Topic: Late February Astronomy Bulletin

[Clive]

HIGHLY ABNORMAL SOLAR ACTIVITY OVER 7,000 YEARS AGO
Nagoya University

An international team of researchers has identified a new type of solar
event and dated it to the year 5480 BC; they did that by measuring
carbon-14 levels in tree rings, which reflect the effects of cosmic
radiation on the atmosphere at the time.  They have also proposed causes
of that event.  When the activity of the Sun changes, it has direct
effects on the Earth.  For example, when the Sun is relatively inactive,
the amount of carbon-14 increases in the Earth's atmosphere.  Because
carbon dioxide in the air is absorbed by trees, carbon-14 levels in tree
rings actually reflect solar activity and unusual solar events in the
past.  The team took advantage of such a phenomenon by analyzing a
specimen from a bristlecone pine tree, a species that can live for
thousands of years, to look back into the history of the Sun.  The team
measured the 14C levels in the pine sample at three different labora-
tories in Japan, the US, and Switzerland, to ensure the reliability of
the results.  It found a change in 14C that was more abrupt than any
found previously, except for cosmic-ray events in AD 775 and AD 994, and
the use of annual data rather than data for each decade allowed the team
to pinpoint exactly when the change occurred.

The team attempted to develop an explanation for the anomalous solar-
activity data by comparing the features of the 14C change with those of
other solar events known to have occurred over the last couple of
millennia.  Although the newly discovered event was more dramatic than
others found to date, comparisons of the 14C data among them can help us
to work out what happened to the Sun at that time.  It is thought that a
change in the magnetic activity of the Sun along with a series of strong
solar bursts, or a very weak Sun, may have caused the unusual tree-ring
data, but the actual mechanism is unknown.


TWO BILLION YEARS OF VOLCANIC ACTIVITY ON MARS 
University of Houston

Analysis of a Martian meteorite found in Africa in 2012 has uncovered
evidence of at least 2 billion years of volcanic activity on Mars.
That confirms that Mars may have some of the longest-lived volcanoes
in the Solar System.  Shield volcanoes and lava plains form from lava
flowing over long distances, as has happened in the Hawaiian Islands.
The largest Martian volcano, Olympus Mons, is nearly 17 miles high.
That is almost three times the height of the Earth's tallest volcano,
Mauna Kea, at 6.25 miles (the height from the base of its structure,
not from sea level).  The findings offer new clues as to how the planet
evolved and insight into the history of volcanic activity on Mars.  Much
of what we know about the composition of rocks from volcanoes on Mars
comes from meteorites found on Earth.  Analysis of different substances
provides information about the age of the meteorite, its magma source,
length of time in space and how long the meteorite was on the Earth's
surface.  Something slammed into the surface of Mars 1 million years
ago, hitting a volcano or lava plain.  The impact ejected rocks into
space.  Fragments of those rocks crossed the Earth's orbit and fell as
meteorites.

The meteorite known as Northwest Africa 7635 and discovered in 2012 was
found to be a type of volcanic rock called a shergottite.  Eleven such
Martian meteorites, with similar chemical composition and ejection time,
have been found, and they came from a similar volcanic source.  Given
that they also have the same ejection time, we can conclude that they
came from the same location on Mars.  Previously analyzed Martian
meteorites range in age from 327 million to 600 million years old.  In
contrast, the 2012 meteorite was formed 2.4 billion years ago and was
ejected from one of the longest-lived volcanic centres in the Solar
System.


PLANETS OF RED DWARFS MAY FACE OXYGEN LOSS
NASA Goddard

The search for life beyond Earth starts in habitable zones, the regions
around stars where conditions could potentially allow liquid water --
which is essential for life as we know it -- to pool on a planet's
surface.  New research suggests that some such zones might not actually
be able to support life, owing to frequent stellar eruptions -- which
spew huge amounts of stellar material and radiation out into space --
from young red-dwarf stars.  Now scientists want to expand how habitable
zones are defined, taking into account the impact of stellar activity,
which can threaten an exoplanet's atmosphere with oxygen loss.  To
determine a star's habitable zone, scientists have previously considered
how much heat and light the star emits.  Stars more massive than the Sun
produce more heat and light, so the habitable zone must be farther out.
Smaller, cooler stars have close-in habitable zones.  But along with heat
and visible light, stars emit X-ray and ultraviolet radiation, and
produce stellar eruptions such as flares and coronal mass ejections --
collectively called space weather.  One possible effect of such radiation
is atmospheric erosion, in which high-energy particles drag atmospheric
molecules, such as hydrogen and oxygen, the ingredients of water, out
into space.  The new model for habitable zones now takes that effect into
account.

