HELIUM PAIR IN VIOLENT FLARE-UPS
RAS
Astronomers at Armagh Observatory have observed violent eruptions from
an interacting pair of stars that orbit around one another every 25
minutes. Unusually, these outbursts take place at regular and
predictable intervals, erupting every two months. The new
observations were made with the robotic Liverpool Telescope sited in
the Canary Islands and the orbiting Swift observatory. The stars are
both helium-rich white dwarfs, the compact remnants that are the end
state of stars like our Sun. The system is known as KL Dra and the
stars are separated by a distance equivalent to just half that between
the Earth and Moon, close enough for the more massive partner to
accrete helium lost by its companion. The stream of helium travels
from one white dwarf and eventually reaches the other at a speed of
about 2000 km/s. Most of the time the material accumulates in a disc
orbiting around the accreting companion, with only a trickle landing
on the star itself, causing it to glow quietly at optical, ultraviolet
and X-ray energies. However, every two months the material in the
disc gets suddenly released in a giant eruption that causes the
stellar system to shine dozens of times more brightly than before.
The binary is one of very few known systems transferring helium. The
hydrogen which was originally in both stars has long been converted
into helium and heavier elements. Almost all other interacting binary
systems so far discovered transfer hydrogen. Since helium is heavier
and has properties differing from those of hydrogen, the eruption
properties of KL Dra could be expected to differ from those of the
more familiar hydrogen binaries. As KL Dra erupts regularly and
predictably, scientists can plan detailed and sensitive observations
with a range of telescopes when it is in outburst. Such observations
will potentially have wide-ranging implications since the same general
process of accretion takes place in many astrophysical systems,
ranging from young stars in the process of forming, to massive black
holes found at the centres of galaxies. The team of astronomers
obtained complementary observations of KL Dra with the Swift
observatory, which showed that the eruption was seen very strongly in
ultraviolet light. Surprisingly, unlike the hydrogen binaries, there
was no change in the system's brightness in X-rays during the
eruption.
RUNAWAY STAR SPEEDING FROM 30 DORADUS
STScI
A massive runaway star is rushing away from a star-forming region at
more than 100 km/s. The object is the most extreme case known of a
massive star that has been thrown out of its cluster by a group of
even-more-massive members. The star is on the outskirts of the 30
Doradus nebula, a dynamic star-formation centre in the Large
Magellanic Cloud. 30 Doradus, also called the Tarantula Nebula, is
roughly 50,000 parsecs away (1 pc = 3.26 l-y). Observations from
three observatories, including the Hubble telescope, suggest that the
star may have travelled more than 100 pc from its suspected home, a
star cluster called R136 in the core of 30 Doradus. R136 contains
several stars believed to exceed 100 solar masses each.
Runaway stars can occur in two ways. A star may encounter one or two
more-massive ones in a massive, dense cluster and get ejected through
exchanges of momentum. Or, a star may get a kick from a supernova
explosion in a binary system, with the more massive star exploding
first. It is generally accepted that R136 is sufficiently young (1 to
2 million years old) that even its most massive stars have not yet
exploded as supernovae. That implies that the runaway star must have
been ejected through dynamical interaction. It is an exceptionally
hot, massive blue-white star and relatively far from any cluster in
which such stars are usually found.
Ultraviolet spectroscopic observations, made with the Hubble telescope
last July, showed that the star has one of the most powerful stellar
winds known, a clear sign that it is extremely massive, perhaps as
much as 90 times the mass of the Sun. Therefore it too must be very
young, about 1 to 2 million years old, because extremely massive stars
live only a few million years. An optical image of the star taken by
Hubble in 1995 shows that it is at one end of an egg-shaped cavity.
The cavity's glowing edges stretch behind the star and point in the
direction of its origin in 30 Doradus. Another spectroscopic study,
from the VLT, showed the star's velocity to be constant and not a
result of orbital motion in a binary system. Its velocity corresponds
to an unusual motion relative to its surroundings, evidence that it is
a runaway star. The study also confirmed that the light from the
runaway is from a single massive star rather than the combined light
of two lower-mass stars. In addition, the observation established that
the star is about 10 times hotter than the Sun, a temperature that is
consistent with a high-mass object. Two other extremely hot, massive
stars have been observed beyond the edges of 30 Doradus. Astronomers
suspect that those stars, too, may have been ejected from the cluster.
