Topic: December Astronomy Bulletin

[Clive]

DEMISE OF COMET ISON
NASA

The much-anticipated perihelion passage Comet ISON on November 28 is
over, and instead of becoming a Great Comet, the comet fell apart; the
remains are now invisible.  As it approached the Sun, the comet
produced a tail that reached a length of about 10°, and the head was a
naked-eye object in the pre-dawn sky.  The perihelion passage was
watched by a number of spacecraft; all that emerged from the vicinity
of the Sun after that close passage was a fan-shaped cloud of debris
that then seemed to dissipate altogether.


PHAETHON SPROUTS A TAIL
NASA

Every year in mid-December there is the Geminid meteor shower.  The
meteors are fast, bright, and reliable, and some observers consider
them to be the finest meteors of the year.  However, their origin has
been somewhat of an enigma.  Meteor showers are supposed to come from
comets, yet the orbit of the Geminids does not match that of any
comet.  It does, however, match the orbit of an object called 3200
Phaethon, that was discovered in 1983 by the IRAS satellite and looks
like a rocky asteroid.  It has an eccentric orbit with a period of
1.4 years, much like a comet's, but it never shows a dusty tail that
might replenish the Geminids -- until now.  Astronomers have been
using the STEREO probes to observe Phaethon at perihelion.  In 2010
one of the probes recorded a doubling of Phaethon's brightness as it
approached the Sun, as if sunlight were shining on a cloud of dust
around the asteroid. The observers began to suspect that Phaethon was
something new: a rock comet -- an asteroid that comes so close to the
Sun that solar heating scorches dusty debris right off its rocky
surface to form a sort of gravelly tail.  Indeed, in further STEREO
observations, astronomers did observe a small tail sticking out behind
the 'asteroid'.

The tail gives incontrovertible evidence that Phaethon ejects dust,
and gives researchers confidence that Phaethon is indeed the source of
the Geminids -- but a question remains as to how such a stubby tail
squares with such a grand meteor shower.  From the luminosity of the
tail, astronomers estimated that it had a total mass of something like
30 tons.  That might sound like a lot of meteoroids but it is in fact
orders of magnitude too small to sustain the massive Geminid debris
stream.  Perhaps Phaethon experienced a big event in the recent past.

This year's Geminid shower peaks on the night of Dec. 13-14 with
dozens of 'rock comet meteors' every hour, but the circumstances are
none too favourable, with the Moon, approaching Full, in Aries.  It
will set at about 4h on Dec. 14, leaving an interval of dark sky at
the end of the night.  Some Geminids should be seen on the nights of
Dec. 12 and 14 too.


GALAXY'S ANCIENT BROWN-DWARF POPULATION
RAS

A team of astronomers has discovered two of the oldest brown dwarfs so
far found in the Galaxy.  They are moving at speeds of 100-200 km/s,
much faster than normal stars and other brown dwarfs, and are thought
to have formed when the Galaxy was young, more than 10 billion years
ago.  The scientists believe that they could be part of a large and
previously unseen population of objects.  Brown dwarfs are star-like
objects but are much less massive (less than 7% of the Sun's mass),
and do not generate internal heat through nuclear fusion as stars do.
They simply cool and fade with time, and very old brown dwarfs become
very cool indeed -- the new discoveries have temperatures of 250-600°C,
much cooler than stars.  The team identified the new objects in a
survey made by the Wide-field Infrared Survey Explorer (WISE).  The
object names are WISE 0013+0634 and WISE 0833+0052, and they lie in
the Pisces and Hydra constellations respectively.  Additional
measurements confirming the natures of the objects came from large
ground-based telescopes.

The infrared spectra of the objects are unusual, reflecting their
ancient atmospheres which are almost entirely made up of hydrogen
rather than having a fair amount of the heavier elements seen in
younger stars.  Stars 'near' to the Sun belong to three overlapping
populations -- the thin disc, the thick disc and the halo.  The thick
disc is much older than the thin disc, and its stars typically have
higher random velocities in the direction perpendicular to the
Galactic plane.  Both of the disc components sit within the halo that
contains the remnants of the first stars that formed in the Galaxy.
About 97% of local stars are thin-disc members, so the other two
populations are quite rare.  Brown-dwarf population numbers probably
follow those of stars, which would explain why fast-moving thick-disc/
halo objects are only now being discovered.  The two brown dwarfs that
have now been identified *may* be the 'tip of an iceberg'.


BIG GAMMA-RAY BURST
University of Copenhagen - Niels Bohr Institute

Gamma-ray bursts (GRBs) are violent bursts of gamma radiation
associated with exploding massive stars.  Gamma rays are a *very*-
short-wavelength form of light rays, even shorter than X rays.  When
astronomers observe them, they never see the original star itself as
it is far too dim to be seen from so far away.  But when the star
dies, they can see the exploding star as a supernova.  Supernovae are
sometimes accompanied by bursts of gamma radiation.  GRBs are
extremely bright and can be seen from across the entire Universe, but
only by spacecraft because the Earth's atmosphere is opaque to gamma
rays.  The Swift satellite, which was launched in 2004, discovers
about 100 GRBs each year.  The bursts are thus quite common, but one
of the most powerful ones ever observed occurred in April.  As soon as
a burst is observed, the satellite directs instruments to measure
X-rays, ultraviolet radiation and optical light in the visible field.
That all happens very quickly, because the burst is over in less than
a minute.

Astronomers follow the so-called afterglow, which usually lasts a few
days to several weeks, from both Swift and ground-based telescopes.
In the April case, the burst was so powerful that they could observe
the afterglow for several months.  The spectra suggest that the
original star was a 'Wolf-Rayet' star -- a very hot star a few times
the radius of the Sun and with a mass of 20-30 times as much, and
rapidly rotating.  It was in a galaxy about 3.75 (american-)billion
light-years away.


