Topic: Early May Astronomy Bulletin

[Clive]

JUPITER'S ATMOSPHERE STILL CONTAINS WATER FROM SL9 COMET IMPACT
Astronomy & Astrophysics

In 1994, Comet Shoemaker-Levy 9 (SL9) hit Jupiter and left scars that
were visible on the Jovian disc for weeks.  That was the first direct
observation of a collision in the Solar System.  SL9 was discovered
orbiting Jupiter by astronomers David Levy and Carolyn and Eugene
Shoemaker on 1993 March 24.  It was the first comet observed orbiting
a planet rather than the Sun, and was found to be composed of at least
21 fragments.  Soon after that, orbital studies showed that the comet
had passed within Jupiter's Roche limit in 1992.  Inside that limit,
the planet's tidal forces are strong enough to disrupt a body held
together by its own gravity, thus explaining SL9's fragmentation.
Observations showed that SL9's orbit, which was substantially changed
by the close approach at which the comet was disrupted, would
intersect Jupiter's surface in 1994 July, so the comet was about to
destroy itself by colliding with the planet, with impact near latitude
44°S.

The SL9 impact and the scars it left on Jupiter were observed for
weeks, but its chemical impact on Jupiter's atmosphere has lasted much
longer.  Water vapour was observed spectroscopically during the
fireball phase of the SL9 impacts, but from that observation it was
difficult to assess how it might modify Jupiter's composition in the
long term.  In 1997, the ESA Infrared Space Observatory (ISO) detected
water vapour in the stratosphere of Jupiter.  At that time,
astronomers suspected that it might be a consequence of the SL9 impact
because comets are known to be water-rich bodies.  However, there were
other possible sources of water -- interplanetary dust particles
produced by cometary activity and asteroid collisions, icy rings, or
one of the 60 Jovian satellites.  Nearly twenty years after the major
impact, astronomers are still observing its consequences on Jupiter.
The Herschel space telescope is sensitive enough to map the abundance
of water vs. latitude and altitude in the Jovian stratosphere.  Its
observations show a clear north-south asymmetry in the distribution of
water, with more of it in the south.  They indicate that 95% of the
water currently observable on Jupiter came from the comet.


COMET ISON METEOR SHOWER
NASA

The 'Sun-grazing' Comet ISON (C/2012 S1), now approaching the inner
Solar System, will make a close encounter with the Sun in November,
and may well become one of the most spectacular comets in many years.
When the Swift spacecraft observed it in January, when it was still
near the orbit of Jupiter, it was already very active -- it was
expelling dust from the nucleus at a rate estimated at nearly a ton a
second.  Computer models suggest that for several days around 2014
January 12 the Earth will pass through a stream of fine-grained debris
from Comet ISON at a relative speed of 56 km/s.  Because the particles
are so small, the Earth's upper atmosphere will slow them to a stop.
Instead of burning up in a flash of light, they will drift gently down
to the surface, probably taking months or even years to settle out.
While the dust is in the upper atmosphere, it may help to produce
noctilucent clouds (NLCs), which are bluish icy clouds, possibly
seeded by space dust, that form very high up (more than 80 km) in the
polar regions in summer time.


HOT SPOTS OBSERVED IN OUTER ATMOSPHERE OF BETELGEUSE
RAS

A new image of the outer atmosphere of Betelgeuse -- one of the
nearest red supergiants -- shows structure in the 'stellar wind' of
material being thrown off the star.  The image was obtained by the
e-MERLIN radio-telescope array operated from Jodrell Bank.  The star
itself is 1,000 times the diameter of the Sun, but at a distance of
about 200 parsecs (650 light-years) it still appears as a tiny dot
in the sky.  Its disc can, however, be resolved by an interfero-
metric technique involving the use of separated telescopes.

