MERCURY'S DARK SURFACE EXPLAINED
Brown University
The darkness of Mercury's surface has long been of interest.
On average, Mercury is much darker than the similarly airless Moon.
Airless bodies are known to be darkened by micrometeorite impacts and
bombardment by the solar wind, processes that create a thin coating of
dark iron nano-particles on the surface. But spectral data from
Mercury suggest that its surface contains very little nano-phase
iron, certainly not enough to account for its dim appearance. Now
researchers suggest that comets may be responsible for Mercury's dark
surface. As comets approach Mercury's neighbourhood, they often start
to break apart. Cometary dust is composed of as much as 25% carbon by
weight, so Mercury would be exposed to a steady bombardment of carbon
from crumbling comets. Using a model of impact delivery and a known
estimate of micrometeorite flux at Mercury, the team was able to
estimate how often cometary material would impact Mercury, how much
carbon would stick to Mercury's surface, and how much would be thrown
back into space. The calculations suggest that after billions of
years of bombardment, Mercury's surface should be anywhere from 3 to 6
per cent carbon.
The next part of the work was to find out how much darkening could be
expected from all that impacting carbon. For that, the researchers
turned to the NASA-Ames Vertical Gun Range. The 14-foot cannon can
simulate celestial impacts by firing projectiles at up to 8 km/s. The
team launched projectiles in the presence of sugar, a complex organic
compound that mimics the organics in cometary material. The heat of
an impact burns the sugar up, releasing carbon. Projectiles were
fired into a material that mimics lunar basalt, the rock that makes up
the dark patches on the nearside of the Moon. The experiments showed
that tiny carbon particles become deeply embedded in the impact-melted
material. The process reduced the amount of light reflected by the
target material to less than 5% -- about the same as the darkest parts
of Mercury. Importantly, spectroscopic analysis of the impact samples
revealed no distinctive spectral fingerprints, again similar to the
flat spectral signatures from Mercury. Carbon is evidently a stealthy
darkening agent -- from the standpoint of spectral analysis, it is
like an invisible paint.
RR LYRAE STARS NOT SO LONELY
RAS
Stars are very often found in binary systems, orbiting around their
common centre of gravity. Some binary systems are of great importance
in astrophysics, as their properties can be inferred with unparallel-
led accuracy from their orbital elements. Puzzlingly, however, an
overwhelming majority of the known members of a very important family
of stars, the RR Lyrae variables, have for long appeared to be single.
Those stars, being among the oldest known, offer information about the
origin and evolution of the stellar systems that harbour them, such as
the Milky Way itself. However, the lack of RR Lyrae stars in binary
systems has made a direct assessment of some of their key properties
difficult. Most often, theory had to be invoked to fill the gap.
Now, however, researchers have found evidence that the stars may not
lack companionship so thoroughly after all. They report the identifi-
cation of as many as 20 candidate RR Lyrae binaries. Twelve of the
candidates have enough measurements to conclude with high confidence
that they do indeed consist of two stars orbiting each other. In the
solar neighbourhood, about every second star is in a binary. The fact
that among 100,000 known RR Lyrae stars only one of them had been
recognized as having a companion was something really intriguing for
astronomers.
The team used the 'light-travel time' method, which exploits changes
in the time it takes starlight to reach us. The RR Lyrae stars
pulsate regularly, significantly increasing and decreasing their
sizes, temperatures, and brightness, in a matter of just a few hours.
