Topic: Late October Astronomy Bulletin

[Clive]

THE CHILL OF SOLAR MINIMUM
Spaceweather.com

The Sun is entering one of the deepest Solar Minima of the Space Age.
Sunspots have been absent for most of 2018, and the Sun's ultraviolet output
has sharply dropped.  New research shows that the Earth's upper atmosphere
is responding.  NASA scientists say that high above the Earth's surface,
near the edge of space, our atmosphere is losing heat energy.  If current
trends continue, it could soon set a Space-Age record for cold.  These
results come from the SABER instrument onboard NASA's TIMED satellite.
SABER monitors infrared emissions from carbon dioxide (CO2) and nitric oxide
(NO), two substances that play a key role in the energy balance of air 100
to 300 kilometres above our planet's surface.  By measuring the infrared
glow of those molecules, SABER can assess the thermal state of gas at the
very top of the atmosphere -- a layer that researchers call the thermo-
sphere.  The thermosphere always cools off during Solar Minimum.  It is one
of the most important ways the solar cycle affects our planet.  When the
thermosphere cools, it shrinks, literally decreasing the radius of the
Earth's atmosphere.  The shrinkage decreases aerodynamic drag on satellites
in low Earth orbit, extending their lifetimes.  That's the good news.  The
bad news is, it also delays the natural decay of space junk, resulting in a
more cluttered environment around the Earth.  SABER is currently measuring
33 US-billion watts of infrared power from NO.  That is 10 times smaller
than we see during more active phases of the solar cycle.  As 2018 nears
its end, the Thermosphere Climate Index is on the verge of setting a Space-
Age record for cold.
 

SPACE MISSIONS TO EUROPA MAY BE DIFFICULT
Cardiff University     
 
New research has revealed that Europa may be a tricky place for spacecraft
to land.  A team of scientists has predicted that fields of sharp ice
growing to almost 15 metres tall could be scattered across the equatorial
regions of the Jupiter moon.  Previous space missions have already
identified Europa as one of the likeliest places for harbouring life
in our Solar System, most notably because of the large seas of liquid water
underneath its surface.  The study shows that any potential landing mission
may have to navigate hazardous obstacles known as 'penitentes' before
touching down on Europa's surface.  Penitentes are tall sharp-edged blades
and spikes made of snow and ice that point towards the midday sun.  They
form through a process known as sublimation, which requires bright,
sustained sunlight as well as cold, dry and still air.  Sublimation is a
process through which ice turns directly into water vapour without melting
into a liquid first.  When sublimation occurs, the distinctive blade-like
formations are left behind.  Penitentes are present on Earth and grow to
between 1 and 5 metres tall, but they are restricted to high-altitude
tropical and sub-tropical conditions, such as in the Andes.  Europa however
has the perfect conditions necessary for penitentes to form more uniformly
-- its surface is dominated by ice; it has the thermal conditions needed for
ice to sublime without melting; and there is very little variation in the
angle in which the Sun shines on the surface.  In their study, the
researchers used observational data to calculate the sublimation rates at
various points on Europa's surface and used them to estimate the size and
distribution of penitentes.  They concluded that the penitentes could
potentially grow to around 15 metres tall with a spacing of around 7.5
metres between them.  It was also inferred that the penitentes would be more
common around Europa's equator.  No spacecraft has yet landed on Europa;
however, NASA intends to undertake a number of fly-bys around the moon with
the Europa Clipper, which is to be launched in 2022.  It is believed that a
landing mission could follow soon after.