The search for habitable planets often homes in on red dwarfs, as those
are the coolest, smallest and most numerous stars in the Universe. 
On the downside, red dwarfs are also prone to more frequent and powerful
stellar eruptions than the Sun.  To assess the habitability of planets
around such stars, we need to understand how those various effects
balance out.  Another important habitability factor is a star's age, say
the scientists, on the basis of observations that they have gathered from
the Kepler mission.  Every day, young stars produce 'superflares' --
flares and eruptions at least 10 times more powerful than those observed
on the Sun.  On their older, matured counterparts resembling our middle-
aged Sun today, such superflares are observed only once every 100 years.
When we look at young red dwarfs in our galaxy, we see they are much
less luminous than the Sun is today.  By the classical definition, the
habitable zone around red dwarfs must be 10 to 20 times closer-in than
the Earth is to the Sun.  Now we know that red-dwarf stars generate a
lot of X-ray and extreme-ultraviolet emissions that trouble the habitable
zones of exoplanets through frequent flares and stellar storms.  Super-
flares cause atmospheric erosion when high-energy X-ray and extreme-
ultraviolet emissions first break molecules into atoms and then ionize
atmospheric gases.  During ionization, radiation strikes the atoms and
knocks off electrons.  Electrons are much lighter than the newly formed
ions, so they escape gravity's pull far more readily and race out into
space.

Opposites attract, so as more and more negatively-charged electrons are
generated, they create a powerful charge separation that lures positively
charged ions out of the atmosphere in a process called ion escape.  The
model estimates the oxygen escape on planets around red dwarfs, assuming
that they do not compensate with volcanic activity or comet bombardment.
Various earlier atmospheric-erosion models indicated that hydrogen is
most vulnerable to ion escape.  As the lightest element, it easily
escapes into space, presumably leaving behind an atmosphere rich with
heavier elements such as oxygen and nitrogen.  But when the scientists
accounted for superflares, their new model indicated that the violent
storms of young red dwarfs generate enough high-energy radiation to
enable the escape even of oxygen and nitrogen -- building blocks for
life's essential molecules.  Considering oxygen escape alone, the model
estimates that a young red dwarf could render a close-in exoplanet
uninhabitable within a few tens to a hundred million years.  The loss
of both atmospheric hydrogen and oxygen would reduce and eliminate the
planet's water supply before life would have a chance to develop.  The
new habitability model has implications for the recently discovered
planet orbiting the red dwarf Proxima Centauri, our nearest stellar
neighbour.  The Earth-sized planet Proxima b orbits Proxima Centauri
20 times closer than Earth is to the Sun.  Considering the age of the
host star and how close the planet is to it, the scientists expect that
Proxima b is subjected to torrents of X-ray and extreme-ultraviolet
radiation from superflares occurring roughly every two hours.  They
estimate that oxygen would escape from Proxima b's atmosphere in 10
million years.  Additionally, intense magnetic activity and stellar wind
-- the continuous flow of charged particles from a star -- exacerbate
already harsh space weather conditions.  The scientists concluded that
Proxima b is quite unlikely to be habitable.


WHITE-DWARF PULSAR DISCOVERED
University of Warwick

An exotic binary star system 380 light-years away has been identified
as an elusive white dwarf pulsar -- the first of its kind ever to be
discovered.  Astronomers have identified the star AR Scorpii (AR Sco) as
the first white-dwarf version of a pulsar -- objects found in the 1960s
and associated with very different objects called neutron stars.  AR Sco
contains a rapidly spinning, burnt-out stellar remnant called a white
dwarf, which lashes its neighbour -- a red dwarf -- with powerful beams
of electrical particles and radiation, causing the entire system to
brighten and fade dramatically twice every two minutes.  The latest
research establishes that the lash of energy from AR Sco is a focussed
'beam', emitting concentrated radiation in a single direction -- much
like a particle accelerator -- something which is unique in the known
Universe.  AR Sco lies in the constellation Scorpius, 380 light-years
away, quite a close neighbour to us in astronomical terms.  The white
dwarf in AR Sco is the size of the Earth but 200,000 times as massive,
and is in a 3.6-hour orbit with a cool star one-third the mass of the
Sun.

With an electromagnetic field 100 million times more powerful than the
Earth's, and spinning in a period just shy of two minutes, AR Sco
produces lighthouse-like beams of radiation and particles, which lash
across the face of the cool star, a red dwarf.  As the researchers
previously discovered, that powerful light-house effect accelerates
electrons in the atmosphere of the red dwarf to close to the speed of
light, an effect never observed before in similar types of binary stars.
The distance between the two stars is around 1.4 million kilometres --
three times the distance between the Moon and the Earth.  The new data
show that AR Sco's light is highly polarized, showing that the magnetic
field controls the emission of the entire system, just as in neutron-star
pulsars.  AR Sco is like a gigantic dynamo: a magnet, the size of the
Earth, with a field that is ~10,000 times stronger than any field that we
can produce in a laboratory, and it is rotating every two minutes.  That
generates an enormous electric current in the companion star, which then
produces the variations that we detect in its light.


