Topic: Mid August Astronomy Bulletin

[Clive]

OLDEST-EVER METEORITE CONTAINS CLUES
University of New Mexico

Scientists believe that the Solar System was formed some 4.6 billion years
ago when a cloud of gas and dust collapsed under gravity, possibly triggered
by a cataclysmic explosion from a nearby massive star or supernova. As the
cloud collapsed, it formed a spinning disc with the Sun in the centre.
Scientists have been able to establish the subsequent formation of the Solar
System piece by piece. Now, new research has enabled them to add another
piece to that puzzle with the discovery of the oldest-ever-dated igneous
meteorite, called NWA 11119. That research provides direct evidence that
chemically evolved silica-rich crustal rocks were forming on planetesimals
within the first 10 million years, prior to the assembly of the terrestrial
planets, and helps scientists to understand further the complexities of
planet formation. Not only is the meteorite an extremely unusual rock type,
it is telling us that not all asteroids look the same. Some of them look
almost like the crust of the Earth because they are so light-coloured and
full of SiO2. Scientists studied crustal rock that had been received from a
meteorite dealer but had been found in a sand dune in Mauritania by a nomad.
The rock was lighter in colour than most meteorites and was laced with green
crystals and cavities and surrounded by quench melt. Using an electron
microprobe and a CT (computed tomography) scan at UNM and Johnson Space
Center facilities, researchers started to examine the composition and
mineralogy of the rock, and found it very different from anything that they
had worked on before. One of the main things they saw first were the large
crystals of tridymite which is a high-temperature polymorph of silica
(quartz). When they conducted further image analyses to quantify the
tridymite, they found that the amount present was a staggering 30 per cent
of the total meteorite -- such an amount is unheard-of in meteorites and is
found only in certain volcanic rocks on the Earth.

The research also involved trying to determine through chemical and isotopic
analyses what body the meteorite could have come from. On the basis of
oxygen isotopes, scientists know that it is from an extra- terrestrial
source somewhere in the Solar System, but they can't actually pinpoint it to
a known body that has been viewed with a telescope. However, through the
measured isotopic values, they have been able to propose possible links to
two other unusual meteorites (Northwest Africa 7235 and Almahata Sitta),
suggesting that they all are from the same parent body -- perhaps a large,
geologically complex body that formed in the early Solar System. One
possibility is that that parent body was disrupted through a collision with
another asteroid or planetesimal, and some of its fragments eventually
reached the Earth's orbit, falling through the atmosphere and ending up as
meteorites on the ground -- in the case of NWA 11119, falling in Mauritania
at an as-yet-unknown time in the past. The oxygen isotopes of NWA 11119,
NWA 7235, and Almahata Sitta are all identical, but this rock -- NWA 11119
-- stands out as something completely different from any of the over 40,000
meteorites that have been found on Earth. Further, research using an
inductively coupled plasma mass spectrometer was performed in the Isotope
Cosmochemistry and Geochronology Laboratory (ICGL) to determine the precise
formation age of the meteorite. The research found that NWA 11119 is the
oldest igneous meteorite ever recorded, at 4.565 billion years. Most
meteorites are formed through the collision of asteroids orbiting the Sun in
the asteroid belt. Asteroids are remnants from the formation of the Solar
System some 4.6 billion years ago. The chemical composition ranges of
ancient igneous meteorites, or achondrites, are key to understanding the
diversity and geochemical evolution of planetary building blocks.
Achondrite meteorites record the first episodes of vulcanism and crust
formation. Meteorites like NWA 11119 were the precursors to planet
formation and represent a critical step in the evolution of rocky bodies in
the Solar System. When we look out of the Solar System today, we see fully
formed bodies, planets, asteroids, comets and so forth. Then, our curiosity
always pushes us to ask the question, how did they form, how did the Earth
form? That is basically a missing part of the puzzle that we have now
found, that tells us that igneous processes act like blast furnaces that are
melting rock and processing all of the Solar-System solids. Ultimately,
that is how planets are forged.


