'dark' matter, and 68.3% is the even more mysterious 'dark
energy'. Complementing those missions, ground-based observatories
have mapped the positions of galaxies and, indirectly, their
associated dark matter over large volumes, showing that they are
located in filaments that make up a 'cosmic web'. The team
investigated that in more detail, using data from the Illustris
project, a large computer simulation of the evolution and formation of
galaxies, to estimate the mass and volume of the filaments and the
galaxies within them. Illustris simulates a cube of space in the
Universe, measuring some 100 million parsecs (350 million light-years)
on each side. It starts when the Universe was just 12 million years
old, a small fraction of its current age, and tracks how gravity and
the flow of matter change the structure of the cosmos up to the
present day. The simulation deals with both normal and dark matter,
with the most important effect being the gravitational pull of the
dark matter.
When the scientists looked at the data, they found that about 50% of
the total mass of the Universe is in the places where galaxies reside,
compressed into a volume of 0.2% of the Universe we see, and a further
44% is in the enveloping filaments. Just 6% is located in the voids,
which make up 80% of the volume. But the team also found that a
surprising fraction of normal matter (20%) is likely to be have been
transported into the voids. The culprit appears to be the super-
massive black holes found in the centres of galaxies. Some of the
matter falling towards the holes is converted into energy. That
energy is delivered to the surrounding gas, and leads to large
outflows of matter, which stretch for hundreds of thousands of light-
years from the black holes, reaching far beyond the extent of their
host galaxies. Apart from filling the voids with more matter than
thought, the result might help to explain the missing-baryon problem,
where astronomers do not see the amount of normal matter predicted by
their models. The simulation, one of the most complicated ever run,
suggests that the black holes at the centres of every galaxy are
helping to send matter into the loneliest places in the Universe.
What is needed now is to refine the model, and confirm those initial
findings. Illustris is now running new simulations, and results from
them should be available in a few months, with the researchers keen to
see whether, for example, their understanding of black-hole output is
right. Whatever the outcome, it will be hard to see the matter in the
voids, as it is likely to be very tenuous, and too cool to emit the
X-rays that would make it detectable by satellites.
OLDEST LIGHT IN THE UNIVERSE
Cosmos Up!
Astronomers have used the Chandra X-ray observatory to discover a
powerful jet from a very distant supermassive black hole that is being
illuminated by the oldest light in the Universe. The discovery shows
that black holes with powerful jets may be more common than previously
thought in the first few billion years after the Big Bang. The light
detected from the jet was emitted when the Universe was only 2.7
billion years old, a fifth of its present age. In that period of the
Universe, the intensity of the cosmic microwave background radiation
(CMB) left over from the Big Bang was much greater than it is today.
Because we are seeing the jet when the Universe was less than three
billion years old, the jet is about 150 times brighter in X-rays than
it would be in the nearby Universe. The jet was found in the system
known as B3 0727+409, and is at least 300,000 light-years long. Many
long jets emitted by supermassive black holes have been detected in
the 'nearby' Universe, but no one knows how the jets give off X-rays.
In B3 0727+409, it appears that the cosmic microwave background is
somehow being boosted at X-ray wavelengths. Astronomers say that as
the electrons in the jet fly from the black hole at close to the speed
of light they moved through the 'sea' of CMB radiation and collide
with microwave photons, boosting the energy of the photons up to the
X-ray band, allowing them to be detected by Chandra. That implies
that the electrons in the B3 0727+409 jet must keep moving at nearly
the speed of light for hundreds of thousands of light-years. Electrons
in black-hole jets are usually brighter at radio wave-lengths, so
typically such things are found by radio observations. The discovery
of the jet here is special because so far almost no radio signal has
been detected from that object, yet it is easily seen in the X-ray
image. So far, astronomers have identified very few jets distant
enough that their X-ray brightness could be amplified by the CMB as
clearly as in this system. If bright X-ray jets can exist with very
faint or undetected radio counterparts, it means that there could be
many more of them. Activity by supermassive black holes, including
the launching of jets, may be different in the early Universe from
what we see later on.
Topic: Early March Astronomy Bulletin (Read 1521 times)