TAIL OF STRAY BLACK HOLE HIDING IN MILKY WAY
National Astronomical Observatory of Japan

By analyzing the gas motion of an extraordinarily fast-moving cosmic
cloud in a corner of the Milky Way, astronomers found hints of a
wandering black hole hidden in the cloud.  That result marks the
beginning of the search for quiet black holes; millions of such objects
are expected to be floating in the Milky Way although only dozens have
been found to date.  It is difficult to find black holes, because they
are completely black, though in some cases black holes cause effects
which can be seen.  For example, if a black hole has a companion star,
gas streaming into the black hole piles up around it and forms a disc. 
The disc heats up through effects of the enormous gravitational pull
of the black hole, and emits intense radiation.  But if a black hole is
floating alone in space, no emissions would be observable coming from it. 
A research team used the ASTE Telescope in Chile and the 45-m radio
telescope at Nobeyama Radio Observatory to observe molecular clouds
around the supernova remnant W44, located 10,000 light-years away from
us.  Their primary goal was to examine how much energy was transferred
from the supernova explosion to the surrounding molecular gas, but they
happened to find signs of a hidden black hole at the edge of W44.

During the survey, the team found a compact molecular cloud with
enigmatic motion.  That cloud, named the 'Bullet', has a speed of more
than 100 km/s, which exceeds the speed of sound in interstellar space by
more than two orders of magnitude.  In addition, that cloud, with the
size of two light-years, moves backward against the rotation of the Milky
Way Galaxy.  To investigate the origin of the Bullet, the team made
intensive observations of the gas cloud.  The data indicate that the
Bullet seems to jump out from the edge of the W44 supernova remnant with
immense kinetic energy.  Most of the Bullet has an expanding motion with
a speed of 50 km/s, but the tip of the Bullet has a speed of 120 km/s. 
Its kinetic energy is a few tens of times larger than that injected by
the W44 supernova.  It seems impossible to generate such an energetic
cloud under ordinary environments.  The team proposed two scenarios for
the formation of the Bullet.  In both cases, a dark and compact gravity
source, possibly a black hole, has an important role.  One scenario is
the 'explosion model' in which an expanding gas shell of the supernova
remnant passes by a static black hole.  The black hole pulls the gas
very close to it, giving rise to an explosion, which accelerates the gas
toward us after the gas shell has passed the black hole.  In that case,
the astronomers estimated that the mass of the black hole to be 3.5 solar
masses or larger.  The other scenario is the 'irruption model' in which a
high-speed black hole storms through a dense gas and the gas is dragged
along by the strong gravity of the black hole to form a gas stream.  In
that case, researchers estimated that the mass of the black hole would be
36 solar masses or larger.  With the present data set, the team can not
distinguish which scenario is more likely.  Theoretical studies have
suggested that more than a hundred million black holes should exist in
the Milky Way, although only 60 or so have been identified to date.


BRIDGE OF STARS CONNECTS MAGELLANIC CLOUDS
University of Cambridge

The Magellanic Clouds, the two largest satellite galaxies of the Milky
Way, appear to be connected by a bridge stretching across 43,000 light-
years.  For the past 15 years, scientists have been eagerly anticipating
the data from Gaia.  The first tranche of information from the satellite
was released three months ago and is freely accessible to everyone.  That
data set, of unprecedented quality, is a catalogue of the positions and
brightnesses of a thousand million stars in our Milky Way galaxy and its
environs.  The satellite's angular resolution is similar to that of the
Hubble space telescope, but owing to its bigger field of view, it can
cover the entire sky rather than a small portion of it.  In fact, Gaia
uses the largest number of pixels of any space-borne instrument to take
digital images of the sky.  Better still, the Observatory has not just
one telescope but two, sharing the metre-wide focal plane.  Unlike
typical telescopes, Gaia does not just point and stare: it constantly
spins slowly around an axis, sweeping the entire sky in less than a
month, so it not only measures the instantaneous properties of the stars,
but also tracks their changes over time.  That provides an opportunity to
find a variety of objects, for example stars that pulsate or explode --
even though that is not what the satellite was primarily designed for.
The Cambridge team concentrated on the area around the Magellanic Clouds
and used the Gaia data to pick out pulsating stars of the type called
RR Lyrae stars, very old and chemically un-evolved.  As those stars have
existed since the earliest days of the Clouds' existence, they offer an
insight into the pair's history.  Studying the Large and Small Magellanic
Clouds (LMC and SMC respectively) has always been difficult, as they
sprawl out over a large area, but with Gaia's all-sky view, it has become
a much easier task.

