THE SUN IS A SOLAR-TYPE STAR
American Association for the Advancement of Science
A new study claims that the Sun is a solar-type star, -- resolving an
ongoing controversy about whether the star at the centre of our Solar
System exhibits the same cyclic behaviour as other 'nearby' solar-type
stars. The results also advance scientists' understanding of how stars
generate their magnetic fields. The Sun's activity -- including changes
in the number of sunspots, levels of radiation and ejection of material --
varies on an eleven-year cycle, driven by changes in its magnetic field.
Understanding that cycle is one of the biggest outstanding problems in
solar physics, in part because it does not appear to match magnetic
cycles observed in other solar-type stars -- leading some astronomers
to suggest that the Sun is fundamentally different. By carrying out
a series of simulations of stellar magnetic fields, researchers show
that the Sun's magnetic cycle depends on its rotation rate and
luminosity. The relationship can be expressed in terms of the
so-called Rossby number. Comparing the results of their simulations
with available observations of cyclic activity in a sample of nearby
solar-type stars, the team further finds that the cycle periods of the
Sun and other solar-type stars all follow the same relationship with
the Rossby number.
NEW EVIDENCE FOR PLANET 9
SINC
Scientists continue to argue about the existence of a ninth planet
in the Solar System. At the beginning of 2016, researchers from the
California Institute of Technology announced that they had evidence of
the existence of such an object, located at an average distance of 700
astronomical units (700 times the Earth-Sun separation) and with a
mass ten times that of the Earth. Their calculations were motivated by
the peculiar distribution of the orbits found for the trans-Neptunian
objects (TNO) of the Kuiper belt, which apparently revealed the
presence of a Planet Nine or X in the confines of the Solar System.
However, scientists from the Canadian-French-Hawaiian project OSSOS
detected biasses in their own observations of the orbits of the TNOs,
which had been systematically directed towards the same regions of the
sky, and considered that other groups, including the Caltech group,
may be experiencing the same issues. According to those scientists,
it is not necessary to propose the existence of a massive perturber
(a Planet Nine) to explain the observations, as they are compatible
with a random distribution of orbits. Now, however, two astronomers
from the Complutense University of Madrid have applied a new tech-
nique, less exposed to observational bias, to study a special type of
trans-Neptunian objects -- the extreme ones (ETNOs, located at average
distances greater than 150 AU and that never cross Neptune's orbit).
For the first time, the distances from their nodes to the Sun have
been analysed, and the results once again indicate that there is a
planet beyond Pluto. The nodes are the two points at which the orbit
of an ETNO, or any other celestial body, crosses the plane of the
Solar System. Those are the precise points where the probability of
interacting with other objects is the largest, and therefore, at those
points the ETNOs may experience drastic changes in their orbits or
even a collision.
If there is nothing to perturb them, the nodes of the extreme trans-
Neptunian objects should be uniformly distributed, as there is nothing
for them to avoid, but if there are one or more perturbers, two
situations may arise. One possibility is that the ETNOs are stable,
and in that case they would tend to have their nodes away from the
path of possible perturbers, but if they are unstable they would
behave as the comets that interact with Jupiter do, that is tending to
have one of the nodes close to the orbit of the hypothetical perturber.
Using calculations and data mining, the Spanish astronomers have found
that the nodes of the 28 ETNOs analysed (and the 24 extreme Centaurs
with average distances from the Sun of more than 150 AU) are clustered
in certain ranges of distances from the Sun; furthermore, they have
found a correlation, where none should exist, between the positions of
the nodes and the inclination, one of the parameters which defines the
orientation of the orbits of the objects in space. Assuming that the
ETNOs are dynamically similar to the comets that interact with Jupiter,
the team interprets those results as signs of the presence of a planet
that is actively interacting with them in a range of distances from
300 to 400 AU. Astronomers believe that what they are seeing here
cannot be attributed to observational bias. Until now, studies that
challenged the existence of Planet Nine using the data available for
trans-Neptunian objects argued that there had been systematic errors
linked to the orientations of the orbits (defined by three angles),
owing to the way in which the observations had been made. Neverthe-
less, the nodal distances mainly depend on the size and shape of the
orbit, parameters which are relatively free from observational bias.
