Topic: Early November Astronomy Bulletin

[Clive]

EARTH'S DUST-CLOUD SATELLITES CONFIRMED
RAS

A team of Hungarian astronomers may have confirmed two elusive clouds of
dust, in semi-stable points just 400,000 kilometres from the Earth. The
clouds, first reported by and named for Polish astronomer Kazimierz
Kordylewski in 1961, are extraordinarily faint, so their existence is
controversial. The Earth--Moon system has five points of stability where
gravitational forces maintain the relative positions of objects located
there. Two of those so-called Lagrange points, L4 and L5, form an
equilateral triangle with the Earth and Moon, and move around the Earth as
the Moon moves along its orbit. L4 and L5 are not completely stable, as
they are disturbed by the gravitational pull of the Sun. Nonetheless they
are thought to be locations where interplanetary dust might collect, at
least temporarily. Kordylewski observed two nearby clusters of dust at L5
in 1961, with various reports since then, but their extreme faintness makes
them difficult to detect and many scientists doubted their existence. In a
paper earlier this year the Hungarian team modelled the Kordylewski clouds
to assess how they form and how they might be detected. The researchers
were interested in their appearance with polarising filters, which transmit
light with a particular direction of oscillation, similar to those in some
types of sunglasses; scattered or reflected light is always more or less
polarised, depending on the angle of scattering or reflection. They then
set out to find the dust clouds. With a linearly polarising filter system
attached to a camera lens and CCD detector, the scientists took exposures of
the purported location of the Kordylewski cloud at the L5 point. The images
they obtained show polarised light reflected from dust, extending well
outside the field of view of the camera lens.

The observed pattern matches predictions made by the same group of
researchers in an earlier paper and is consistent with the earliest
observations of the Kordylewski clouds 60 years ago. Astronomers were able
to rule out optical artefacts and other effects, so the presence of the dust
cloud is confirmed. The Kordylewski clouds are two of the toughest objects
to find, and though they are as close to the Earth as the Moon are largely
overlooked by researchers in astronomy. It is intriguing to confirm that
our planet has dusty pseudo-satellites in orbit alongside our lunar
neighbour. Given their stability, the L4 and L5 points are seen as
potential sites for orbiting space probes, and as transfer stations for
missions exploring the wider Solar System. There are also proposals to
store pollutants at the two points. Future research will look at L4 and L5,
and the associated Kordylewski clouds, to understand how stable they really
are, and whether their dust presents any kind of threat to equipment and
future astronauts.

 

MOON TO REVEAL SECRETS OF THE EARLY UNIVERSE
RAS

The Moon may be the key to unlocking how the first stars and galaxies shaped
the early Universe. A team of astronomers observed the Moon with a radio
telescope to help search for the faint signal from hydrogen atoms. Before
there were stars and galaxies, the Universe was pretty much just hydrogen,
floating around in space. Since there are no sources of the optical light
visible to our eyes, that early stage of the Universe is known as the
'cosmic dark ages'. The astronomers describe how they have used the
Murchison Widefield Array (MWA) radio telescope to help search for radio
signals given off by the hydrogen atoms.

The radio signal from the early Universe is very weak compared to the
extremely bright objects in the foreground, which include accreting black
holes in other galaxies and electrons in our own Milky Way. The key to
solving that problem is being able precisely to measure the average
brightness of the sky. However, built-in effects from the instruments and
radio-frequency interference make it difficult to get accurate observations
of the very faint radio signal. In their work, the astronomers used the
Moon as a reference point of known brightness and shape. That allowed them
to measure the brightness of the Milky Way at the position of the occulting
Moon. The astronomers also took into account 'earthshine' radio waves from
the Earth that reflect off the Moon and back onto the telescope. Earthshine
corrupts the signal from the Moon and the team had to remove that contam-
ination from their analysis. If astronomers can detect that radio signal it
will tell us whether our theories about the evolution of the Universe are
correct. With more observations, the astronomers hope to uncover the
hydrogen signal and put theoretical models of the Universe to the test.