Around the Milky Way, the Clouds are the brightest, and largest, examples
of dwarf satellite galaxies.  Known to humanity since the dawn of history
(and to Europeans since their first voyages to the Southern Hemisphere),
the Magellanic Clouds have remained an enigma to date.  Even though the
Clouds have been a constant fixture of the heavens, astronomers have only
recently been able to study them in any detail.  Whether the Clouds fit
the conventional theory of galaxy formation or not depends critically on
their mass and the time of their first approach to the Milky Way.  The
researchers at Cambridge's Institute of Astronomy found clues that could
help answer both of those questions.  First, the RR Lyrae stars detected
by Gaia were used to trace the extent of the Large Magellanic Cloud.  The
LMC was found to possess a fuzzy low-luminosity 'halo' stretching as far
as 20 degrees from its centre.  The LMC would only be able to hold on to
the stars at such large distances if it is substantially more massive
than was previously thought, totalling perhaps as much as a tenth of the
mass of the entire Milky Way.  An accurate timing of the Clouds' arrival
to the galaxy is impossible without knowledge of their orbits.  Unfortun-
ately, the orbits of satellite galaxies are difficult to measure: at
large distances, an object's motion in the sky is so minute that it is
simply unobservable over a human life-span.  In the absence of an orbit,
the team found the next-best thing: a stellar stream.

Streams of stars form when a satellite -- a dwarf galaxy or a star
cluster -- starts to feel the tidal force of the body around which it
orbits.  The tides stretch the satellite in the direction towards and
away from the host.  As a result, on the periphery of the satellite,
two openings form -- small regions where the gravitational pull of the
satellite is balanced by the pull of the host.  Satellite stars that
enter those regions find it easy to leave the satellite altogether and
start orbiting the host.  Slowly, star after star abandons the satellite,
leaving a luminous trace on the sky, and thus revealing the satellite's
orbit.  Stellar streams around the Clouds were predicted but never
observed.  Marked by the locations of the Gaia RR Lyrae stars on the sky,
a narrow bridge-like structure connecting the two Clouds could be seen.
It is thought that, at least in part, the 'bridge' is composed of stars
stripped from the Small Cloud by the Large.  The rest may actually be
LMC stars pulled from it by the Milky Way.


RECORD-BREAKING BLACK-HOLE ACTIVITY
University of New Hampshire

Astronomers using data from a trio of orbiting X-ray telescopes, NASA's
Chandra X-ray Observatory and Swift satellite as well as ESA's XMM-
Newton, say that a giant black hole ripped apart a nearby star and then
continued to feed off its remains for close to a decade.  That black-hole
meal lasted more than 10 times longer than any other previous episode of
a star's death.  Dozens of such tidal-disruption events have been
detected since the 1990s, but none that remained bright for nearly as
long as this one.  Tidal forces, arising from the intense gravity of the
black hole, can destroy an object -- such as a star -- that wanders too
close.  During a TDE, some of the stellar debris is flung outward at high
speeds, while the rest falls toward the black hole.  As it travels inward
and is ingested by the black hole, the material heats up to millions of
degrees and generates a distinct X-ray flare.  Such multi-wavelength
flares, which can be viewed by the satellites, help to study otherwise
dormant massive back holes.  Previous flares were short-lived, typically
becoming very faint in a year, but this super-long X-ray flare has been
persistently bright for close to a decade.  The extraordinary long bright
phase of this TDE means that either this was the most massive star ever
to be torn apart during one of these events, or the first where a smaller
star was completely torn apart.

The X-ray source containing the black hole, known by its abbreviated name
of XJ15000154, is located in a small galaxy about 1.8 billion light-
years from the Earth.  The X-ray data indicate that radiation from
material surrounding the black hole has consistently surpassed the so-
called Eddington limit, defined by a balance between the outward pressure
of radiation from the hot gas and the inward pull of the gravity of the
black hole.  The conclusion that supermassive black holes can grow, from
TDEs and perhaps other means, at rates above those corresponding to the
Eddington limit has important implications.  Such rapid growth may help
to explain how supermassive black holes were able to reach masses of about
10 to the 9 solar masses when the Universe was 'only' about one
billion years old.  According to modelling by the researchers, the black
hole's feeding supply should be significantly reduced in the next decade
and begin to fade in the next several years.