WALL OF HYDROGEN SURROUNDS SOLAR SYSTEM
NASA

Scientists believe that they have found new evidence of a mysterious 'wall'
surrounding our Solar System. Observations from the New Horizons spacecraft
appear to show that a layer of hydrogen gas encircles all the planets and
objects orbiting the Sun. The Voyager spacecraft first gathered data which
indicated the existence of a barrier at the edge of the 'heliosphere', the
name for a bubble, formed by the solar wind, around the Solar System. The
wind is actually a stream of charged particles gushing from the Sun, which
itself is travelling through an interstellar medium that exists between star
systems. Right now, the Solar System is speeding through a huge 'local
cloud' of gas which is 30 light-years across and also has its own gusts
called 'interstellar wind'. The wall is believed to exist where those two
tempests meet. Hydrogen atoms which make up the barrier produce ultraviolet
light which was picked up by the New Horizons probe, which beamed back
remarkable images of Pluto in 2015. Long-term observations made with the
Alice instrument on the New Horizons spacecraft confirm measurements made
30 years earlier by the Voyager spacecraft. Both sets of data are best
explained if the observed ultraviolet light is not only a result of the
scattering of sunlight by hydrogen atoms within the Solar System but
includes a substantial contribution from a distant source. That distant
source could be the signature of a 'wall' of hydrogen, formed near where
the interstellar wind encounters the solar wind, or could be more distant.
Similar future observations from New Horizons are planned about twice each
year.


LARGEST HAUL OF EXTRASOLAR PLANETS
University of Tokyo

Forty-four planets in solar systems beyond our own have been unveiled in one
go, dwarfing the usual number of confirmations from extra-solar surveys,
which is typically a dozen or less. The findings will improve our models of
solar systems and may help researchers investigate exo-planet atmospheres.
Novel techniques developed to validate the find could hugely accelerate the
confirmation of more extra-solar planet candidates. An international team
of astronomers pooled data from NASA's Kepler and ESA's Gaia space
telescopes, as well as ground-based telescopes in the U.S. A portion of the
findings yields some surprising characteristics. For example, four of the
planets orbit their host stars in less than 24 hours. They contribute to a
small but growing list of 'ultra-short-period' planets, so it could turn out
that they are not as unusual as they might seem. Sixteen were in the same
size class as the Earth, one in particular turning out to be extremely small
-- about the size of Venus -- which was a nice affirmation as it is close to
the limit of what is possible to detect. The source observations for this
study were made by Kepler, and they would not have happened were it not for
a fault in 2013, which prevented accurate control of that space telescope.
Two out of the four control-reaction wheels failed, which meant that Kepler
could not perform its original mission to stare at one specific patch of the
sky. That led to its contingent mission, 'K2' -- these observations came
from campaign 10 of that mission. The planets observed by K2 are known as
transiting planets because their orbits bring them in front of their host
stars, slightly reducing their brightness. However, other astrophysical
phenomena can cause similar signals, so follow-up observations and detailed
statistical analyses were performed to confirm the planetary nature of the
signals. The combination of detailed analyses of data from ground-based
telescopes, K2 and Gaia enabled the precise determination of the planets'
sizes and temperatures. The team's findings include 27 additional
candidates that are likely to be real planets, which will be the subject of
future research. Scientists hope to understand what kinds of planets might
be out there, but can only draw valid conclusions if there are enough
planets for robust statistical analysis. The addition of a large number of
new planets, therefore, leads directly to a better theoretical understanding
of solar-system formation. The planets also provide good targets for
detailed individual studies to yield measurements of planetary composition,
interior structure and atmospheres -- in particular, the 18 planets in
several multi-planet systems.