When a pulsating star is in a binary system, the timing of the changes
perceived by us are affected by where exactly the star is in the
course of its orbit around its companion. The light takes longer to
reach us when the star is at the farthest point in its orbit, and
vice-versa. That subtle effect is what has been detected in the
candidates. The measurements were based on data published by the
Polish OGLE Project. The OGLE team has obtained its data from the
1.3-m Warsaw telescope, at Las Campanas Observatory in northern Chile,
repeatedly observing the same patches of the sky for many years. The
20 candidates were found by analysis of the roughly 2000 best-observed
RR Lyrae stars towards the central parts of the Milky Way -- about 5%
of the known ones. It was only thanks to the high quality of the OGLE
data and the long time span of those observations that the team could
finally find signs of companions around a few of the stars. Indeed,
the systems detected have orbital periods of several years, which
indicates that the pairs, though bound together by gravity, are not
very close to one another. Binaries with even longer periods may also
exist, but the OGLE data do not extend long enough to reach strong
conclusions in that respect. RR Lyrae stars are easy to identify,
since they show characteristic cyclical brightness variations which
make them excellent distance indicators for the nearby Universe.
However, a lot of what we know about them relies on theoretical
modelling. We can now exploit the orbital information about them to
infer their physical properties, especially their masses but possibly
also their diameters, thus opening doors to discoveries that until
recently seemed impossible.
DUSTY CLOUD PASSING GALACTIC CENTRE BLACK HOLE
ESO
A super-massive black hole with a mass four million times that of the
Sun lies at the heart of the Milky Way galaxy. It is orbited by a
small group of bright stars and, in addition, by an enigmatic dusty
cloud, known as G2, which has been tracked in its fall towards the
black hole over the last few years. Closest approach, known as
peribothron, was predicted to be in May 2014. The great tidal forces
in that region of very strong gravity were expected to tear the cloud
apart and disperse it along its orbit. Some of the material would
feed the black hole and lead to sudden flaring and other evidence of
the monster enjoying a rare meal. To study those expected events, the
region of the Galactic centre has been very carefully observed over
the last few years by many teams using large telescopes around the
world. A team from the University of Cologne has observed the region
with the Very Large Telescope (VLT) over many years, including new
observations during the critical period from February to September
2014, just before and after the peribothron event in May 2014. The
new observations are consistent with earlier ones made with the Keck
telescope in Hawaii. Images of infrared light coming from glowing
hydrogen show that the cloud was compact both before and after its
closest approach, as it swung round the black hole. As well as
providing sharp images, the SINFONI instrument on the VLT includes a
spectrograph and allows the velocity of the cloud to be estimated.
Before closest approach, the cloud was found to be travelling away
from us at about 2800 km/s, and after swinging round the black hole,
it was measured to be approaching at about 3300 km/s. Although
earlier observations had suggested that the G2 object was being
stretched, the new observations did not show evidence that the cloud
had become significantly smeared out, either by becoming visibly
extended, or by showing a larger spread of velocities. In addition to
the observations with the SINFONI instrument, the team has also made a
long series of measurements, with the NACO instrument on the VLT, of
the polarization of the light coming from the region of the super-
massive black hole. They show that the behaviour of the material
being accreted onto the black hole is very stable, and so far has not
been disrupted by the arrival of material from the G2 cloud. The
resilience of the dusty cloud to the extreme gravitational tidal
effects so close to the black hole strongly suggest that it surrounds
a dense object with a massive core, rather than being a free-floating
cloud. That is also supported by the lack, so far, of evidence of
flaring and increased activity of the black hole.
SUPERNOVA EXPLOSION INVOLVED SINGLE WHITE DWARF
NASA/Goddard Space Flight Center
Using archival data from the Japan-led Suzaku X-ray satellite,
astronomers have determined the pre-explosion mass of a white dwarf
star that blew up thousands of years ago. The measurement strongly
suggests that the explosion involved only a single white dwarf, ruling
out an alternative scenario involving a pair of merging white dwarfs.
The researchers analyzed archival observations of a supernova remnant
named 3C 397, about 33,000 light-years away in the constellation
Aquila. Astronomers estimate that the cloud of stellar debris has
been expanding for between 1,000 and 2,000 years, making 3C 397 a
middle-aged remnant. The team made clear detections with Suzaku's
X-ray imaging spectrometer of elements crucial to determining the mass
of the white dwarf. The observation, made in October 2010 at energies
between 5,000 and 9,000 electron volts, provided a total effective
exposure of 19 hours. Infrared data from the Spitzer space telescope
provided an estimate of the amount of gas and dust the expanding
remnant has gathered up as it expands into interstellar space. The
observations, from April 2005, indicate that 3C 397 has swept up a
mass some 18 times greater than that of the original white dwarf. As
a result, the team concludes that shock waves have thoroughly heated
the remnant's innermost parts.