EXTREMELY DISTANT SOLAR SYSTEM OBJECT FOUND
Carnegie Institution for Science     
 
A team of astronomers has discovered a new extremely distant object far
beyond Pluto, with an orbit that supports the presence of an even-farther-
out, Super-Earth or larger Planet X.  The newly found object, called 2015
TG387, was discovered about 80 astronomical units (AU) from the Sun.  Pluto
is around 34 AU, so 2015 TG387 is about two and a half times further away
from the Sun than Pluto is right now.  The new object is on a very elongated
orbit and never comes closer to the Sun than about 65 AU.  Only 2012 VP113
and Sedna at 80 and 76 AU respectively have more-distant perihelia than
2015 TG387. Although 2015 TG387 has the third-most-distant perihelion, its
orbital semi-major axis is larger than those of 2012 VP113 and Sedna,
meaning that it travels much farther from the Sun than they do.  At its
furthest point, it reaches out to about 2,300 AU.  2015 TG387 is one of the
few known Solar-System objects that never come close enough to the giant
planets, like Neptune and Jupiter, to have significant gravitational
interactions with them.  Astronomers think that there could be thousands of
small bodies like 2015 TG387 out in fringes of the Solar System, but their
distance makes finding them very difficult.  Currently we would only detect
2015 TG387 when it is near its closest approach to the Sun.  For some 99
percent of its 40,000-year orbit, it would be too faint to see.  The object
was discovered as part of the team's ongoing hunt for unknown dwarf planets
and Planet X.  That is the largest and deepest survey ever conducted for
distant Solar-System objects.  Such distant objects are like breadcrumbs
leading us to Planet X.  The more of them we can find, the better we can
understand the outer Solar System and the possible planet that we think is
shaping their orbits -- a discovery that would redefine our knowledge of the
Solar System's evolution.

It took the team a few years of observations to obtain a good orbit for 2015
TG387 because it moves so slowly and has such a long orbital period.  They
first observed it in 2015 October with the Japanese Subaru 8-m telescope on
Mauna Kea in Hawaii.  Follow-up observations at the Magellan telescope at
Carnegie's Las Campanas Observatory in Chile and the Discovery Channel
Telescope in Arizona were obtained in 2015, 2016, 2017 and 2018 to determine
its orbit.  2015 TG387 is probably at the small end of dwarf-planet sizes,
since it has a diameter near 300 km.  The location in the sky where 2015
TG387 reaches perihelion is similar to that of 2012 VP113, Sedna, and indeed
of those of most other known extremely distant trans-Neptunian objects,
suggesting that something is pushing them into similar types of orbits.
Astronomers ran computer simulations for how different hypothetical Planet X
orbits would affect the orbit of 2015 TG387.  The simulations included a
Super-Earth-mass planet at several hundred AU in an elongated orbit.  Most
of the simulations showed that not only was 2015 TG387's orbit stable for
the age of the Solar System, but it was actually shepherded by Planet X's
gravity, which keeps the smaller 2015 TG387 away from the massive planet.
That gravitational shepherding could explain why the most-distant objects in
the Solar System have similar orbits.  Those orbits prevent them from ever
approaching the proposed planet too closely, which is similar to how Pluto
never gets too close to Neptune even though their orbits cross.  Planet X
seems to affect 2015 TG387 the same way as all the other extremely distant
Solar-System objects.  These simulations do not prove that there is another
massive planet in the Solar System, but they are further evidence that
something big could be out there.
 

VOYAGER 2 NEARING INTERSTELLAR SPACE
NASA

The Voyager 2 probe, currently on a journey towards interstellar space,
has detected an increase in cosmic rays that originate outside the Solar
System.  Launched in 1977, Voyager 2 is 17.7 US-billion kilometres from the
Earth, or more than 118 times the distance from the Earth to the Sun.  Since
2007 the probe has been travelling through the outermost layer of the helio-
sphere -- the vast bubble around the Sun and the planets dominated by solar
material and magnetic fields.  Voyager scientists have been watching for the
spacecraft to reach the outer boundary of the heliosphere, known as the
heliopause.  Once Voyager 2 exits the heliosphere, it will become the second
man-made object, after Voyager 1, to enter interstellar space.  Since late
August, the Cosmic Ray Subsystem instrument on Voyager 2 has measured about
a 5% increase in the rate of cosmic rays hitting the spacecraft compared to
early August.  The probe's Low-Energy Charged Particle instrument has
detected a similar increase in higher-energy cosmic rays.  Cosmic rays are
fast-moving particles that originate outside the Solar System.  Some of them
are blocked by the heliosphere, so mission planners expect that Voyager 2
will measure an increase in the rate of cosmic rays as it approaches and
crosses the boundary of the heliosphere.  In 2012 May, Voyager 1 experienced
an increase in the rate of cosmic rays similar to the one that Voyager 2 is
now detecting.  That was about three months before Voyager 1 crossed the
heliopause and entered interstellar space.  However, Voyager team members
note that the increase in cosmic rays is not a definitive sign that the
probe is about to cross the heliopause.  Voyager 2 is in a different
location in the heliosheath -- the outer region of the heliosphere -- from
Voyager 1, and possible differences in their locations means Voyager 2 may
experience a different exit time-line from Voyager 1.  The fact that Voyager
2 may be approaching the heliopause six years after Voyager 1 is also
relevant, because the heliopause moves in and out during the Sun's 11-year
activity cycle.  Solar activity refers to emissions from the Sun, including
solar flares and eruptions of material called coronal mass ejections.
During the 11-year solar cycle, the Sun reaches both a maximum and a minimum
level of activity.