The authors note that their study supports the existence of a planet-
ary object within the range of parameters considered both in the
Planet-Nine hypothesis of Mike Brown and by Konstantin Batygin from
Caltech. The hypothetical Planet Nine suggested in this study has
nothing to do with another possible planet or planetoid situated much
closer to us, and hinted at by other recent findings. Also, applying
data mining to the orbits of the TNOs of the Kuiper Belt, astronomers
from the University of Arizona have found that the plane on which
these objects orbit the Sun is slightly warped, a fact that could be
explained if there is a perturber of the size of Mars at 60 AU from
the Sun. Given the current definition of planet, that other putative
object might not be a true planet, even if it has a size similar to
that of the Earth, as it could be surrounded by huge asteroids or
dwarf planets. In any case, scientists are convinced that the work
has found solid evidence of the presence of a massive body beyond the
so-called Kuiper Cliff, the furthest point of the trans-Neptunian
belt, at some 50 AU from the Sun.
LARGE,DISTANT COMETS MORE COMMON THAN THOUGHT
NASA
Comets that take more than 200 years to make one revolution around the
Sun are notoriously difficult to study. Because they spend most of
their time far from our part of the Solar System, many 'long-period
comets' will never approach the Sun in any particular person's
lifetime. In fact, those that travel inward from the Oort Cloud --
a group of icy bodies beginning roughly 5000 AU away from the Sun --
can have periods of thousands or even millions of years. NASA's WISE
space-craft, scanning the entire sky at infrared wavelengths, has
delivered new insights about those distant wanderers. Scientists
found that there are about seven times more long-period comets
measuring at least 1 kilometre across than had been predicted
previously. They also found that long-period comets are on average up
to twice as large as 'Jupiter-family comets', whose orbits are shaped
by Jupiter's gravity and have periods less than 20 years. Researchers
also observed that in eight months, three to five times as many long-
period comets passed by the Sun as had been predicted. The Oort Cloud
is too distant to be seen by current telescopes, but is thought to be
a spherical distribution of small icy bodies at the outermost edge of
the Solar System. The density of comets within it is low, so the odds
of comets colliding within it are rare. Long-period comets that WISE
observed probably got kicked out of the Oort Cloud millions of years
ago. The observations were carried out during the spacecraft's
primary mission before it was renamed NEOWISE and reactivated to
target near-Earth objects (NEOs). Astronomers already had broader
estimates of how many long-period and Jupiter-family comets exist in
the Solar System, but had no good way of measuring the sizes of long-
period comets. That is because a comet has a 'coma', a cloud of gas
and dust that appears hazy in images and obscures the cometary
nucleus. But by using the WISE data showing the infrared glow of the
coma, scientists were able to subtract the coma from the overall
comet and estimate the nucleus sizes of the comets. The data came
from 2010 WISE observations of 95 Jupiter family comets and 56 long-
period comets.
The results reinforce the idea that comets that pass by the Sun more
often tend to be smaller than those spending much more time away from
the Sun. That is because Jupiter-family comets get more heat exposure,
which causes volatile substances like water to sublimate and drag away
other material from the comet's surface as well. The existence of so
many more long-period comets than predicted suggests that more of them
are likely to have impacted planets, delivering icy materials from the
outer reaches of the Solar System. Researchers also found clustering
in the orbits of the long-period comets that they studied, suggesting
that there could have been larger bodies that broke apart to form
those groups. The results will be important for assessing the likeli-
hood of comets impacting planets, including the Earth. Comets travel
much faster than asteroids, and some of them are very big. Studies
such as the one reported here may help us to define what kind of
hazard long-period comets may pose.
SMALLEST-EVER STAR DISCOVERED
University of Cambridge
The smallest star yet measured has been discovered by a team of
astronomers led from the University of Cambridge. With a size just
slightly larger than Saturn, the gravity at its surface is about 300
times stronger than gravity on the Earth. The star is probably about
as small as stars can possibly be, as it has just enough mass to
enable the fusion of hydrogen nuclei into helium. If it were any
smaller, the pressure at the centre of the star would no longer be
sufficient to enable that process to take place. Hydrogen fusion is
also what powers the Sun. Such very small and dim stars are also the
best possible candidates for detecting Earth-sized planets which can
have liquid water on their surfaces, such as TRAPPIST-1, an ultra-cool
dwarf surrounded by seven temperate Earth-sized worlds. The newly-
measured star, called EBLM J0555-57Ab, is located about 600 light
years away. It is part of a binary system, and was identified as it
passed in front of its much larger companion, a method which usually
detects planets, not stars. EBLM J0555-57Ab was identified by WASP, a
planet-finding experiment run by the Universities of Keele, Warwick,
Leicester and St Andrews.
