RHEA HAS AN 'ATMOSPHERE', BUT IT IS AN INCREDIBLY THIN ONE
BBC News
Scientists using the Cassini probe say that Rhea, the second-biggest
of Saturn's moons, has an excessively thin atmosphere of oxygen and
carbon dioxide. Oxygen exospheres have previously been found on
Jupiter's moons Europa and Ganymede. The tenuous envelope around Rhea
is maintained by high-energy particles that constantly bombard the
moon's icy surface. As Saturn's magnetic field rotates with the
planet, particles carried in the field impinge on the hemisphere of
Rhea that faces their flow and break water molecules on the surface.
The atoms then rearrange themselves to make O2 molecules, which are
sputtered from the surface by other impacting particles. It is an
ongoing process -- as fast as the oxygen is created, it is being lost
into space. The researchers say that the mechanism driving the
production of carbon dioxide is less obvious. Like the O2, it could
be being produced as a result of high-energy particle impacts, if
carbon- containing compounds are present in the surface ice. It is
possible also that the carbon dioxide was made in deep sub-surface
processes and the CO2 is slowly escaping the moon's body. Cassini's
mass spectrometer measured peak densities of oxygen and carbon dioxide
of about 50 and 20 molecules per cubic millimetre, respectively. Such
a density of oxygen is about 10*(-14) of that at sea-level on the
Earth, so it is hardly likely to support life as we know it.
ASTRONOMERS FIND 'ROSETTA STONE' FOR T-DWARF STARS
RAS
An international team of astronomers has discovered a star system with
a very cool methane-rich (or T-) dwarf star and a white-dwarf stellar
remnant in orbit around one another. The system is the first of its
kind to be found, and gives scientists a means to estimate the mass
of a T dwarf. The two stars are low in mass and have a weak mutual
gravitational attraction, as they are separated by about a quarter of a
light-year. Despite the frailty of the system it has stayed together
for billions of years, but its stars are cooling down to a dark
demise.
Methane dwarfs are on the star/planet boundary and are about the size
of Jupiter. They have temperatures of less than 1000°C. Methane is a
fragile molecule destroyed at higher temperatures, so it is seen only
in very cool stars and objects like Jupiter. Neither giant planets
nor T-dwarf stars are hot enough for the hydrogen fusion that powers
the Sun to take place, so they simply cool and fade over time. White
dwarfs are the end state of stars similar to and including the Sun.
Once such stars have exhausted the available nuclear fuel in their
cores, they expel most of their outer layers into space, forming a
remnant planetary nebula, and leave behind a hot, but cooling, core
or white dwarf about the size of the Earth. In the newly-discovered
binary, the remnant nebula has long since dissipated, and all that is
left is the steadily cooling white dwarf and its methane-dwarf
companion. The present temperature of the white dwarf provides an
estimate of the age of both objects, and calibrates properties of the
methane dwarf such as its mass.
The methane dwarf was identified by UKIRT as part of a project to
identify the coolest objects in the Galaxy. Its temperature and
spectrum were measured by the Gemini North telescope in Hawaii. The
team then found that the methane dwarf shares its proper motion with a
nearby blue object catalogued as LSPM 1459+0857, which they studied
with the VLT, finding it to be a cool white dwarf. The objects were
then re-named LSPM 1459+0857 A and B.
MASS OF ONE CEPHEID VARIABLE STAR DETERMINED
European Southern Observatory
Astronomers have used the 3.6-m NTT telescope in Chile to observe the
first double star to be discovered in which a pulsating Cepheid
variable and another star eclipse one another, and have determined
accurately the mass of the Cepheid. Up to now astronomers had two
incompatible theoretical predictions of Cepheid masses; the new result
shows that the prediction from stellar-pulsation theory is correct,
while the prediction from stellar-evolution theory is not. Classical
Cepheid variables, usually just called Cepheids, are unstable stars
that are much larger and brighter than the Sun. They expand and
contract in a regular way, taking anything from a few days to a month
or so to complete the cycle. There is a relationship between the
stars' luminosities and the periods of their variability, making the
study of Cepheids a way to estimate the distances to 'nearby' galaxies
and from there to extrapolate the distance scale to the whole Universe.