INFANT GALAXIES MERGING NEAR 'COSMIC DAWN'
NASA

Astronomers using the Atacama Large Millimetre/sub-millimetre Array
(ALMA) telescope and the Hubble telescope have discovered a trio of
primitive galaxies within a cloud of primordial gas nearly 13 billion
light-years away.  That system, seen as it was when the Universe was
'only' 800 million years old, offers a vignette of the earliest stages
of galaxy formation during the period when the Universe was first
bathed in starlight.  The three galaxies appear poised to merge into a
single massive one, which might eventually evolve into something akin
to the Milky Way.  Researchers first detected the object, which
appeared to be a great bubble of hot, ionized gas, in 2009.  Dubbed
Himiko (after a legendary queen of ancient Japan), it is nearly ten
times larger than typical galaxies of that era and comparable in size
to the Milky Way.  Subsequent observations with the Spitzer space
telescope suggested that Himiko might represent a single galaxy, which
would make it uncharacteristically massive for that period of the
Universe; but the new ALMA observations showed that, rather than being
a single galaxy, Himiko harbours three distinct, bright sources, whose
intense star formation is heating and ionizing the enveloping cloud of
gas.  Areas of such furious star formation should be brimming with
heavy elements such as carbon, silicon, and oxygen, elements forged in
the nuclear furnaces of massive, short-lived stars like those in the
three galaxies detected by Hubble.  At the end of their relatively
brief lives, such stars explode as supernovae, seeding the inter-
galactic medium with dust containing heavy elements.  When the dust
is heated by ultraviolet radiation from massive newborn stars, it
re-radiates at radio wavelengths.  Such radiation is not detected in
Himiko.  Surprisingly, observations with ALMA revealed a complete
absence of the signal from carbon, which is rapidly synthesized in
young stars.  Exactly how the intense activity can be reconciled with
the primitive chemical composition of Himiko is quite puzzling.  The
astronomers speculate that a large fraction of the gas in Himiko could
be primordial, a mixture of the light elements hydrogen and helium,
which were created in the Big Bang.  If so, in Himiko we are seeing a
primordial galaxy in course of formation.


THE MOST DISTANT GALAXIES
National Radio Astronomy Observatory

For quite a few years, certain soi-disant scientists seem to have been
engaged in an unseemly (if undeclared) competition to discover the
most distant galaxies -- galaxies that existed the soonest after the
Big Bang.  Records have been set and supposedly broken, and the
protagonists have jousted over which of the alleged early galaxies are
real and which are embarrassing errors.  In 2009, astronauts fitted
the Hubble telescope with a new camera, called Wide-Field Camera 3,
that is far more efficient than its predecessors and can see very red
light of the right wavelengths for detecting the most distant galaxies.
When it produced its first batch of data in 2009, the competing groups
pounced on it and quickly found dozens of galaxies much deeper in time
than had been seen before.  At first, the farthest galaxies detected
were at 650 million years after the Big Bang, then 'only' 500 million
years, and now even 450 million years or less, though those figures
are the subject of debate.  Some results, however, seem largely to
have been agreed.  Scientists now believe that the hydrogen fog that
permeated the early Universe was cleared by the starlight from huge
numbers of rather faint, small galaxies, not from a few very large
ones.


THE ERA OF NEUTRINO ASTRONOMY HAS BEGUN
University of Maryland

Astrophysicists using a telescope embedded in Antarctic ice have
succeeded in a quest to detect and record cosmic neutrinos -- nearly
massless particles that stream to Earth at the speed of light from
outside our Solar System.  Next, they hope to build on the early
success of the IceCube Neutrino Observatory to detect the source of
the neutrinos.  By studying the neutrinos that IceCube detects,
scientists may be able to learn about the nature of phenomena
occurring millions or even billions of light-years from the Earth.
The sources of neutrinos, and what could accelerate them, are still
unknown.  Now scientists have an instrument that can detect
astrophysical neutrinos, and they expect it to run for another 20
years.  Neutrinos are among the basic building blocks of the Universe.
Millions of them pass through our bodies unnoticed every second.  They
have high energies and maintain their speed and direction unaffected
by magnetic fields.  The vast majority of them originates either in
the Sun or in the Earth's own atmosphere.  Far rarer are astrophysical
neutrinos, which come from the outer reaches of our Galaxy or beyond.
The origin and cause of astrophysical neutrinos are unknown, though
gamma-ray bursts, active galactic nuclei and black holes are potential
sources.  Better understanding of such matters is important both to
particle physics and to astronomy, and scientists have hoped for a
long time to have a neutrino detector such as IceCube.

IceCube has two major scientific goals: to measure the flux, or rate,
of high-energy neutrinos, and to identify some of their sources.  The
observatory is buried deep in the Antarctic ice but looks out at the
entire Universe, detecting neutrinos coming through the solid Earth
from the northern skies as well as from the southern ones.  In 2012
April, IceCube detected two high-energy events above 1 peta-electron-
volt (PeV, 10 to the power 15 electron volts, or 1.6 x 10*-4 joules),
the first astrophysical neutrinos definitely recorded by a terrestrial
detector.  The IceCube team then searched their records of 2010--2012
for events that fell slightly below the energy level of their original
search.  They discovered 26 more high-energy events, all at levels of
30 tera-electron-volts (TeV, 10*12 eV) or higher, indicative of
astrophysical neutrinos.  Since astrophysical neutrinos move in
straight lines unimpeded by outside forces, they can act as pointers
to the place where they originated.  The 28 events recorded so far are
too few to point to any one location.  Over the coming years, the
IceCube team will watch as more measurements fill in a picture that
may reveal the points of origin.