COLDEST PLACE IN SOLAR SYSTEM
BBC News
The Moon has the coldest place yet measured in the Solar System by a
spacecraft. The Lunar Reconnaissance Orbiter has been observing the
insides of permanently shadowed lunar craters, and has found that
surface temperatures inside the coldest craters in the northern polar
region can dip as low as -249 C. That occurred near the 'winter
solstice', which took place in October. The Moon does have seasons --
just about. The tilt of the lunar axis with respect to the normal to
the ecliptic is 1.54 degrees, so it causes a small, three-degree
change in the elevation of the Sun through the course of a year. For
most places, that makes negligible difference, but at the poles, it
results in a significant variation in the extent of shadows and
temperatures. Calculations suggest that one would have to travel to a
distance beyond the Kuiper Belt -- well beyond the orbit of Neptune --
to find objects with surfaces so cold. The measurement tends to
support the idea that some craters on the Moon could harbour water-ice
for extended periods.
SPECULAR REFLECTION SEEN FROM A LAKE SURFACE ON TITAN
NASA
The Cassini spacecraft has photographed a flash of sunlight reflecting
from a lake on Saturn's moon Titan, confirming the presence of liquid
hydrocarbons on a part of the moon dotted with many lake-like basins.
Cassini scientists have been hoping to see such a specular reflection
ever since the spacecraft began orbiting Saturn in 2004, but until
recently Titan's northern hemisphere, where most of the lakes are
located, has been veiled in winter darkness. Now, however, the
seasons are changing and sunlight has returned to the north, allowing
Cassini to capture a picture which shows sunlight reflecting from the
smooth surface of a liquid. Scientists have theorized for 20 years
that Titan's cold surface could have seas or lakes of liquid
hydrocarbons, making it the only other planetary body besides the
Earth believed to have liquid on its surface. While data from Cassini
have not indicated any vast seas, they have shown what appeared to be
large lakes near Titan's north and south poles.
In 2008, Cassini scientists using infrared data identified the
presence of liquid in Ontario Lacus, the largest lake in Titan's
southern hemisphere. But they still wanted to confirm liquid in the
northern hemisphere, where the basins are larger and more numerous.
At last, in July, they saw a reflection at the southern shoreline of a
lake called Kraken Mare, which covers about 400,000 square kilometres,
an area larger than the Caspian Sea, the largest lake on Earth. By
comparing that new image to radar and near-infrared images acquired
since 2006, scientists were able to show that the shoreline of Kraken
Mare has been stable over the last three years and that Titan has an
ongoing hydrological cycle that brings liquid (in this case probably
methane) to the surface.
HUBBLE FINDS SMALLEST KUIPER BELT OBJECT YET DETECTED
Space Science Institute
The Hubble telescope has discovered the smallest object yet
detected in the Kuiper Belt, a ring of icy debris that circulates
in the Solar System just beyond Neptune. The object is only 1 km
across and is 6.8 billion kilometres away. The smallest Kuiper Belt
Object (KBO) seen previously, in reflected light, is roughly 50 km
across. This is the first observational evidence for a population of
comet-sized bodies in the Kuiper Belt that are being ground down
through collisions. The Kuiper Belt is therefore collisionally
evolving, meaning that the region's icy content has been modified over
the past 4.5 billion years.
The object detected by Hubble is so faint, at 35th magnitude, it is
100 times dimmer than anything the telescope can see directly. So
then how did the space telescope uncover such a small body? Hubble
has three optical instruments called Fine Guidance Sensors (FGS). The
FGSs provide navigational information to the telescope's attitude-
control systems by looking at selected guide stars for pointing. They
would also be able to see the effects of a small object passing in
front of a star. That would cause a brief occultation and diffraction
signature in the FGS data as the light from the guide star was bent
around the intervening foreground KBO. They selected 4.5 years of FGS
observations for analysis. Hubble spent a total of 12,000 hours
during that period looking along a strip of sky within 20° of the
Solar System's ecliptic plane, where the majority of KBOs should be.
The team analyzed the FGS observations of 50,000 guide stars in total,
and found one single 0.3-second-long occultation event. In
interpreting it, they had to make some assumptions about the nature of
the object's orbit. The duration of the occultation was short largely
because of Earth's orbital motion around the Sun. The distance was
estimated from the duration of the occultation, and the amount of
dimming was used to calculate the size of the object. Hubble
observations of nearby stars show that a number of them have Kuiper
Belt-like discs of icy debris encircling them. The discs are thought
to be the remnants of planetary formation. The prediction is that
over billions of years the debris should collide, grinding the
KBO-type objects down to smaller pieces that were not part of the
original population.
'SUPER-EARTH' ORBITS RED DWARF
Science Daily
Astronomers have discovered a 'super-Earth' transiting in front of a
red dwarf star 40 light-years from Earth. A super-Earth is defined as
a planet between one and ten times the mass of the Earth. The
new-found planet, GJ1214b, is about 6.5 times as massive as the Earth.
Its host star, GJ1214, is a small, red M-type star about one-fifth the
size of the Sun. It has a surface temperature of only about 2700C and
a luminosity only three-thousandths that of the Sun. GJ1214b orbits
its star once every 38 hours at a distance of only 1.3 million miles.