The image shows that the Betelgeuse atmosphere extends out to five
times the size of the visual surface of the star.  There are two hot
spots within the outer atmosphere, and a faint arc of cool gas even
farther out, beyond the radio surface of the star.  The hot spots,
separated by about half the visual diameter of the star, have a
temperature of about 4,000-5,000 K, much higher than the average
temperature of the radio surface of the star (about 1,200 K) and even
higher than the visual surface (3,600 K).  There is an arc of cool gas
almost 7.4 billion kilometres away from the star -- about the same
distance as the furthest Pluto gets from the Sun.  It is estimated to
have a mass almost two-thirds that of the Earth and a temperature of
about 150 Kelvin.

Researchers said it was not clear why the hot spots are so hot.
They seem not to have recognized that the hot regions in the high
atmosphere are where the atmospheric gas is very tenuous; temperature
there is not something that could be measured by a thermometer, but is
a measure of the mean rate at which the atoms move.  There is probably
an analogy to be drawn with the solar corona, which has a temperature
that is numerically 200 times higher than that of the photosphere, but
it is not heated by conduction from somewhere hotter, like a joint
being cooked by hot gas in an oven -- the rapid motions of coronal
atoms and ions are induced by other means, by deposition of energy
from mechanical and electromagnetic waves that originate in regions
that are themselves at much more moderate temperatures.  According to
the item being reproduced here (albeit in edited form), a possibility
is that shock waves, caused either by the star pulsating or by
convection in its outer layers, are compressing and heating the gas.
Another is that the outer atmosphere is patchy and we are seeing
through to hotter regions within.  The arc of cool gas is thought to
be the result of a period of increased mass loss from the star at some
point in the last century, but its relationship to structures like the
hot spots, which lie much closer in, within the star's outer
atmosphere, is unknown.

The mechanism by which supergiant stars like Betelgeuse lose matter
into space is not understood, although it obviously plays an important
role in enriching the interstellar material from which future stars
will form.  Betelgeuse produces an outward-blowing 'stellar wind'
equivalent to losing the mass of the Earth every three years, enriched
with the elements that will go into the next generation of stars.


KEPLER DISCOVERS SMALLEST 'HABITABLE ZONE' PLANETS
NASA

The Kepler mission is reported to have discovered two new planetary
systems that include three super-Earth-size planets in the 'habitable
zone'.  It ought to be made clear -- but isn't, except here -- that
the misleading expression 'habitable zone' has been coined, and is
deliberately used, entirely mischievously by the planet enthusiasts,
in an effort to conjure up wholly unsubstantiated vignettes of alien
populations of 'little green men' or other imaginary beings.  When the
matter is considered with normal, scientifically becoming, sobriety,
it is immediately clear that *any* celestial object that has a
temperature above the boiling point of water must be surrounded, at a
certain range of distances, with places where another body that is in
some sort of radiative equilibrium with it will have a surface
temperature within the range where water is liquid.  It is that range
of distances, which astronomically is of no particular interest or
significance, which is what the planet people try to imbue with
special importance by their grandiloquent characterization of it as
the 'habitable zone'.  The Gilbertian punishment would be to send them
there; it would not take long after their arrival before they realised
that they had omitted to consider, or at any rate to mention to their
public, that there is lot more to the concept of being habitable than
merely being within the range of temperatures from freezing to
boiling!

The two planetary systems are Kepler 62 and Kepler 69.  The Kepler 62
system has five planets; it may seem surprising that, in contrast to
the semantic initiative that originated the term habitable zone, the
planets suffer the prosaic designations 62b, c, d, e and f.  Likewise
the Kepler 69 system's two planets are called 69b and c.  Kepler 62e,
62f and 69c are the 'habitable-zone' 'super-Earths'.  The planets of
the Kepler 62 system orbit a star classified as a K2 dwarf, which is
two-thirds the size of the Sun and only one-fifth as bright.  At seven
(American-)billion years old, the star is somewhat older than the Sun.
It is about 400 parsecs (1,200 light-years) away, in the constellation
Lyra.  Kepler 62f is only 40% larger than the Earth, making it the
exo-planet closest to the size of our planet known in the 'habitable
zone' of another star.  Kepler 62f may have a rocky composition.  We
are told that "Kepler 62e orbits on the inner edge of the habitable
zone" [just a planet-astronomer's circuitous way of saying that it is
boiling hot!] and is roughly 60% larger than the Earth.  Kepler 69 is
a star in the same class (G) as our Sun.  It is 93% the size of the
Sun and 80% as luminous and is about 800 parsecs away, in Cygnus.
Kepler 69c is 70% larger than the Earth.  Astronomers are uncertain
about the composition of Kepler 69c, but it has an orbital period of
242 days.