PLANET IS MAGNETIC POWERHOUSE
National Radio Astronomy Observatory

Astronomers using the VLA have made the first radio-telescope detection of a
planetary-mass object beyond the Solar System. The object, about a dozen
times more massive than Jupiter, is a surprisingly strong magnetic
powerhouse and a 'rogue', travelling through space unaccompanied by any
parent star. The object is right at the boundary between a planet and a
brown dwarf or 'failed star', and is giving us some surprises that can
potentially help us to understand magnetic processes on both stars and
planets. Brown dwarfs are objects too massive to be considered planets, yet
not massive enough to sustain nuclear fusion of hydrogen in their cores --
the process that powers stars. Theorists suggested in the 1960s that such
objects would exist, but the first one was not discovered until 1995. They
were originally thought not to emit radio waves, but in 2001 a VLA discovery
of radio flaring in one revealed strong magnetic activity. Subsequent
observations showed that some brown dwarfs have strong aurorae, similar to
those that occur in our own Solar System's giant planets. The aurorae seen
on Earth are caused by our planet's magnetic field interacting with the
solar wind. However, solitary brown dwarfs do not have a solar wind from a
nearby star with which to interact. How the aurorae are caused in brown
dwarfs is not clear, but the scientists think that one possibility is an
orbiting planet or moon interacting with the brown dwarf's magnetic field,
such as happens between Jupiter and its moon Io. The strange object in the
latest study, called SIMP J01365663+0933473, has a magnetic field more than
200 times stronger than Jupiter's. The object was originally detected in
2016 as one of five brown dwarfs that the scientists studied with the VLA to
gain new knowledge about magnetic fields and the mechanisms by which some of
the coolest such objects can produce strong radio emission. Brown-dwarf
masses are notoriously difficult to measure, and at the time the object was
thought to be an old and much more massive brown dwarf.

Last year, an independent team of scientists discovered that SIMP
J01365663+0933473 was part of a very young group of stars. Its young age
meant that it was in fact so much less massive that it could be a free-
floating planet -- only 12.7 times the mass of Jupiter, with a radius 1.22
times that of Jupiter. At 200 million years old and 20 light-years from
Earth, the object has a surface temperature of about 825 degrees C.
By comparison, the Sun's surface temperature is about 5,500 degrees C.
The difference between a gas-giant planet and a brown dwarf remains debated
among astronomers, but one rule of thumb that astronomers use is the mass
below which deuterium fusion ceases, known as the 'deuterium-burning limit',
around 13 Jupiter masses. Simultaneously, the Caltech team that originally
detected the brown dwarf's radio emission in 2016 had observed it again in a
new study at even higher radio frequencies and found that its magnetic field
was even stronger than first measured. The VLA observations provided both
the first radio detection and the first measurement of the magnetic field of
a possible planetary-mass object beyond the Solar System. Such a strong
magnetic field presents huge challenges to our understanding of the dynamo
mechanism that produces the magnetic fields in brown dwarfs and exo-planets
and help to drive the aurorae we see. This particular object is exciting
because studying its magnetic dynamo mechanisms can give us new insights on
how the same type of mechanisms can operate in extra-solar planets. We
think that such mechanisms can work not only in brown dwarfs, but also in
both gas-giant and terrestrial planets. Detecting SIMP J01365663+0933473
with the VLA through its auroral radio emission also means that we may have
a new way of detecting exo-planets, including the elusive rogue ones not
orbiting a parent star.


HISTORIC STELLAR BLAST
Association of Universities for Research in Astronomy (AURA)

