HOW NEUTRON STAR KICKS CAN BREAK UP CLUSTERS
RAS
A supernova explosion at the end of a large star's life can leave the
collapsed core, or neutron star, hurtling away from its dust and gas
envelope at hundreds of kilometres per second. Now, astronomers have
found that even a tiny number of such neutron-star 'natal kicks' can
have a dramatic effect on the lifetime of surrounding star clusters.
The fast-moving neutron stars can cause star clusters to lose mass and
break apart up to four times more quickly. Astronomers have known for
some time that there are two distinct scenarios for the break-up of
star clusters: a smooth, gradual loss and a sudden discarding of mass.
It is analogous to the difference between skiing down a gentle slope
and jumping off a cliff. Because the dividing line between the two
modes is very sharp, it is not surprising that a small effect can make
a big difference. The study suggests that natal kicks from neutron
stars could be one of the triggers that sends star clusters into
'jumping' modes of rapid dissolution rather than the more gradual
'skiing' one. While neutron-star natal kicks are known to occur,
their frequency, distribution and cause are still uncertain. The high
velocities from the kicks should send neutron stars beyond the
gravitational control of the star clusters in which they are born.
However, because the neutron stars are very hard to observe, their
retention fraction in globular clusters is still unknown. The team
ran a series of computer simulations of the evolution of clusters of
different sizes, either with or without neutron-star natal kicks.
They found that even though neutron stars accounted for less than 2
per cent of the star cluster by mass, the presence of kicks could have
a big effect on the cluster's evolution. In particular, the presence
of kicks and the 'jumping' mode of rapid mass loss meant that the
clusters never achieved the high degree of central concentration of
stars, observed in about 20 per cent of clusters, which is often
considered to be an outcome of the process of 'core collapse'. On the
other hand, all longer-lived ('skiing') models showed signatures of
core collapse, a phenomenon resulting from the slow dynamical
evolution of a stellar system, driven by gravitational encounters
between individual stars. It seems that apparently minor differences
can have very large effects.
LUMINOUS RED NOVA OUTBURST IS COLLIDING RED GIANT
RAS
Observations of a rare astronomical phenomenon, called a luminous red
nova, suggest that the bright outburst was caused by a red giant
colliding with another star. Astronomers used the Liverpool Telescope
to track a nova outburst over several months and hunted through the
Hubble telescope archive to identify possible progenitors. The
outburst was first observed in 2015 January in the Andromeda galaxy
(M31) by the Global MASTER Robotic Network, a Russian-led network of
telescopes dedicated to looking for transient objects in the night
sky. Initially, astronomers thought it was a classical nova, but,
watching the way its brightness evolved at different wavelengths, they
soon realised that the object was unusual and was a luminous red nova.
Classical novae are not particularly rare, with around 30 observed
each year in M31 alone. They are thought to occur in binary systems
when material falls onto the surface of a white dwarf from its larger
companion star, causing a relatively short burst of nuclear fusion.
By comparison, very few luminous red novae have been found to date.
Their cause is still uncertain, but they may be the result of two
stars merging together, causing a very sudden and dramatic brightening
of the system.
The Liverpool team first observed the new system, dubbed M31LRN 2015,
three days after its discovery. The outburst brightened over several
days. After reaching a peak, the nova faded quickly at blue
wavelengths, but remained bright at the redder wavelengths for several
weeks. Spectral analysis showed an initial burst of hydrogen
emission, and as it faded, features resembling those of a cool red
star emerged. The outburst was observed again in May, and had all but
disappeared in the optical, but was still bright in the infrared.
The team searched the Hubble archives for objects at the same place.
An image taken in 2004 showed the likely progenitor star: a red giant.
Interestingly, the system appears to have shown evidence of hydrogen
emission many years before the outburst, although the source of the
emission is not clear. The team found that M31LRN 2015 showed strong
similarities to other objects classified as luminous red novae. If a
single mechanism is responsible for all such events, the evidence
suggests merging stars as the cause.
BEST CANDIDATE SUPERNOVA?
RAS
Using the robotic Liverpool Telescope, an international team of
scientists has found what looks like the best pre-explosion candidate
yet for a type-1a supernova, where a massive and extremely dense
star in the Andromeda galaxy is dragging material away from its
companion. The star is destined to be completely destroyed in the
(astronomically) near future in a catastrophic explosion. Our Sun is
expected to have a relatively gentle end to its life, but some stars
have a more violent demise in prospect -- they are destined to explode
as supernovae, briefly shining as brightly as a whole galaxy of stars.
One class of such explosions, type-1a supernovae (SN1a), is
fundamental to our understanding of the evolution of the Universe.