The parent star became dimmer in a periodic fashion, the signature of
an orbiting object. Thanks to that special configuration, researchers
can measure accurately the mass and size of any orbiting companions,
in this case a small star. The mass of EBLM J0555-57Ab was established
by the Doppler method, using data from the CORALIE spectrometer. The
newly-measured star has a mass comparable to the current estimate for
TRAPPIST-1, but has a radius that is nearly 30% smaller. The smallest
stars provide optimal conditions for the discovery of Earth-like
planets, and for the remote exploration of their atmospheres. However,
before we can study planets, we need to understand their stars.
Although they are the most numerous stars in the Universe, stars with
sizes and masses less than 20% that of the Sun are poorly understood,
since they are difficult to detect owing to their small size and low
brightness. The EBLM project, which identified the star in this
study, aims to plug that gap in knowledge.
COSMIC 'DUST FACTORY' REVEALS CLUES TO HOW STARS ARE BORN
RAS
Researchers at Cardiff University have discovered a rich inventory of
molecules at the centre of an exploded star for the very first time.
Two previously undetected molecules, formylium (HCO+) and sulphur
monoxide (SO), were found in the cooling aftermath of Supernova 1987A,
located 163,000 light-years away in the Large Magellanic Cloud - a
'nearby' neighbour of our own Milky Way galaxy. The newly identified
molecules were accompanied by previously detected compounds such as
carbon monoxide (CO) and silicon oxide (SiO). The researchers estimate
that about 1 in 1000 silicon atoms from the exploded star can be found
in SiO molecules and only a few out of every million carbon atoms are
in HCO+ molecules. It was previously thought that the massive
explosions of supernovae would completely destroy any molecules and
dust that may already have been present. However, the detection of
those unexpected molecules suggests that the explosive death of stars
could lead to clouds of molecules and dust at extremely cold tempera-
tures -- conditions similar to those seen in stellar nurseries where
stars are born. The results show that as the leftover gas from a
supernova begins to cool down to below 200°C, the many heavy elements
that are synthesized can begin to harbour rich molecules, creating a
dust factory. What is most surprising is that this factory of rich
molecules is usually found in conditions where stars are born. The
deaths of massive stars may therefore lead to the birth of a new
generation.
The team used the Atacama Large Millimeter/submillimeter Array (ALMA)
to observe the heart of Supernova 1987A in remarkably fine detail.
Astronomers have been studying SN 1987A since it was first discovered
30 years ago, but have found it difficult to analyze the supernova's
innermost core. ALMA's ability to observe at millimetre wavelengths
-- a region of the electromagnetic spectrum between infrared light and
radio waves -- made it possible to see through the intervening dust
and gas, and study the abundance and location of the newly formed mol-
ecules. The ALMA observations of molecules such as silicon monoxide
in SN 1987A have enabled isotopic abundance ratios to be measured for
the first time in supernova material, allowing comparisons to be made
with models for the explosive nuclear reactions that take place in
such supernovae. In an accompanying paper, a second research team
has used ALMA's data to create the first 3D model of SN 1987A,
offering important insights into the original star itself and the way
supernovae create the basic building blocks of planets. It is well
understood that massive stars, those more than 10 times the mass of
the Sun, end their existence in spectacular fashion. When such a star
runs out of fuel, there is no longer enough heat and energy to sustain
the star against the force of its own gravity. The outer reaches of
the star, once held up by the power of nuclear fusion, then come
crashing down on the core with tremendous force. The rebound from
that collapse triggers an explosion that blasts material into space.
Building on its current findings, the team hopes to use ALMA to find
out exactly how abundant the molecules of HCO+ and SO are, and to see
if there are within the supernova any other molecules that have yet to
be detected.