BURSTS OF GAMMA RAYS POINT TO NEW WAY TO DESTROY A STAR
University of Warwick

Astronomers have identified a new type of exceptionally powerful and
long-lived cosmic explosions, prompting a theory that they arise in
the violent demise of a supergiant star.  The explosions create blasts
of high-energy gamma-rays, known as gamma-ray bursts, but while most
bursts are over in about a minute, the new type can last for hours.
The first example was observed on Christmas Day 2010, but it lacked a
measurement of distance, so its nature remained uncertain.  A new
study has found several analogous examples of unusual cosmic
explosions and shows that the Christmas-Day burst took place in a very
distant galaxy.

Using data from the Gemini telescope in Hawaii, the scientists found
that that long gamma-ray burst was in a galaxy with a redshift of
0.847, giving it a location approximately half-way to the edge of the
observable Universe, or 7 billion light-years away.  The scientists
now think that that kind of burst is caused by a supergiant, a star 20
times the mass of the Sun, which evolves to become among the biggest
and brightest of all stars, with a radius up to 1,000 times that of
the Sun.  They think that the durations of the Christmas gamma-ray
burst and two other similar ones are simply due to the sheer size of
the supergiants exploding as supernovae.  Most stars that create
gamma-ray bursts are thought to be relatively small and dense, and the
explosion that destroys them punches through the star in a matter of
seconds.  In the case of the newly recognized long bursts the
explosion takes much longer to propagate through the star, so the
gamma-ray burst lasts for a much longer time.


ALMA MAPS EARLY GALAXIES QUICKLY
RAS

A team of astronomers has used the new ALMA (Atacama Large Millimetre
Array) telescope to determine the positions of over 100 of the most
fertile star-forming galaxies in the early Universe.  ALMA made the
observations, which would be time-consuming with other instruments,
in a total of just a few hours.

The most fertile bursts of star birth in the early Universe took place
in distant galaxies containing lots of cosmic dust.  They are of some
importance to our understanding of galaxy formation and evolution, but
the dust obscures them and makes them difficult to identify with
visible-light telescopes, so they need to be observed with telescopes
that work at much longer wavelengths, such as ALMA.  The best map of
them so far was made with the Atacama Pathfinder Experiment telescope
(APEX). It surveyed a patch of the sky about the size of the Full
Moon, and detected 126 such galaxies.  In the APEX images, however,
each burst of star formation appeared as a relatively fuzzy blob.
While APEX has a single 12-metre antenna, ALMA combines the signals
from many APEX-like antennae spread over considerable distances and
obtains a resolving power equivalent to that of an aperture equal to
that of the whole array.

The team used ALMA to observe the galaxies from the APEX map during
ALMA's first phase of scientific observations, with the telescope
still under construction.  Using less than a quarter of the final
complement of 66 antennae, spread over distances of up to 125 metres,
ALMA needed just two minutes per galaxy to determine their positions
much more accurately than APEX, and with higher sensitivity.  Then
the team could identify unambiguously which galaxies had regions
of active star formation, and in some cases they found that multiple
star-forming galaxies had been blended into a single blob in the
previous observations.  It had been thought that the brightest of the
galaxies were forming stars a thousand times more vigorously than our
own galaxy, the Milky Way, putting them at risk of blowing themselves
apart.  The ALMA images show multiple, smaller galaxies forming stars
at somewhat more reasonable rates.