X-RAYS POINT TO LOCATION OF MISSING MATTER
Chandra X-ray Center
Astronomers using the Chandra X-ray Observatory and XMM-Newton have
detected a reservoir of intergalactic gas about 120 Mpc (megaparsecs)
away. The discovery is seen as evidence that the 'missing matter' in
the 'nearby' Universe is located in a web of hot, diffuse gas. The
missing matter, which is different from dark matter, is composed of
baryons, particles such as protons and electrons, that are found on
Earth, in stars, gas, galaxies, and so on. A variety of measurements
of distant gas clouds and galaxies has provided an estimate of the
amount of 'normal' matter present when the Universe was only a few
billion years old. However, an inventory of the much older 'nearby'
Universe has turned up only about half as much normal matter, an
embarrassingly large shortfall. The new work supports ideas that it
is mostly found in a web of hot, diffuse gas known as the Warm-Hot
Intergalactic Medium (WHIM). Some people think that the WHIM is
material left over after the formation of galaxies, which was later
enriched by elements blown out of galaxies.
To look for the WHIM, the researchers examined X-ray observations of a
rapidly growing super-massive black hole known as an active galactic
nucleus (AGN). The AGN, which is about 600 Mpc away, generates a lot
of X-rays as it pulls matter inward. Lying along the line of sight to
the AGN, at a distance of about 120 Mpc, is the 'Sculptor Wall', which
is a diffuse structure stretching across many Mpc and contains
thousands of galaxies and potentially a significant reservoir of the
WHIM if the theoretical simulations are correct. The WHIM in the wall
should absorb some of the X-rays from the AGN as they travel across
intergalactic space to the Earth.
In the new data, absorption of X-rays by oxygen atoms in the WHIM has
clearly been detected. The characteristics of the absorption are
consistent with the distance of the Sculptor Wall as well as the
predicted temperature and density of the WHIM. The result gives
scientists confidence that the WHIM will also be found in other
large-scale structures. Several previous claimed detections of the
hot component of the WHIM have been controversial because the
detections had been made with only one X-ray telescope, and the
statistical significance of many of the results had been questioned.
In addition to having corroborating data from both Chandra and
XMM-Newton, the new study also removes another uncertainty from
previous claims. Because the distance of the Sculptor Wall is already
known, the statistical significance of the absorption detection is
greatly enhanced over previous 'blind' searches, which attempted to
find the WHIM by observing bright AGN at random directions on the sky
in the hope that their lines of sight intersected previously
undiscovered large-scale structures.
SCIENTISTS DETERMINE MASS OF ABELL 3827
Gemini Observatory
A newly discovered gravitational lens in a relatively 'nearby' galaxy
cluster suggests that the cluster has the most massive galaxy known in
our 'local' Universe. The study also reaffirms that galactic merging
is one reason that that galaxy is so massive, at up to 3 times 10*13
solar masses. The super-massive galaxy is located at the core of the
cluster called Abell 3827, which lies about 400 Mpc away. Although
the galaxy, ESO 146-IG 005, dominates the core of Abell 3827, its
magnitude had not been fully appreciated. New Gemini observations
have shown for the first time the effects of gravitational lensing
near the core of ESO 146-IG 005. Light from a background galaxy, in
this case two galaxies, that is passing by the lensing object is
deflected from its original path. From our perspective, we see the
background galaxies' light re-shaped as a ring-like structure that arcs
around the lensing object. Arcs from both galaxies are clearly
visible in the Gemini images. The gravitational lens allowed
astronomers to estimate the mass of the lensing galaxy. The inferred
mass is a factor of 10 greater than previous estimates derived from
X-ray observations.
MARS ROVERS SURPASS LONGEVITY RECORD
Science Daily
The Mars Exploration Rover Project passed a longevity record on May
20. The Opportunity rover will surpass the duration record set by the
Viking 1 Lander of six years and 116 days operating on the surface of
Mars. The effects of favourable weather could also help the rovers
generate more power. Opportunity's twin rover, Spirit, began working
on Mars three weeks before Opportunity. However, Spirit has been out
of communication since March 22. If it awakens from hibernation and
resumes communication, that rover will attain the Martian-surface
longevity record. Unless dust interferes, which is unlikely in the
coming months, the solar panels on both rovers should gradually
generate more electricity. Operators hope that Spirit will recharge
its batteries enough to awaken from hibernation, start communicating
and resume scientific tasks. Unlike recent operations, Opportunity
will not have to rest to regain energy between driving days. The
gradual increase in available sunshine will eventually improve the
rate of Opportunity's progress across a vast plain toward its
long-term destination, Endeavour Crater. The record for the longest
working lifetime by a spacecraft at Mars belongs to Mars Global
Surveyor, which operated for more than 9 years after arriving in 1997.
Mars Odyssey, in orbit since in 2001, has been working at Mars longer
than any other current mission and is on track to take the Mars
longevity record late this year.
Topic: Late May Astronomy Bulletin (Read 593 times)