HIGH-ENERGY TRAP IN CENTRE OF MILKY WAY
NASA/Goddard Space Flight Center
A combined analysis of data from NASA's Fermi Gamma-ray Space
Telescope and the High-Energy Stereoscopic System (HESS), a
ground-based observatory in Namibia, suggests that the centre of our
Milky Way contains a 'trap' that concentrates some of the highest-
energy cosmic rays, among the fastest particles in the galaxy. Cosmic
rays are high-energy particles moving through space at almost the
speed of light. About 90% of them are protons, with electrons and the
nuclei of various atoms making up the rest. In their journey across
the Galaxy, those electrically charged particles are affected by
magnetic fields, which alter their paths and make it impossible to
know where they originated. But astronomers can learn about cosmic
rays when they interact with matter and emit gamma rays, the highest-
energy form of light. In 2016 March, scientists with the HESS
Collaboration reported gamma-ray evidence of the extreme activity in
the Galactic Centre. The team found a diffuse glow of gamma rays
reaching nearly 50 trillion electron volts (TeV). That's some 50
times greater than the gamma-ray energies observed by Fermi's Large
Area Telescope (LAT). To put those numbers in perspective, the energy
of visible light ranges from about 2 to 3 electron volts.
The Fermi spacecraft detects gamma rays when they enter the LAT. On
the ground, HESS detects the emission when the atmosphere absorbs
gamma rays, which triggers a cascade of particles resulting in a flash
of blue light. In a new analysis, an international team of scientists
combined low-energy LAT data with high-energy HESS observations.
The result was a continuous gamma-ray spectrum describing the Galactic
Centre emission across a thousandfold span of energy. Once the bright
point sources were subtracted, good agreement was found between the
LAT and HESS data, which was somewhat surprising owing to the
different energy windows and observing techniques used. The agreement
indicates that the same population of cosmic rays -- mostly protons --
found throughout the rest of the Galaxy is responsible for gamma rays
observed from the Galactic Centre. But the highest-energy share of
those particles, those reaching 1,000 TeV, move through the region
less efficiently than they do everywhere else in the Galaxy. That
results in a gamma-ray glow extending to the highest energies HESS
observed. The most energetic cosmic rays spend more time in the
central part of the Galaxy than previously thought, so they make a
stronger impression in gamma rays. That effect is not included in
conventional models of how cosmic rays move through the Galaxy.
But the researchers show that simulations incorporating that change
display even better agreement with Fermi data. The same breakneck
particle collisions responsible for producing the gamma rays should
also produce neutrinos, the fastest, lightest and least-understood
fundamental particles. Neutrinos travel straight to us from their
sources because they barely interact with other matter and because
they carry no electrical charge, so magnetic fields don't sway them.
Experiments like IceCube in Antarctica are detecting high-energy
neutrinos from beyond the Solar System, but pinpointing their sources
is much more difficult. The findings from Fermi and HESS suggest
that the Galactic Centre could be detected as a strong neutrino source
in the near future.
ONE OF BRIGHTEST GALAXIES EVER DISCOVERED
Instituto de Astrofisica de Canarias (IAC)
According to Einstein's theory of General Relativity, when a ray of
light passes close to a very massive object the gravity of the object
attracts the photons and deviates them from their initial path. That
phenomenon, known as gravitational lensing, is comparable to that
produced by lenses on light rays, and acts as a sort of magnifier,
changing the size and intensity of the apparent image of the original
object. Taking advantage of that effect, a team of scientists has
discovered a very distant galaxy, some 10 billion light-years away,
about a thousand times brighter than the Milky Way. It is the
brightest of the sub-millimetre galaxies, so called because of their
very strong emission in the far infrared. To measure it they used the
Gran Telescopio Canarias (GTC) on La Palma. Thanks to the
gravitational lens produced by a cluster of galaxies between ourselves
and the source, which acts as if it were a telescope, the galaxy appears
11 times bigger and brighter than it really is, and appears as several
images on an arc centred on the densest part of the cluster, which is
known as an 'Einstein Ring'. An advantage of that kind of
amplification is that it does not distort the spectral properties of the light,
which can be studied for even very distant objects as if they were much
nearer.