A CHRISTMAS COMET
Spaceweather.com

Astronomers are calling Comet 46P/Wirtanen the "comet of the year". Seven
weeks from now, on Dec. 16, the kilometre-wide ball of dirty ice will come
within 11.5 million km of the Earth -- making it one of the 10 closest-
approaching comets of the Space Age. Comet 46P/Wirtanen will probably
become a naked-eye object for several weeks around Christmas. Pictures
taken with a 12-inch telescope show the comet's green atmosphere which is,
impressively, almost twice the angular size of the planet Jupiter. The
green colour comes from diatomic carbon (C2) -- a gaseous substance common
in comet atmospheres that glows green in the near-vacuum of space. At the
moment, the integrated brightness of the comet is similar to that of a
10th-magnitude star. However, it is expected to brighten more than 200-fold
by December. If current trends hold, 46P could ultimately reach magnitude
+3, making it not a Great Comet but a very good one, visible to the unaided
eye and an easy target for binoculars or small telescopes. Comet Wirtanen
passes through the inner Solar System every 5.4 years. Right now it is near
the orbit of Mars, and is heading in our direction.


ULTRA-CLOSE STARS INSIDE PLANETARY NEBULA
RAS

Astronomers have discovered two stars in a binary pair that complete an
orbit around one another in a little over three hours, residing in the
planetary nebula M3-1. Remarkably, the stars could drive a nova explosion,
an entirely unexpected event according to current understanding of binary-
star evolution. Planetary nebulae are the glowing shells of gas and dust
formed from the outer layers of stars like our own Sun, which they throw off
during the final stages of their evolution. In many cases, interaction with
a nearby companion star plays an important role in the ejection of that
material and the formation of the elaborate structures seen in the resulting
planetary nebulae. The planetary nebula M3-1 is located in the constella-
tion Canis Major, at a distance of roughly 14,000 light-years. M3-1 was a
firm candidate to host a binary central star, as its structure with prom-
inent jets and filaments is typical of binary-star interactions. Using the
telescopes of the European Southern Observatory in Chile, the team looked
at M3-1 over a period of several years. In the process they discovered and
studied the binary star in the centre of the nebula.

The two stars are so close together that they cannot be resolved from the
ground, so instead the presence of the second star is inferred from the
variation of their observed combined brightness -- most obviously by
periodic eclipses of one star by the other, which produce marked drops in
the brightness. The team discovered that the central star of the planetary
nebula M3-1 has one of the shortest orbital periods of binary central stars
known to date, at just over three hours. The ESO observations also show
that the two stars -- most likely a white dwarf with a low-mass main-
sequence companion -- are almost touching. As a result, the pair is likely
to undergo a nova eruption, the result of the transfer of material from one
star to the other. When the recipient star reaches a critical mass, a
violent thermonuclear explosion takes place and the system temporarily
increases in brightness by up to a million times. Theory suggests that
binary stars should be well separated after the formation of a planetary
nebula. It should then take a long time before they begin to interact again
and events such as novae become possible. In 2007, astronomers observed a
different nova explosion, known as Nova Vul 2007, inside another planetary
nebula. The 2007 event was particularly difficult to explain. By the time
the two stars are close enough for a nova, the material in the planetary
nebula should have expanded and dissipated so much that it is no longer
visible. The new event adds to the conundrum. Among the stars in the
centre of M3-1, there is another candidate for a similar nova eruption in
the relatively near future.


SUPERMASSIVE BLACK HOLE MERGERS
RAS

New research has found evidence for a large number of double supermassive
black holes, probably precursors of gigantic black-hole merging events.
That confirms the current understanding of cosmological evolution -- that
galaxies and their associated black holes merge over time, forming bigger
and bigger galaxies and black holes. Astronomers have looked at radio maps
of powerful jet sources and found signs that would usually be present when
looking at black holes that are closely orbiting each other. Before black
holes merge they form a binary black hole, where the two black holes orbit
around one another. Gravitational-wave telescopes have been able to record
the merging of smaller black holes since 2015, by measuring the strong
bursts of gravitational waves that are emitted when binary black holes
merge, but current technology cannot be used to demonstrate the presence of
supermassive binary black holes. Supermassive black holes emit powerful
jets. When supermassive binary black holes orbit, they cause the jet
emanating from the nucleus of a galaxy periodically to change its direction.
Astronomers studied the directions in which such jets are emitted, and
variances in those directions; they compared the direction of the jets with
that of one of the radio lobes (that store all the particles that ever went
through the jet channels) to demonstrate that that method can be used to
indicate the presence of supermassive binary black holes. The fact that
the most powerful jets are associated with binary black holes could have
important consequences for the formation of stars in galaxies: stars form
from cold gas, jets heat that gas and thus suppress the formation of stars.
A jet that always heads in the same direction only heats a limited amount of
gas in its vicinity. However, jets from binary black holes change direction
continuously. Therefore, they can heat much more gas, suppressing the
formation of stars much more efficiently, and thus contributing towards
keeping the number of stars in galaxies within the observed limits.