 
A MOON OUTSIDE THE SOLAR SYSTEM
Columbia University

Astronomers using the Hubble Space Telescope and Kepler Space Telescope
have assembled compelling evidence for the existence of a moon orbiting a gas-
giant planet 8,000 light-years away.  The detection of a candidate exo-moon
-- that is, a moon orbiting a planet in another star system -- is unusual
because of its large size, comparable to that of Neptune.  Such gargantuan
moons do not exist in our own Solar System, where nearly 200 natural
satellites have been catalogued.  In looking for exo-moons, the researchers
analyzed data from 284 Kepler-discovered planets that were in comparatively
large orbits, with periods greater than 30 days, around their host stars.
The observations measured the momentary dimming of starlight as a planet
passed in front of its star, called a transit.  The researchers found one
instance, in Kepler 1625b, that had intriguing anomalies.  The Kepler
results were enough for the team to get 40 hours of time with the Hubble
telescope to study the planet intensively, obtaining data four times more
precise than those of Kepler.  The researchers monitored the planet before
and during its 19-hour-long transit across the face of the star.

After it ended, Hubble detected a second and much smaller decrease in the
star's brightness 3.5 hours later, consistent with an origin from a moon
trailing the planet.  Unfortunately, the scheduled Hubble observations ended
before the complete transit of the moon could be measured.  In addition to
that dip in light, Hubble provided supporting evidence for the moon hypothe-
sis by measuring that the planet began its transit 1.25 hours earlier than
predicted.  That is consistent with the planet and moon orbiting a common
centre of gravity that would cause the planet to wobble from its predicted
location.  An extra-terrestrial civilization watching the Earth and Moon
transit the Sun would note similar anomalies in the timing of the Earth's
transit.

The researchers note that in principle the anomaly could be caused by the
gravitational pull of a hypothetical second planet in the system, although
Kepler found no evidence for additional planets around the star during its
four-year mission.  A companion moon is the simplest and most natural
explanation for the second dip in the light-curve and the orbit-timing
deviation.  The moon is estimated to be only 1.5 per cent the mass of its
companion planet, which itself estimated to be several times the mass of
Jupiter.  That value is close to the mass-ratio between the Earth and its
Moon.  But in the case of the Earth--Moon system and the Pluto-Charon system
-- Charon is the largest of the five known satellites of Pluto -- an early
collision with a larger body is hypothesized to have blasted off material
that later coalesced into a moon.  Kepler 1625b and its satellite, however,
are gaseous, not rocky, and, therefore, such a collision may not lead to the
condensation of a satellite.  Exo-moons are difficult to find because they
are smaller than their companion planet and so their transit signal is weak;
they also shift position with each transit because the moon is orbiting the
planet.  In addition, the ideal candidate planets hosting moons are in large
orbits, with long and infrequent transit times.  In this search, the
Neptune-sized moon would have been among the easiest to detect because of
its large size.  The host planet and its moon lie within the solar-mass
star's (Kepler 1625's) habitable zone, where moderate temperatures allow the
existence of liquid water on any solid planetary surface.