Astronomers estimate the planet's temperature to be about 200 C, which
is less hot than any other known transiting planet, because it orbits
a very dim star. Since GJ1214b crosses in front of its star,
astronomers were able to measure its radius, which is about 2.7 times
that of Earth, makes it one of the two smallest transiting planets
known so far.
Astronomers found the planet with the MEarth (pronounced 'mirth')
Project -- an array of eight identical 16-inch telescopes that monitor
a pre-selected list of 2,000 red dwarf stars. MEarth looks for stars
that change brightness. The goal is to find a planet that crosses in
front of, or transits, its star. The Kepler mission also uses
transits to look for Earth-sized planets orbiting Sun-like stars.
However, such systems dim by only one part in ten thousand. The higher
precision required to detect the drop means that such planets can be
found only from space. In contrast, a super-Earth transiting a small,
red dwarf star yields a greater proportional decrease in brightness,
detectable from the ground. Astronomers then use instruments like the
HARPS (High-Accuracy Radial-Velocity Planet Searcher) spectrograph at
the European Southern Observatory to measure the companion's mass and
confirm that it is a planet, as they did with this discovery.
GIANT PLANET FOUND IN CATACLYSMIC BINARY SYSTEM
RAS
A team of Chinese astronomers has discovered a giant planet close to
the binary-star system QS Virginis. Although dormant now, the two
stars will one day erupt in a violent nova-like outburst. QS Virginis
is 157 light-years from the Sun and the system is made up of a cool
red dwarf and a hot dense white dwarf just 840,000 km apart or about
twice the distance from the Earth to the Moon. They take just 3 hours
and 37 minutes to complete each orbit. No telescope can see them as
separate stars, but as they move in their orbit the two stars
successively eclipse one another, leading to characteristic periodic
changes in the brightness of the system. In many close binaries,
described as cataclysmic variables (CVs), material flows from the red
dwarf star onto its much hotter and denser companion. The stars of
QS Virginis are only slightly too far apart for this to happen, so the
system is 'hibernating' -- relatively quiet at the moment.
The astronomers measured the light fluctuations of QS Virginis
throughout the orbit. By timing the eclipses, they found that the
duration of the orbit changed, with the time of mid-eclipse
periodically advanced or delayed. The shift is explained by an unseen
third object exerting a gravitational pull on the two stars, so
sometimes the light has to travel a little further and sometimes a
little less far to reach us. From their measurements, the Chinese
team deduced that there is a giant planet, with a minimum of 6.4 times
the mass of Jupiter, at an average distance from the stars of 4.2
times that from the Earth to the Sun. The red dwarf star is being
braked by the interaction between its stellar wind and magnetic field
and is decelerating. As it loses energy it must move closer to the
white dwarf, and some time in the next few thousand years it will be
near enough that hydrogen will start to flow from it onto its small
but hot companion star.
The hydrogen will then slowly accumulate on the white dwarf and build
up in a hot dense layer close to the star's surface. In due course
the density and temperature at the base of the hydrogen layer will
become high enough for nuclear reactions to take place. The resulting
explosion will lead to a spectacular outburst of radiation, and the
binary system will temporarily become many times brighter than normal.
Such an explosion is only in a superficial layer and does not
fundamentally change the star, as in the case of a supernova explosion;
it can, and in many systems does, take place repeatedly. Cataclysmic
variable stars all operate in that fashion, with repeated explosions
that take place on characteristic time-scales ranging from about a
fortnight upwards. If the typical interval between outbursts is as
long as decades, the object is called a recurrent nova; a nova is
simply an object of the same type that has been seen to explode only
once.
BLUE STRAGGLERS IN NGC 188
University of Wisconsin at Madison.
For some 50 years it has been recognized that many star clusters
include a small proportion of objects called blue stragglers, which
are too hot and lumninous to be as old as the rest of the stars in the
cluster - they ought to have expired by now. It has become apparent
that they must have been rejuvenated by receiving a substantial
donation of additional material from another star, usually the
companion in a binary system of which the star we now see as a blue
straggler is a member.
A recent discussion has featured the galactic star cluster NGC 188.
It has incorporated radial-velocity measurements donated by the
moderator of these Bulletins and his collaborator James Gunn, made
about 30 years with the 200-inch telescope at Palomar. NGC 188 is
situated in the sky near Polaris, and is some 6,000 light-years from
the Earth; it consists of several thousand old stars, all about the
same age, and it includes 21 blue stragglers.
The team noted that at least three-quarters of the blue stragglers in
NGC 188 occur in binary systems, including one binary system made up
of two blue stragglers. That object, they argue, is emblematic of the
complex binary dances and exchanges, including 'partner swapping',
occurring in the NGC 188 environment. Almost certainly the blue
stragglers formed in separate binary systems, and then the two
binaries concerned encountered one another, ejecting two of the stars
and leaving behind that unusual object. The survey of NGC 188's blue
stragglers also reveals that the stars are spinning much faster than
an average star, a quality that the astronomers hope might lead to
estimates of how recently the blue stragglers were formed.
Topic: Early January Astronomy Bulletin (Read 1476 times)