Most low- and medium-mass stars similar to the Sun will end their days
as white dwarfs. A typical white dwarf is about as massive as the Sun
yet roughly the size of Earth. That puts white dwarfs among the
densest objects scientists know of, surpassed only by neutron stars
and black holes. White dwarfs remain stable if their masses are leass
than about 1.4 solar masses. White dwarfs near that limit are on the
verge of a catastrophic explosion. Until recently, astronomers
thought that the most likely way for a white dwarf to gain mass would
be as a member of a close binary system with a normal Sun-like star.
By accumulating matter from its companion, the white dwarf can, over
millions of years, close in on the limit and explode. The companion
stars are expected to survive, but astronomers find scant evidence for
them, suggesting the need for an alternative model. In the merger
scenario, the blast is triggered by a pair of lower-mass white dwarfs,
whose orbits tighten over time until the stars eventually merge and
explode. Astronomers can distinguish which of those scenarios is
responsible for a given supernova remnant by tallying the nickel and
manganese in the expanding cloud. An explosion from a single white
dwarf near its mass limit will produce significantly different amounts
of those elements from a merger. The team also measured iron and
chromium, which are produced in all type-Ia explosions, as a way to
standardize their calculations. The finding shows that at least some
type-Ia supernovae must have surviving stellar companions, and the
team emphasizes that the search for such stars should continue.
HUBBLE FINDS PHANTOM OBJECTS NEAR DEAD QUASARS
Space Telescope Science Institute (STScI)
The Hubble space telescope has photographed a set of wispy, green-
goblin objects that are the ephemeral ghosts of quasars that flickered
to life and then faded. The glowing structures have looping, helical,
and braided shapes, and astronomers believe that the features offer
insights into the behaviour of galaxies with energetic cores. The
ethereal wisps outside the host galaxy are believed to have been
illuminated by powerful ultraviolet radiation from a super-massive
black hole at the core of the galaxy. The most active of such galaxy
cores are called quasars, where infalling material is heated to a
point where a brilliant searchlight shines into deep space. The beam
is produced by a disc of glowing, super-heated gas encircling the
black hole. However, the quasars are not bright enough now to account
for what we are seeing; that is a record of something that happened in
the past. The glowing filaments are telling us that the quasars were
once emitting more energy, or they are changing very rapidly, which
they were not supposed to do. One possible explanation is that pairs
of co-orbiting black holes are powering the quasars, and that could
change their brightness. The quasar beam caused the once-invisible
filaments in deep space to glow through a process called photo-
ionization. Oxygen atoms in the filaments absorb light from the
quasar and slowly re-emit it over many thousands of years. Other
elements detected in the filaments are hydrogen, helium, nitrogen,
sulphur, and neon. The green filaments are believed to be long tails
of gas pulled apart under gravitational forces resulting from a merger
of two galaxies. Rather than being blasted out of the quasar's black
hole, those immense structures, tens of thousands of light-years long,
are slowly orbiting their host galaxy long after the merger was
completed. The ghostly green structures are so far outside the galaxy
that they may not light up until tens of thousands of years after the
quasar outburst, and would likewise fade only tens of thousands of
years after the quasar itself does. That is the length of time it
would take for the quasar light to reach them. Not coincidentally,
galaxy mergers would also trigger the birth of a quasar by pouring
material into the central super-massive black hole.