COLLISION OF WHITE AND BROWN DWARF STARS
Keele University
     
A team of astronomers using the Atacama Large Millimeter/submillimeter Array
(ALMA) in Chile has found evidence that a white dwarf (the remains of a star
like the Sun at the end of its life) and a brown dwarf (a 'failed' star
without sufficient mass to sustain thermonuclear fusion) collided in a
short-lived blaze of glory that was witnessed on Earth in 1670 as Nova Cygni
-- 'a new star below the head of the Swan'.  The brown-dwarf star was
'shredded' and dumped on the surface of a white-dwarf star, leading to the
1670 eruption.  In July of 1670, observers on Earth witnessed a 'new star',
or nova, in the constellation Cygnus.  Where previously there was no obvious
star, there abruptly appeared a star as bright as those in the Plough, that
gradually faded, reappeared, and finally disappeared from view.  Modern
astronomers studying the remains of that cosmic event initially thought it
was triggered by the merging of two main-sequence stars -- stars on the same
evolutionary path as our Sun.  The so-called 'new star' was long referred to
as 'Nova Vulpeculae 1670', and later became known as CK Vulpeculae.
However, we now know that CK Vulpeculae was not what we would today
describe as a 'nova', but is in fact the result of a merger of two stars -- a white
dwarf and a brown dwarf.  By studying the debris from that event -- which
takes the form of dual rings of dust and gas, resembling an hourglass with a
compact central object -- the research team concluded that a brown dwarf had
merged with a white dwarf.

The team of astronomers examined the remains of the merger, with some
interesting findings.  By studying the light from two more distant stars as
they shine through the dusty remains of the merger, the researchers were
able to detect the signature of the element lithium, which is easily
destroyed in stellar interiors.  The presence of lithium, together with
unusual isotopic ratios of the elements C, N, and O, indicate that an
(astronomically!) small amount of material, in the form of a brown-dwarf
star, crashed onto the surface of a white dwarf in 1670, leading to
thermonuclear 'burning', an eruption that led to the brightening seen by the
Carthusian monk Anthelme and the astronomer Hevelius, and in the hourglass
that we see today.  Stellar collisions are among the most violent events in
the Universe.  Most attention is given to collisions between neutron stars,
between two white dwarfs -- which can give a supernova -- and star-planet
collisions.  But it is very rare actually to see a collision, and where we
believe one occurred, it is difficult to know what kind of stars collided.
The type that we believe happened in this case is a new one, not previously
considered or ever seen before.  It is an extremely exciting discovery.
The white dwarf would have been about 10 times more massive than the brown
dwarf, so as the brown dwarf spiralled into the white dwarf it would have
been ripped apart by the intense tidal forces exerted by the white dwarf.

When the two objects collided, they spilled out a cocktail of molecules and
unusual element isotopes, which provide compelling evidence of the true
origin of the blast.  This is the first time that such an event has been
conclusively identified.  Intriguingly, the hourglass is also rich in
organic molecules such as formaldehyde (H2CO), methanol (CH3OH) and
methanamide (NH2CHO).  Those molecules would not survive in an environment
undergoing nuclear fusion and must have been produced in the debris from the
explosion.  That lends further support to the conclusion that a brown dwarf
met its demise in a star-on-star collision with a white dwarf.  Since most
star systems in the Milky Way are binary, stellar collisions are not that
rare.  The material will eventually become part of a new planetary system.

 
CENTRE OF MILKY WAY MYSTERY SOLVED
Lund University   

Astronomers have now found the explanation of a recent mystery at the centre
of the Milky Way galaxy: the high levels of scandium discovered last spring
near the galaxy's giant black hole were in fact an optical illusion.  Last
spring, researchers published a study about the apparent presence of aston-
ishing and dramatically high levels of three different elements in red giant
stars located less than three light-years away from the massive black hole
at the centre of our galaxy.  Various possible explanations were presented,
for example that the high levels were a result of earlier stars being
disrupted as they fell into the black hole, or a result of debris from the
collisions of neutron stars.  Now another group of astronomers has found an
explanation for the high levels of scandium, vanadium and yttrium.  They
argue that the so-called spectral lines presented last spring were actually
an optical illusion.  According to the new study, the lower temperatures of
the giant stars helped to create the optical illusion that appeared in the
measurements of spectral lines.  Specifically, it means that the electrons
in the elements behave differently at different temperatures, which in turn
can be misleading when measuring the spectral lines of elements in different
stars.  The conclusion is the result of a close collaboration between
astronomers and atomic physicists.  The research team is currently conduct-
ing a comprehensive mapping of the central area of the Milky Way, exploring
the spectral lines in the light from different stars to find out which ele-
ments they contain.  The purpose is to gain an understanding of the events
that have occurred in the history of the Milky Way, but also to understand
how galaxies in general have formed.  The spectral lines of different
elements are recorded by a high-resolution spectrometer.  The research team
has studied the near-infrared part of the spectrum, i.e. the heat radiation
emitted by the stars.  The reason for that is that infrared light can
penetrate the dust that obstructs the line of sight between us and the
centre of the Milky Way, approximately 25,000 light-years away. The
technology for recording such light is quite novel, and has only recently
become available to astronomers.