Imagine travelling to the Moon in just 20 seconds! That's how fast material
from a 170-year-old stellar eruption sped away from the unstable, eruptive,
and extremely massive star Eta Carinae. Astronomers conclude that that is
the fastest jettisoned gas ever measured from a stellar outburst that did
not result in the complete annihilation of the star. The blast, from the
most luminous star known in our Galaxy, released almost as much energy as a
typical supernova explosion that would have left behind a stellar corpse.
However, in this case a double-star system remained and played a critical
role in the circumstances that led to the colossal blast. Over the past
seven years a team of astronomers has determined the extent of that extreme
stellar blast by observing light echoes from Eta Carinae and its surround-
ings. Light echoes occur when the light from bright, short-lived events is
reflected from clouds of dust, which act like distant mirrors redirecting
light in our direction. Owing to the finite speed of light, the arriving
signal of the reflected light has a time delay after the original event.
In the case of Eta Carinae, the bright event was a major eruption of the
star that expelled a huge amount of mass back in the mid-1800s during what
is known as the 'Great Eruption'. The delayed signal of those light echoes
allowed astronomers to decode the light from the eruption with modern
astronomical telescopes and instruments, even though the original eruption
was seen from Earth back in the mid-19th century. That was before modern
tools like the astronomical spectrograph were invented. The Great Eruption
temporarily promoted Eta Carinae to the second-brightest star (after Sirius)
visible in the night sky, and vastly outshining the energy output of every
other star in the Milky Way, after which the star faded from naked-eye
visibility. The outburst expelled material (about 10 times the mass of the
Sun) that also formed the bright glowing gas cloud known as the Homunculus.
That dumbbell-shaped remnant is visible surrounding the star from within a
vast star-forming region. The eruptive remnant can even be seen in small
amateur telescopes from the Earth's Southern Hemisphere and equatorial
regions, but is best seen in images obtained with the Hubble Space
Telescope.

The team used instruments on the 8-m Gemini South telescope, the CTIO 4-m
Blanco telescope, and the Magellan Telescope at Las Campanas Observatory to
observe the light echoes and measure the expansion speeds in the historical
explosion. Astronomers see such really high velocities all the time in
supernova explosions where the star is obliterated. However, in this case
the star survived, and explaining that led the researchers into new
territory. The researchers suggest that the most straightforward way
simultaneously to explain a wide range of observed facts surrounding the
eruption and the remnant star system seen today is with an interaction of
three stars, including a dramatic event where two of the three stars merged
into one monster star. If that was the case, then the present-day binary
system must have started out as a triple system, with one of those two stars
being the one that swallowed its sibling. Eta Carinae is an unstable type
of star known as a Luminous Blue Variable (LBV), located about 7,500 light-
years away in a young star-forming nebula found in the southern constella-
tion Carina. The star is one of the intrinsically brightest in our Galaxy;
it shines some five million times brighter than the Sun and has a mass about
a hundred times greater. Stars like Eta Carinae have the greatest mass-loss
rates prior to undergoing supernova explosions, but the amount of mass
expelled in Eta Carinae's 19th-century Great Eruption exceeds all others
known. Eta Carinae will probably undergo a true supernova explosion
sometime within the next half-million years at most, but possibly much
sooner. Some types of supernovae have been seen to experience eruptive
blasts like that of Eta Carinae only a few years or decades before their
final explosions, so some astronomers speculate that Eta Carinae might blow
sooner rather than later.


SMALL COLLIDING GALAXIES MAY SEED FUTURE STARS
Columbia University

A pair of dwarf galaxies closely circling the Milky Way, the Large and Small
Magellanic Clouds, were in the throes of merging into one when they fell
into our Galaxy. The duo is thought to hold enough gas to replenish half
of the Milky Way's supply of star-making fuel, and now a study offers new
insights into how galaxies like ours are able to capture such gas so easily.
Home to millions of stars, dwarf galaxies are outshone by bigger galaxies
like the Milky Way with hundreds to thousands of times more stars. But what
dwarf galaxies lack in brightness they make up for in their sheer abundance
of star-making fuel. The hydrogen gas in the Large and Small Magellanic
Clouds and dwarf galaxies like them is thought to play a key role in
creating new stars and other small galaxies. To explore the star-making
potential of dwarf-galaxy pairs, a research team investigated a remote pair
-- NGC 4490 and NGC 4485 -- 23 million light-years away. Similar to the
Large Magellanic Cloud, NGC 4490 is several times larger than its companion
galaxy. But its isolated location allowed the researchers to simulate its
eventual merger with NGC 4485 without interference from the Milky Way's
gravitational pull. In their simulations, they watched the bigger galaxy,
NGC 4490, peel off gas from its smaller sibling, a gravitational effect due
to their difference in size. As the pair circled ever closer to one
another, the smaller galaxy's tail of gas was swept further and further
away, a finding that supports a study earlier this year that fingerprinted
the gas streaming from the Magellanic Clouds into the Milky Way as belonging
to the Small Magellanic Cloud.