Some binary systems of stars are particularly close together. Where
one of the stars is a white dwarf (the long-extinguished super-dense
remnant of a star that was once similar to the Sun), and the other is
a more normal companion, the gravity of the white dwarf fundamentally
changes both objects. The outer atmosphere of the normal star, mostly
hydrogen and helium, flows towards the white dwarf, forming a highly
compressed layer on its surface. Under the right conditions, that
material will heat up enough for runaway nuclear fusion to take place,
similar to that in a hydrogen bomb, but far more powerful than
anything possible on Earth. The explosion is a nova (meaning 'new
star'), and for a short period the system will have the brightness of
between 100 and 500 thousand Suns. Some, but by no means all, of the
accumulated material from the companion star will be ejected into
space.
Of the 400 novae that have been seen in our Galaxy, a few have been
seen to erupt more than once. Those 'recurrent novae' erupt
frequently, as the mass of the white dwarf is already high from the
millions of years of transfer of material and its companion star is
losing material at a high rate. In the Milky Way, the most active
recurrent nova is U Scorpii, which erupts about once a decade. But
the cycle of explosions cannot go on for ever. Once a white dwarf
accumulates close to 1.4 times the mass of the Sun, the 'critical
mass', its core temperature will have risen to around 500 million
degrees (30 times hotter than the centre of the Sun). The stellar
material subsequently undergoes another and much more powerful
thermonuclear reaction, in an enormous explosion that destroys the
white dwarf in a few seconds, releasing vast amounts of energy in the
process. That is a type-1a supernova, and for a number of days it has
the brightness of billions of Suns.
In 2008 scientists observed the eruption of a star, later confirmed to
be a nova, in the Andromeda galaxy (M31), the nearest large galaxy to
our own, some 2.5 million light-years away. Remarkably the same star,
catalogued as M31N 2008-12a, erupted again in 2009, 2011, 2012, 2013
and 2014. The team initiated a follow-up programme in 2013 and 2014,
using the Liverpool Telescope and X-ray observations from the orbiting
Swift observatory. Their work shows that in astronomical terms, M31N
2008-12a is on the brink of catastrophe. With explosions in rapid
succession, the white dwarf must be just a fraction under the critical
mass and could be torn to pieces in a supernova any time in the next
few hundred thousand years. The system is right on the cusp of total
destruction, so we are getting a first look at how it is changing
right before it erupts as a supernova. That could happen tomorrow, or
hundreds of thousands of years in the future. The international team
hopes to continue to monitor M31N 2008-12a for the foreseeable future.
Type-1a supernovae are all thought to have similar brightnesses, so
they are used as 'standard candles' to gauge the distances to galaxies
and measure the properties of the Universe as a whole. Understanding
systems like M31N 2008-12a is a key part of that.
FAR FEWER GALAXIES THAN MIGHT BE EXPECTED
Michigan State University
Over the years, the Hubble space telescope has allowed astronomers to
look deep into the Universe. The long view stirred theories of untold
thousands of distant, faint galaxies. New research, however, offers a
theory that reduces the estimated number of the most distant galaxies
by 10 to 100 times. Earlier estimates placed the number of faint
galaxies in the early Universe to be hundreds or thousands of times
larger than the few bright galaxies that we can actually see with
Hubble. Astronomers now think that the number could be closer to only
ten times larger. The team ran computer simulations to examine the
formation of galaxies in the early Universe. The team simulated
thousands of galaxies at a time, including the galaxies' interactions
through gravity or radiation. The simulated galaxies were consistent
with observed distant galaxies at the bright end of the distribution
-- in other words, those that have been discovered and whose existence
is confirmed. The simulations did not, however, show an exponentially
growing number of faint galaxies, as had previously been predicted.
The number of those at the lower end of the brightness distribution
was flat rather than increasing sharply. The simulations will be
tested further when the much-anticipated James Webb space telescope
comes into operation in late 2018. The improved technology will
afford astronomers even-more-detailed views of space than those that
Hubble has produced in recent years. The Hubble telescope can see
just the 'tip of the iceberg' of the most-distant galaxies. While the
Webb telescope will improve views of distant galaxies, it has a
relatively small field of view. As a result, the observations must
take into account cosmic variability -- the statistical variation in
the number of galaxies from place to place.
HIDDEN SUPER-MASSIVE BLACK HOLES REVEALED
RAS
Astronomers have found evidence for a large population of hidden
super-massive black holes in the Universe. Using NASA's Nuclear
Spectroscopic Telescope Array (NuSTAR) satellite observatory,
scientists detected the high-energy X-rays from five super-massive
black holes that are shielded from direct view by dust and gas.