KIN OF GRAVITATIONAL-WAVE SOURCE DISCOVERED
University of Maryland

On 2017 October 16, an international group of astronomers and physicists
excitedly reported the first simultaneous detection of light and
gravitational waves from the same source -- a merger of two neutron stars.
Now, a of astronomers has identified a direct relative of that historic
event. The newly described object, named GRB150101B, was reported as a
gamma-ray burst localized by NASA's Neil Gehrels Swift Observatory in 2015.
Follow-up observations suggest that GRB150101B shares remarkable similari-
ties with the neutron-star merger, named GW170817, discovered by the Laser
Interferometer Gravitational-wave Observatory (LIGO) and observed by
multiple light-gathering telescopes in 2017. A new study suggests that
those two separate objects may, in fact, be directly related. The team
suspects that both GRB150101B and GW170817 were produced by the same
 type of event: a merger of two neutron stars. Such catastrophic coalescences
each generated a narrow jet, or beam, of high-energy particles. The jets each
produced a short, intense gamma-ray burst (GRB) -- a powerful flash that
lasts only a few seconds. GW170817 also created ripples in space-time
called gravitational waves, suggesting that that might be a common feature
of neutron-star mergers. The apparent match between GRB150101B and
GW170817 is striking: both produced an unusually faint and short-lived
gamma-ray burst and both were sources of bright, blue optical light and
long-lasting X-ray emission. The host galaxies are also remarkably similar,
according to HST observations. Both are bright elliptical galaxies with a
population of stars a few billion years old that display no evidence of new star
formation.

In the cases of both GRB150101B and GW170817, the explosion was probably
viewed 'off-axis', that is, with the jet not pointing directly towards the
Earth. So far, those events are the only two off-axis short GRBs that
astronomers have identified. The optical emission from GRB150101B is
largely in the blue portion of the spectrum, providing an important clue
that that event is another kilonova, as seen in GW170817. A kilonova is a
luminous flash of radioactive light that produces large quantities of
important elements like silver, gold, platinum and uranium. While there are
many commonalities between GRB150101B and GW170817, there are two very
important differences. One is their location: GW170817 is relatively close,
at about 130 million light-years from the Earth, while GRB150101B lies about
1.7 billion light-years away. The second important difference is that,
unlike GW170817, gravitational-wave data do not exist for GRB150101B.
Without that information, the team cannot calculate the masses of the two
objects that merged. It is possible that the event resulted from the merger
of a black hole and a neutron star, rather than two neutron stars. It is
possible that a few mergers like the ones seen in GW170817 and GRB150101B,
have been detected previously, but were not properly identified using
complementary observations in different wavelengths of light, according to
the researchers. Without such detections -- in particular, at longer
wavelengths such as X-rays or optical light -- it is very difficult to
determine the precise location of events that produce gamma-ray bursts. In
the case of GRB150101B, astronomers at first thought that the event might
coincide with an X-ray source detected by Swift in the centre of the galaxy.
The most likely explanation for such a source would be a supermassive black
hole devouring gas and dust. However, follow-up observations with Chandra
placed the event further away from the centre of the host galaxy. According
to the researchers, even if LIGO had been operational in early 2015, it
would very likely not have detected gravitational waves from GRB150101B
because of the event's greater distance from the Earth. All the same, every
new event observed with both LIGO and multiple light-gathering telescopes is
likely to fit important new pieces into the puzzle.