Unfortunately, Cepheid masses derived from the theory of pulsating
stars are 20-30% less than predictions from the theory of the
evolution of stars. That embarrassing discrepancy has been known
since the 1960s. The discovery of an eclipsing binary that includes a
Cepheid has now allowed the mass of one such star to be determined
accurately from the orbital parameters derived from radial-velocity
measurements. The system is in the Large Magellanic Cloud and
contains a Cepheid pulsating every 3.8 days. The other star is
slightly bigger and cooler, and the two stars orbit one another in 310
days. The mass of the Cepheid was found to be 4.14 ± 0.05 solar
masses, in excellent agreement with expectations from the theory of
stellar pulsation.
PLANET FROM ANOTHER GALAXY
ESO
In the last 15 years, astronomers have detected nearly 500 planets
orbiting stars in our cosmic neighbourhood, but none outside our Milky
Way. Now, a planet with a minimum mass 1.25 times that of Jupiter has
been discovered orbiting a star that is thought to be of extragalactic
origin, although it is now in our own Galaxy. It is part of the
'Helmi stream' -- a group of stars that may have belonged originally
to a dwarf galaxy that merged with the Milky Way billions of years ago.
The star, known as HIP 13044, is about 2000 light-years away in the
southern constellation Fornax. The astronomers detected the planet
from radial-velocity measurements made with the 2.2-m telescope at La
Silla in Chile. The planet is one of the few such objects known to
have survived the period when its host star passed through the
red-giant phase of stellar evolution and became very much bigger after
exhausting the hydrogen fuel supply in its core. The star has now
contracted again and become a so-called horizontal-branch star,
burning helium in its core. Even so, the planet is less than one
stellar diameter from the surface of the star at periastron (the
closest point in its elliptical orbit). Its 'year' -- the time taken
to complete an orbit -- is only 16.2 days.
POSSIBLE COLLISION IN LOCAL GROUP AGES AGO
Paris Observatory
Numerical simulations suggest that a major collision between two
massive galaxies occurred in the Local Group 6 billion years ago.
It seems that the Andromeda Galaxy (M31), as well as the Magellanic
Clouds, may have formed as a result of such a collision between
two galaxies. The Local Group includes nearly 40 galaxies and is
dominated by two giant spiral ones -- Andromeda and our own Galaxy,
the Milky Way. Some astronomers believe that Andromeda may have
formed through a collision and merger event between two galaxies, one
slightly more massive than the Milky Way, the other about 3 times less
massive. The epoch suggested for the event is that the first passage
and the final fusion occurred about 9 and 5.5 billion years ago,
respectively. Such a collision would be the most important event that
has ever occurred in the Local Group, because the Andromeda galaxy and
its satellites gather the largest fraction of the mass of the Group.
The collision must have been violent enough to generate the large
amount of angular momentum (rotation) needed to form the great
galactic disc of Andromeda.
The researchers also considered the possible consequences of such a
major event in the neighbourhood of our Galaxy, and proposed
that the Magellanic Clouds could have formed within one of the tidal
tails produced during the interaction 9 billion years ago. The Clouds
might have been ejected towards the Milky Way, at a velocity that has
recently been re-evaluated at about 300 km/s. That might explain why
the Magellanic Clouds are the only gas-rich and irregular companions
of the Milky Way.
MASSIVE GALAXIES MAY HAVE BEEN FORMED WHEN THE UNIVERSE WAS YOUNG
ScienceDaily
Astronomers have identified a large number of galaxies five to ten
times more massive than our own Milky Way among a sample studied at
red-shifts 3 to 4, when the Universe was between 1.5 and 2 billion
years old. More than 80% of the massive galaxies show very high
infrared luminosities, indicating that they were extremely active,
probably in a phase of intense growth. Current ideas about the
physical processes responsible for forming such massive galaxies has
difficulty in producing them so quickly. Before too much excitement
is generated by that difficulty, however, the caveat is made that the
red-shifts were estimated only from spectral energy distributions and
not by actual spectroscopy, and may be mistaken.
Topic: Early December Astronomy Bulletin (Read 664 times)