The research, led by astronomers at Durham, supports the theory that
millions more super-massive black holes potentially exist, but are
hidden from view. The scientists pointed NuSTAR at nine candidate
hidden super-massive black holes that were thought to be extremely
active at the centres of galaxies, but where the full extent of such
activity was potentially obscured from view. High-energy X-rays found
from five of the galaxies confirmed that they possessed holes hidden
by dust and gas. The five were much brighter and more active than
previously thought, as they rapidly captured surrounding material and
emitted large amounts of radiation. Such observations were not
possible before NuSTAR, which was launched in 2012 and is able to
detect X-rays of much higher energy than previous satellites.
High-energy X-rays are more penetrating than low-energy ones, so we
can see deeper into the gas surrounding the black holes. NuSTAR
allows us to see how big the hidden objects are, and is helping us to
learn why only some black holes appear obscured.
RINGS AND LOOPS IN THE STARS
RAS
A ring of dust 200 light-years across, and a loop covering a third of
the sky, are among the results in a new map from the Planck satellite.
Planck, launched in 2009 to study the ancient light of the Big Bang,
has also made maps of our Galaxy in microwaves (cm to mm wavelengths).
Microwaves are generated by electrons spiralling in the Galaxy's
magnetic field at nearly the speed of light (the synchrotron process),
by collisions in interstellar plasma, by thermal vibration of inter-
stellar dust grains, and by 'anomalous' microwave emission (AME),
which may be from spinning dust grains. The relative strengths of
those processes change with wavelength, and can separated by the use
of multi-wavelength measurements from Planck, from the WMAP satellite,
and from ground-based radio telescopes, giving maps of each component.
The new maps show regions covering huge areas of our sky that produce
AME; that process, discovered only in 1997, could account for a large
amount of Galactic microwave emission with a wavelength near 1 cm.
One place where it is particularly bright is the 200-light-year-wide
dust ring around the Lambda Orionis nebula (the 'head' of Orion).
This is the first time that the ring has been seen in such a way. A
wide-field map also shows synchrotron loops and spurs (where charged
particles spiral around magnetic fields), including the huge Loop 1,
discovered more than 50 years ago. Remarkably, astronomers are still
very uncertain about its distance -- it could be anywhere between 400
and 25,000 light-years away -- and though it covers about a third of
the sky it is impossible to say exactly how big it is.
NEW SURVEY TO SHINE LIGHT ON DARK MATTER
ESO
Around 85% of the matter in the Universe is dark, and of a type not
understood by physicists. Although it does not shine or absorb light,
astronomers can detect dark matter through its effect on stars and
galaxies, specifically from its gravitational pull. A major project
with powerful survey telescopes is now showing more clearly than
before the relationships between dark matter and the shining galaxies
that we can observe directly. The project, known as the Kilo-Degree
Survey (KiDS), uses imaging from the VLT Survey Telescope (VST) at
Paranal and its huge camera,OmegaCAM. Sited at the Paranal Observa-
tory in Chile, that telescope is dedicated to surveying the night sky
in visible light, and it is complemented by the infrared survey
telescope VISTA. The survey may allow astronomers to make measure-
ments of dark matter, the structure of galaxy haloes, and evolution of
galaxies and clusters. The first KiDS results show to some extent how
the characteristics of the observed galaxies are determined by the
vast invisible clumps of dark matter surrounding them. One of the
major goals of the VST is to map out dark matter and to use the maps
to understand the mysterious 'dark energy' that is said to be causing
the Universe's expansion to accelerate.
The best way to work out where the dark matter lies is through
gravitational lensing -- the distortion of the Universe's fabric by
gravity, which deflects the light coming from distant galaxies far
beyond the dark matter. By studying that effect it is possible to map
out the places where gravity is strongest, and hence where the matter,
including dark matter, resides. The KiDS team has used that approach
to analyse images of over two million galaxies, typically 5.5 billion
light-years away. It has studied the distortion of light emitted from
the galaxies, which is deflected as it passes massive clumps of dark
matter during its journey. The first results come from only 7% of the
final survey area and concentrate on mapping the distribution of dark
matter in clusters of galaxies. Most galaxies live in clusters,
including our own Milky Way, which is part of the 'Local Group', and
understanding how much dark matter they contain is a test of the whole
theory of how galaxies form in the cosmic web. From the gravitational
lensing effect, the groups turn out to contain around 30 times more
dark than visible matter. Interestingly, the brightest galaxy nearly
always sits in the middle of the dark-matter clump. That feature of
galaxy formation, in which galaxies are sucked into groups and pile up
in the centre, has never been demonstrated so clearly before. The
findings are just the start of a major programme to exploit the data
coming from the survey telescopes.
Topic: Late July Astronomy Bulletin Part 2 (Read 1584 times)