LARGEST GALAXY PROTO-SUPERCLUSTER FOUND
ESO

A team of astronomers has used the VIMOS instrument on the Very Large
Telescope (VLT) to identify a gigantic proto-supercluster of galaxies
forming in the early Universe, just 2.3 billion years after the Big Bang.
That structure, which the researchers nicknamed Hyperion, is the largest and
most massive structure to be found so early in the formation of the
Universe. The enormous mass of the proto-supercluster is calculated to be
more than 10 to the 15 times that of the Sun. That titanic mass is similar
to that of the largest structures observed in the Universe today, but
finding such a massive object in the early Universe surprised astronomers.
Located in the COSMOS field in the constellation Sextans, Hyperion was
identified by analyzing the vast amount of data obtained from the VIMOS
Ultra-deep Survey which provides a 3D map of the distribution of over 10,000
galaxies in the distant Universe. The team found that Hyperion has a very
complex structure, containing at least 7 high-density regions connected by
filaments of galaxies, and its size is comparable to that of nearby super-
clusters, though it has a very different structure. Superclusters closer to
the Earth tend to have a much more concentrated distribution of mass, with
clear structural features. But in Hyperion, the mass is distributed much
more uniformly in a series of connected blobs, populated by loose associa-
tions of galaxies. That contrast is most likely due to the fact that nearby
super-clusters have had billions of years for gravity to gather matter
together into denser regions -- a process that has been acting for far less
time in the much younger Hyperion. Given its size so early in the history
of the Universe, Hyperion is expected to evolve into something similar to
the immense structures in the local Universe such as the superclusters
making up the Sloan Great Wall or the Virgo Supercluster that contains our
own galaxy, the Milky Way. Understanding Hyperion and how it compares to
similar recent structures may give insights into how the Universe developed
in the past and will evolve into the future, and allow us the opportunity to
challenge some models of supercluster formation.


HUBBLE SPACE TELESCOPE BACK IN ACTION
Cornell University

The Hubble Space Telescope has resumed normal operations after an anxious
few weeks when it looked as if the stalwart spacecraft was on its last legs.
Hubble reported itself offline on its Twitter account early in October and
NASA explained that the craft had entered safe mode after one of the three
gyroscopes used to point and steady the telescope had failed. According to
the space agency, the gyro that failed had been "exhibiting end-of-life
behaviour", which is no surprise given that Hubble was originally a 15-year
mission and has now been scanning the Universe for over 28 years. A backup
enhanced gyro should have seamlessly taken over when the original failed,
but it initially refused to perform, at a level required for operations,
after being switched off for 7.5 years. To get it going again, Hubble
technicians basically instructed the telescope to jiggle around so as to
shake any blockages out of the gyro, and then switched it off and on again
and into different modes a few times. That strategy worked, and Hubble
returned to normal operations on October 26.

Hubble's life-span is now almost twice what was originally intended, and its
successor, the James Webb Space Telescope, will be capable of feats that
Hubble can only dream of. However, the powerful new spacecraft has been
beset by delays over the last 20 years and its launch date has been
postponed once again from May 2020 to March 2021. The project has over-run
its budget many times over and NASA now believes the end total will creep
over $8bn. If that happens, the agency will have to apply for re-author-
ization from Congress for the space telescope. That might seem like an
astronomical sum to pay to further our understanding of the Universe, but
the James Webb could potentially revolutionize that understanding by making
observations of the earliest moments after the Big Bang and examining our
Solar System and nearby exoplanets in detail that has been impossible until
now. In the meantime, however, scientists are still reliant on booking time
with observatories like Hubble.


SPACECRAFT MAKES RECORD CLOSE APPROACH TO THE SUN
Spaceweather.com

NASA's Parker Solar Probe is now closer to the Sun than any other spacecraft
in history, breaking the previous record of 26.6 million miles set by the
Helios 2 spacecraft in 1976. The probe is now well inside the orbit of
Mercury. At closest approach, the solar disc will seem 6 times wider than
it does from the Earth, as the probe is hit by "brutal heat and radiation".
Parker's carbon-composite heat shield is expected to heat up to 2000 deg. F.
The prime mission is to investigate the origin of the solar wind -- a project
best done uncomfortably close to the star. Parker will trace the solar wind
back to its source and find out how it escapes the Sun's gravity and
magnetic confinement. The probe's wide-field camera system, WISPR, can
actually see the solar wind, allowing it to image clouds and shock waves as
they approach and pass the spacecraft. Other sensors on the spacecraft will
sample the structures that WISPR sees, making measurements of particles and
fields that researchers can use to test competing theories. Parker will
plunge towards the Sun 24 more times in the next 8 years.