The galaxy is notable for having a high rate of star formation. It is
forming stars at a rate of 1000 solar masses per year, compared to the
Milky Way which is forming stars at a rate of some two a solar masses
a year. Such objects harbour the most powerful star-forming regions
known in the Universe. The next step will be to study their molecular
content. The fact that the galaxy is so bright, its light is gravi-
tationally amplifed, and has multiple images, allows us to look into
its internal properties, which would otherwise not be possible with
such distant galaxies. In the future we will be able to make more
detailed studies of its star formation using interferometers such as
the Northern Extended Millimeter Array (NOEMA/IRAM), in France, and
the Atacama Large Millimeter Array (ALMA), in Chile.
SUPERLUMINOUS SUPERNOVAE
RAS
The death of a massive star in a distant galaxy 10 billion years ago
created a rare superluminous supernova that astronomers say is one of
the most distant ever discovered. The brilliant explosion, more than
three times as bright as the 100 billion stars of our Milky Way
galaxy combined, occurred about 3.5 billion years after the Big Bang
when the rate of star formation in the Universe reached its peak.
Superluminous supernovae are 10 to 100 times brighter than the typical
supernova resulting from the collapse of a massive star. But astron-
omers still don't know exactly what kinds of stars give rise to their
extreme luminosity or what physical processes are involved. The
supernova known as DES15E2mlf is unusual even among the small number
of superluminous supernovae astronomers have detected so far. It was
initially detected in 2015 November by the Dark Energy Survey (DES)
collaboration using the Blanco 4-metre telescope at the Cerro Tololo
Inter-American Observatory in Chile. Follow-up observations to
measure the distance and obtain detailed spectra of the supernova were
conducted with the Gemini Multi-Object Spectrograph on the 8-metre
Gemini South telescope. The new observations may provide clues to the
nature of stars and galaxies during peak star formation. Supernovae
are important in the evolution of galaxies because their explosions
enrich the interstellar gas, from which new stars form with elements
heavier than helium (which astronomers call "metals"). Previous
observations of superluminous supernovae found they typically reside
in low-mass or dwarf galaxies, which tend to be less enriched in
metals than more massive galaxies. The host galaxy of DES15E2mlf,
however, is a fairly massive, normal-looking galaxy.
The current idea is that a low-metal environment is important in
creating superluminous supernovae, and that is why they tend to occur
in low-mass galaxies, but DES15E2mlf is in a relatively massive galaxy
compared to the typical host galaxy for superluminous supernovae.
Stars with fewer heavy elements retain a larger fraction of their
mass when they die, which may cause a bigger explosion when the star
exhausts its fuel supply and collapses. Metallicity affects the life
of a star and how it dies, so finding this superluminous supernova in
a higher-mass galaxy goes counter to current thinking. But we are
looking so far back in time, that galaxy would have had less time to
create metals, so it may be that at those earlier times in the
Universe's history, even high-mass galaxies had low enough metal
content to create such extraordinary stellar explosions. At some
point, the Milky Way also had those conditions and might also have
produced a lot of such explosions. Although many puzzles remain, the
ability to observe the unusual supernovae at such great distances
provides valuable information about the most massive stars and about
an important period in the evolution of galaxies. The Dark Energy
Survey has discovered a number of superluminous supernovae and
continues to see more distant cosmic explosions, revealing how stars
exploded during the strongest period of star formation.
AUSTRALIA ENTERS INTO PARTNERSHIP WITH ESO
ESO
An arrangement has been signed to begin a ten-year strategic partner-
ship between ESO and Australia. The partnership will further streng-
then ESO's programme, both scientifically and technically, and will
give Australian astronomers and industry access to the La Silla
Paranal Observatory. It may also be the first step towards Australia
becoming an ESO Member State. It means that Australia will contribute
financially to ESO for ten years, with the potential of then obtaining
full membership. The partnership will allow Australian astronomers to
participate in all activities relating to ESO's La Silla Paranal
Observatory facilities -- specifically, the Very Large Telescope, the
Very Large Telescope Interferometer, VISTA, VST, the ESO 3.6-metre
telescope, and the New Technology Telescope. The partnership will
also open opportunities for Australian scientists and industry to
collaborate with ESO Member State institutions on upcoming instruments
at those observatories.
Topic: Late July Astronomy Bulletin (Read 2763 times)