In their simulation, the researchers found that, long after NGC 4490
collided with its smaller companion and they merged into one, their gas
footprint continued to expand. In five billion years, they found, the
pair's gas tails would extend over a distance of 1 million light-years,
nearly twice its current length. At that time, 10% of the gas envelope
still resided more than 260,000 light-years from the merged galaxies, so it
evidently takes a very long time before all the gas falls back onto the
merged remnant. When the researchers compared their results to real-world
telescopic observations of NGC 4490/4485, the results matched, indicating
that their model might well be accurate. Their findings are also consistent
with what astronomers know about the recycling of gas in the Universe. As
gas clouds grow more extended, the looser the gas becomes, thus making it
easier for a bigger galaxy to come along and gobble it up. The simulation
suggests that that dispersal process has helped the Milky Way efficiently
to strip gas from the Small Magellanic Cloud, and that this sort of gas-
transfer may be fairly common elsewhere in the Universe. The study further
suggests that declining gas density on the outskirts of colliding dwarf
galaxies makes it hard for new stars to form, a conclusion matched by
observations. The researchers plan to continue studying other pairs of
dwarf-galaxy collisions to try to refine their model.


SOLAR PROBE LAUNCHES ON HISTORIC JOURNEY
NASA

The Parker Solar Probe has begun its journey to the Sun, from where it
will transmit its first scientific observations in December, beginning a
revolution in our understanding of the star that makes life on Earth
possible. The mission's findings will help researchers toimprove their
forecasts of space-weather events, which have the potential to damage
satellites and harm astronauts in orbit, disrupt radio communications and,
at their most severe, overwhelm power grids. During the first week of its
journey, the spacecraft will deploy its high-gain antenna and magnetometer
boom. It also will perform the first part of a two-part deployment of its
electric-field antennae. Instrument testing will begin in early September
and last approximately four weeks, after which the probe can begin its main
scientific operations. Over the next two months, the Parker Solar Probe
will fly towards Venus, obtaining its first Venus 'gravity assist' in early
October -- a manoeuvre that whips the spacecraft around the planet, using
Venus's gravity to trim the spacecraft's orbit tighter around the Sun. That
first fly-by will place the Solar Probe in position in early November to fly
as close as 15 million miles from the Sun -- within the blazing solar
atmosphere, the corona, closer than anything made by humanity has ever gone
before. Throughout its 7-year mission, Parker Solar Probe will make six
more Venus fly-bys and 24 total passes by the Sun, journeying steadily
closer to the Sun until it makes its closest approach at 3.8 million miles.
At that point, the probe will be moving at roughly 430,000 miles per hour,
setting the record for the fastest-moving object made by humanity. Parker
Solar Probe will set its sights on the corona to solve long-standing,
fundamental mysteries of our Sun. What is the secret of the scorching
corona, which is more than 300 times hotter than the Sun's surface,
thousands of miles below? What drives the supersonic solar wind -- the
constant stream of solar material that blows through the entire Solar
System? And finally, what accelerates solar energetic particles, which can
reach speeds up to more than half the speed of light as they rocket away
from the Sun? Scientists have sought answers to those questions for more
than 60 years, but the investigation requires sending a probe right through
the unrelenting heat of the corona. Parker Solar Probe carries four
instrument suites designed to study magnetic fields, plasma and energetic
particles, and capture images of the solar wind. The mission is named for
Eugene Parker, the physicist who first theorized the existence of the solar
wind in 1958. It is the first NASA mission to be named for a living
researcher.