STARS FLYING BETWEEN GALAXIES
RAS
 
A team of astronomers using the latest set of data from ESA's Gaia mission
to look for high-velocity stars being kicked out of the Milky Way was
surprised to find stars instead rushing inwards -- perhaps from another
galaxy.  Stars circle around our Galaxy at hundreds of kilometres per
second, and their motions contain a wealth of information about the past
history of the Galaxy.  The fastest class of those stars are called hyper-
velocity stars, which are thought to start their lives near the Galactic
centre, later to be flung towards the edge of the Milky Way by interactions
with the black hole at its centre.  Only a small number of hyper-velocity
stars has ever been discovered, and Gaia's recently published second data
release provides an opportunity to look for more of them.  Of the seven
million Gaia stars with full 3D velocity measurements, scientists found
twenty that could be travelling fast enough to escape eventually from the
Milky Way.  However, the team was in for a surprise: rather than flying
away from the Galactic centre, most of the high-velocity stars seem to be
racing towards it.

It is possible that those intergalactic interlopers come from the Large
Magellanic Cloud, a relatively small galaxy orbiting the Milky Way, or they
may originate from a galaxy even further afield.  If that is the case, they
carry the imprint of their site of origin, and studying them at much closer
distances than their parent galaxy could provide unprecedented information
on the nature of stars in another galaxy -- similar in a way to studying
Martian material brought to our planet by meteorites.  Stars can be
accelerated to high velocities when they interact with super-massive black
holes.  So the presence of these stars might be a sign of such black holes
in nearby galaxies.  But the stars may also once have been parts of binary
systems, flung towards the Milky Way when their companion stars exploded as
supernovae.  Either way, studying them could tell us more about that kind of
process in nearby galaxies.  An alternative explanation is that the newly
identified sprinting stars could be native to our Galaxy's halo, accelerated
and pushed inwards through interactions with one of the dwarf galaxies that
fell towards the Milky Way during its build-up history.  Additional inform-
ation about the age and composition of the stars could help the astronomers
clarify their origin.  New data will help to nail down the nature and origin
of these stars with more certainty, and the team will use ground-based
telescopes to find out more about them.  At least two more Gaia data releases
are planned in the 2020s, and each will provide both more-precise and new
information on a larger set of stars.
 
 
ANOTHER 20 FAST RADIO BURSTS OBSERVED
Swinburn University of Technology

Another 20 fast radio bursts (FRBs) have been observed by Australian
astronomers.  The astrophysical mysteries -- brief, bright flashes of radio
waves that last a few milliseconds -- are thought to originate in distant
galaxies.  Their source, however, remains unclear.  FRBs have the potential
to help scientists understand the structure of matter in the Universe.  A
fast radio burst leaves a distant galaxy and travels to Earth over billions
of years, occasionally passing through a cloud of gas during its journey.
Each time hot gas (or plasma) is encountered, the different wavelengths that
make up a burst are slowed by different amounts.  Short radio waves, for
example, arrive at a terrestrial telescope before long ones.  This is called
dispersion.  The amount of dispersion tells us how much matter the bursts
have travelled through, and until now it has not been clear where that
matter is.  The data, collected by the Australian Square Kilometre Array
Pathfinder (ASKAP) radio telescope, points to the cosmic web -- the region
between star systems.  It also says that the bursts are coming from vast
distances -- from galaxies halfway across the Universe.  None of the FRBs
repeated.  That makes the bursts different from the best-studied, known as
FRB 121102 -- aptly called the repeater -- from which hundreds of pulses
have been detected.  Moving forward, the team hopes to tie bursts to host
galaxies and accurately measure their distances, eventually making a 3D map
of the cosmic web.  This has been a banner year for fast radio bursts.
Australia's Parkes Observatory in March reported three, including one (FRB
180309) with the highest signal-to-noise ratio to date.  And this summer's
FRB 180725A marks the first detection of a blast under 700 megahertz --as
low as 580 MHz.  Last month, researchers used machine learning to discover
72 new fast radio bursts from a mysterious source 3 US-billion light-years
from the Earth.