The first 'green goblin' type of object was found in 2007 by Dutch
schoolteacher Hanny van Arkel. She discovered the ghostly structure
in the online Galaxy Zoo project. The project has enlisted the public
to help classify more than a million galaxies catalogued in the Sloan
Digital Sky Survey (SDSS), and moved on to add galaxies seen in Hubble
images probing the distant Universe. The bizarre feature was dubbed
Hanny's Voorwerp, Dutch for Hanny's object. Because the follow-up
Hubble images of Hanny's Voorwerp were so intriguing, astronomers
started a deliberate hunt for more objects like it. They would share
the rare and striking colour signature of Hanny's Voorwerp on the SDSS
images. The team used 200 volunteers specifically to look at over
15,000 galaxies hosting quasars. Each candidate had to have at least
10 views that collectively reveal weirdly-coloured clouds. The team
took the galaxies that looked the most promising and further studied
the by spectroscopy. In follow-up observations from Kitt Peak and the
Lick Observatory, the team found 20 galaxies that had gas that was
ionized by radiation from a quasar, rather than from the energy of
star formation. The clouds extended more than 30,000 light-years
outside the host galaxies. Eight of the newly discovered clouds were
more energetic than would be expected from the amount of radiation
coming from the host quasar, even when observed in infrared light by
the Wide-field Infrared Survey Explorer (WISE) space telescope. The
host quasars were as little as one-tenth the brightness needed to
provide enough energy to ionize the gas. Astronomers presume that the
brightness changes are governed by the rate at which material is
falling onto the central black hole. The quasar variability might be
explained if there are two massive black holes circling each other at
the host galaxy's centre. That could conceivably happen after two
galaxies merged. A pair of black holes whirling about each other
could disrupt the steady flow of infalling gas, causing abrupt spikes
in the accretion rate and triggering blasts of radiation. When our
Milky Way galaxy merges with the Andromeda galaxy (M31) in about 4
billion years, the black holes in each galaxy could wind up orbiting
each other. So in the far future our Galactic system could have its
own version of Hanny's Voorwerp encircling it.
LIKELY PRECURSORS OF GALAXY CLUSTERS WE SEE TODAY
University of Arizona
Galaxies like our Milky Way with its 100 billion stars are usually not
found in isolation. In the Universe today, 13.8 billion years after
the Big Bang, many are in dense clusters of tens or even hundreds of
galaxies. However, the clusters have not always existed, and a key
question in modern cosmology is how such massive structures assemble,
to provide insight into the process of galaxy cluster evolution,
including the role played by dark matter in shaping the clusters.
Now, using the combined strengths of the Herschel space observatory
and the Planck satellite, astronomers have found objects in the
distant Universe, seen at a time when it was only three billion years
old, which could be precursors of the clusters seen around us today.
Because we are looking so far back in time, and because the Universe
is assumed to be homogeneous in all directions, researchers think that
it is very similar to looking at the equivalent of what a baby cluster
might look like. In contrast to previous observations, for which the
odd one or two baby clusters were found, astronomers have now found a
good sample of 200 such clusters. The main goal of Planck, a space
observatory operated by the European Space Agency, was to provide the
most precise map of the relic radiation of the Big Bang, the cosmic
microwave background. To do so, it surveyed the entire sky in nine
different wavelengths from the far-infrared to radio, in order to
eliminate foreground emission from our galaxy and others in the
Universe. But those foreground sources can be important in other
fields of astronomy, and it was in Planck's short-wavelength data that
scientists were able to identify 234 bright sources with characteris-
tics that suggested that they were located in the distant, early
Universe. Another space observatory -- Herschel -- then observed the
objects across the far-infrared to sub-millimetre wavelength range
(just a bit shorter than microwaves), but with much higher sensitivity
and angular resolution. Herschel revealed that the vast majority of
the Planck-detected sources are consistent with dense concentrations
of galaxies in the early Universe, vigorously forming new stars. Each
of the young galaxies is seen to be converting gas and dust into stars
at a rate of a few hundred to 1,500 solar masses a year.
Topic: Mid April Astronomy bulletin (Read 1340 times)