FIRST SUCCESS FOR OLD PLANET-HUNTING METHOD
JPL
Astrometry, a long-tried but hitherto unsuccessful method of fishing
for planets outside the Solar System, has netted its first catch:
a Jupiter-like planet orbiting one of the least-massive stars known.
It involves measuring the position of a star on the sky as it orbits
with an unseen planet. But the method requires very precise
measurements over long periods of time, and until now has failed to
turn up any exo-planets.
For the past 12 years astronomers have been observing 30 stars with an
astrometric instrument on the 200" telescope at Palomar. The
newly found exo-planet, called VB 10b, is about 20 light-years away in
the constellation Aquila. It is a gas giant, with a mass 6 times that
of Jupiter and an orbit about as far from its star as Mercury is from
the Sun -- far enough away from its star to be regarded as a 'cold
Jupiter' analogous to our own. In reality, the planet's own internal
heat might give it an Earthlike temperature. The planet's star, VB
10, is an M-type dwarf and has only one-twelfth the mass of our Sun,
just barely big enough to fuse atoms at its core and shine with
starlight. For years, VB 10 was the least massive star known; now it
the least massive star known to host a planet. In fact, even though
the star is much more massive than the planet, it is not a great deal
bigger.
BETELGEUSE IS SHRINKING
AAS
The red supergiant star Betelgeuse, in the constellation Orion, has
steadily shrunk over the past 15 years, according to University of
California, Berkeley, researchers. Long-term monitoring by an
interferometer on Mount Wilson in California shows that Betelgeuse,
which is so big that in our Solar System it would reach to the orbit
of Jupiter, has shrunk in diameter by more than 15% since 1993.
Despite the star's diminished size, its visual brightness has shown no
significant decrease during that period.
Betelgeuse was the first star (after the Sun) to have its angular
size directly measured, and even today it is one of only a handful of
stars that can be identified by the Hubble telescope as being slightly
extended rather than an unresolved point of light. In 1921, Francis
Pease and Albert Michelson used optical interferometry to estimate
that its diameter was equivalent to the orbit of Mars. Last year, new
ideas of the distance to Betelgeuse raised it from 430 light-years to
640, which increased the diameter derived from it from about 3.7 to
about 5.5 astronomical units (Earth--Sun distances). Since 1921, its
size has been re-measured by many different interferometer systems.
At a given wavelength the variation has been scarcely beyond the
measurement uncertainties, but different wavelengths give diameters
differing by up to 30%, because the tenuous gas in the outer regions
of the star emits light at some wavelengths and absorbs it at others
-- the star looks bigger in the light of an emission line than it does
in an absorption line.
The interferometer built at Berkeley in the early 1990s sidesteps the
confounding emission and absorption lines by observing in the mid-
infrared with a narrow bandwidth that can be tuned between spectral
lines. The interferometer combines signals from pairs of telescopes
in order to determine path-length differences between light that
originates at the star's centre and light that originates at the
star's edge. Its observing wavelength of about 11 microns, in the
mid-infrared, penetrates the dust, and the narrow bandwidth avoids
any spectral lines, so the star is seen relatively undistorted.
The first measurements showed the size to be quite close to Pease and
Michelson's result, but over 15 years it has decreased about 15%,
changing smoothly but faster as the years have progressed.
CLUSTER IN CROWDED NEIGHBOURHOOD PROVES TO BE NORMAL
Science Daily
Using ESO's Very Large Telescope, astronomers have been observing the
Arches Cluster, which is about 25 000 light-years away towards the
constellation Sagittarius and contains about a thousand young, massive
stars, less than 2.5 million years old. It is close to the centre of
our Milky Way, where it must be subject to unusual disturbances from
the stars, gas and the super-massive black hole there. The Arches
Cluster is ten times more massive than typical young star clusters
scattered throughout the Milky Way, and it has unusually high
abundances of chemical elements heavier than helium. Observing the
Arches Cluster is challenging because of the huge quantities of
absorbing dust, which visible light cannot penetrate, between the
Earth and the Galactic Centre. The new study, made in the infrared,
confirms the Arches Cluster to be the densest cluster of massive young
stars known. It is about three light-years across, with more than a
thousand stars packed into each cubic light-year -- an extreme density
a million times greater than in the Sun's neighbourhood.
Astronomers studying clusters of stars have always found that
high-mass stars are rarer than less-massive ones, and their relative
numbers are the same everywhere. but the Arches Cluster has seemed to
be a striking exception. With the extreme conditions in the cluster,
one might indeed imagine that stars would not form in the same way as
in our quiet solar neighbourhood. However, the new observations show
that the distribution of masses in the cluster actually does follow
the same universal law. The most massive star has a mass of about 120
times that of the Sun. Astronomers conclude from that that, if stars
more massive than 130 solar masses exist, they must live for less than
2.5 million years and end their lives without exploding as supernovae,
as massive stars usually do. The total mass of the cluster seems to
be about 30 000 times that of the Sun, much more than was previously
thought.