BLACK HOLES ARE NOT MISSING DARK MATTER
University of California - Berkeley

For one brief shining moment after the 2015 detection of gravitational waves
from colliding black holes, astronomers held out hope that the Universe's
mysterious dark matter might consist of a plenitude of black holes sprinkled
throughout the Universe.  Now physicists have dashed those hopes.  On the
basis of a statistical analysis of 740 of the brightest supernovae discover-
ed as of 2014, and the fact that none of them appears to be magnified or
brightened by hidden black-hole gravitational lenses, the researchers
concluded that primordial black holes can make up no more than about 40 per
cent of the dark matter in the Universe.  Primordial black holes could only
have been created within the first milliseconds of the Big Bang, as regions
of the Universe with a concentrated mass tens or hundreds of times that of
the Sun collapsed into objects a hundred kilometres across.  The results
suggest that none of the Universe's dark matter consists of massive black
holes, or any similar objects, including massive compact halo objects,
so-called MACHOs.  Dark matter is one of astronomy's most embarrassing
conundra: despite its comprising 84.5% of the matter in the Universe, no one
can find it.  Proposed dark-matter candidates span nearly 90 orders of
magnitude in mass, from ultralight particles like axions to MACHOs.  Several
theorists have proposed scenarios in which there are multiple types of dark
matter.  But if dark matter consists of several unrelated components, each
would require a different explanation for its origin, which makes the models
very complex.  An as-yet-unpublished re-analysis by the same team using an
updated list of 1,048 supernovae cuts the limit to a maximum of about 23%,
further slamming the door on the dark-matter--black-hole proposal.

Their conclusions are based on the fact that an unseen population of
primordial black holes, or any massive compact object, would gravitationally
bend and magnify light from distant objects on its way to the Earth.  There-
fore, gravitational lensing should affect the light from distant Type Ia
supernovae.  Those are the exploding stars that scientists have used as
standard brightness sources to measure cosmic distances and document the
expansion of the Universe.  The team conducted a complex statistical
analysis of data on the brightness and distance of supernovae catalogued in
two compilations -- 580 in the Union and 740 in the joint light-curve
analysis (JLA) catalogues -- and concluded that eight should be brighter by
a few tenths of a per cent than predicted on the basis of observations of
how those supernovae brighten and fade over time.  No such brightening has
been detected.  Other researchers have performed similar but simpler
analyses that yielded inconclusive results.  But the new research incorpor-
ated the precise probability of seeing all magnifications, from small to
huge, as well as uncertainties in brightness and distance of each supernova.
Even for low-mass black holes -- those 1% the mass of the Sun -- there
should be some highly magnified distant supernovae, but there is none.  You
cannot see this effect on one supernova, but when you put them all together
and do a full Bayesian analysis you start putting very strong constraints on
the dark matter, because each supernova counts and you have so many of them.
The more supernovae included in the analysis, and the farther away they are,
the tighter the constraints.  Data on 1,048 bright supernovae from the
Pantheon catalogue provided an even lower upper limit -- 23% -- than the
newly published analysis.  A paper was published proposing this type of
analysis in the late 1990s, but when interest shifted from looking for big
objects, MACHOs, to looking for fundamental particles, in particular weakly
interacting massive particles, or WIMPs, follow-up plans fell by the
wayside.  By then, many experiments had excluded most masses and types of
MACHOs, leaving little hope of discovering such objects.  At the time, too,
only a small number of distant Type-Ia supernovae had been discovered and
their distances measured.  Only after the LIGO observations brought up the
issue again did the team embark on the complicated analysis to determine the
limits on dark matter.  What was intriguing is that the masses of the black
holes in the LIGO event were right where black holes had not yet been
excluded as dark matter.  That was an interesting coincidence that got
everyone excited.  But it was only a coincidence.