NEW CLASS OF DIM SUPERNOVAE
Science Daily
Core-collapse (or gravitational) supernovae are the final tremendous
explosions that end the life-cycles of stars more massive than
approximately 8 times the Sun. After running out of fuel, the core of
such a star collapses and forms a neutron star or a black hole. At
the same time, the outer layers are ejected at high velocity and
briefly shine as brightly as billions of stars together. The total
energy suddenly released by such a supernova exceeds the total energy
released by the Sun during its whole past and future life time of 10
billion years. However, some core-collapse supernovae are up to 100
times less energetic and luminous than usual. Such 'low'-power
explosions normally show the presence of hydrogen gas, but a recent
event, supernova SN 2008ha, is the first dim supernova in which no
hydrogen could be detected. Taken together, the dimness and lack of
hydrogen leave room for at least two possibilities for the origin of
SN 2008ha. One is that the progenitor star may have been a moderately
massive star in a binary system, and lost its outer layers through
interaction with the companion. Alternatively, it may have been a
very massive star which shed its envelope through stellar winds.
CARBON STAR (?) EXPLODES
ScienceDaily
Astrophysicists at the University of Warwick think that a star that
appeared temporarily in 2006 may have been a supernova explosion of a
carbon star -- a type of cool giant star with an excessive abundance
of carbon in its atmosphere. The object, known as SCP 06F6, was first
noticed in 2006 in images taken with the Hubble telescope; it
appeared, and faded away again beyond detection, over the course of
120 days. The distance of the object was indeterminate. According to
the new research, identifying the event as a carbon-star supernova, it
must have been at a distance of about 2 billion light-years. It would
be an unusual type of supernova in several aspects: SCP 06F6 is
located in a blank part of the sky, with no visible host galaxy. If
the star did explode as a type-II supernova, why then did it take up
to four times as long to brighten and fade as other such supernovae,
and why did it emit up to 100 times more X-ray energy than would be
expected? There is no answer to those questions, but the leader of
the research at Warwick, Boris Gänsicke, remarks that several
telescopes are now being designed and built to monitor the entire sky
continuously for short appearances of new stars, and he is sure that
SCP 06F6 will not remain alone in puzzling astronomers over the coming
years.
NEW MASS ESTIMATE FOR M87 BLACK HOLE
AAS
To try to understand how galaxies form and grow, astronomers need to
start with basic census information about today's galaxies. What are
they made of? How big are they? What are their masses? Astronomers
assess that last category, galaxy masses, from the speeds of stars
orbiting within a galaxy. Studies of the total mass are important,
but so is the question as to whether the mass is in a black hole, in
the stars, or in a dark halo. A new computer model has been made of
M87, one of the largest 'nearby' galaxies; it is more complicated than
previous ones because, in addition to modelling its stars and black
hole, it takes into account the galaxy's dark halo -- a spherical
region surrounding it that extends beyond its main visible structure,
and contains dark matter -- matter with a substantially smaller ratio
of luminosity to mass than the Sun's. The new model suggests that the
black hole at the heart of M87 is two to three times more massive than
previously thought; at 6.4 billion times the Sun's mass, it is the
most massive black hole estimated by a supposedly robust technique,
There is a suggestion that the 'accepted' masses of black holes in
other large galaxies may be off by similar factors. That would put
them more into line with the masses that have been attributed to the
black holes that are supposed to power quasars at the centres of
extremely distant galaxies seen at a much earlier cosmic epoch.
GHOST REMAINS AFTER BLACK-HOLE ERUPTION
Chandra X-ray Center
The Chandra X-ray Observatory has found what has been called a cosmic
'ghost', because it is supposed to be all that is left to mark the
site of an earlier quasar outburst after other radiation from the
presumed outburst has died away, It is in the Chandra Deep Field
North, one of the deepest X-ray images taken by Chandra. The source,
called HDF 130, is over 10 billion light-years away and existed at a
time 3 billion years after the Big Bang.
Chandra astronomers think that the X-ray glow from HDF 130 is evidence
for a powerful outburst from a central black hole in the form of jets
of energetic particles travelling at almost the speed of light. When
the eruption was ongoing, it would have produced prodigious amounts of
radio and X radiation, but after several million years the radio
signal would have faded as the electrons radiated away their energy.
However, even less-energetic electrons can still produce X-rays by
interacting with the pervasive sea of photons remaining from the Big
Bang (the 'cosmic background radiation'). Collisions between the
electrons and the background photons can impart enough energy to the
photons to boost them into the X-ray energy band. That process can
produce an extended X-ray source that lasts for another 30 million
years or so.
If the observations have been correctly interpreted, this is the
first X-ray ghost seen after the demise of radio-bright jets.
Astronomers have observed extensive X-ray emission of similar origin,
but only from galaxies with radio emission on large scales, signifying
continuing eruptions. In HDF 130, only a point source is detected in
radio images, coinciding with the elliptical galaxy seen in its
optical image. The radio source is taken to indicate the presence of
a growing super-massive black hole. The details of Chandra's data on
HDF 130 helped to identify its nature. In X-rays, HDF 130 has a
cigar-like shape that extends for some 2.2 million light-years. The
linear shape of the X-ray source is consistent with the shape of radio
jets and not with that of a cluster of galaxies, which is expected to
be circular. The energy distribution of the X rays is also consistent
with the interpretation of an X-ray ghost.
Topic: Mid June Astronomy Bulletin (Read 1237 times)