Posted on by Fraser Cain

Early Earth Wasn’t So Hellish

The Earth. Image credit: NASA. Click to enlarge
New ANU research is set to radically overturn the conventional wisdom that early Earth was a hellish planet barren of continents.

An international research team led by Professor Mark Harrison of the Research School of Earth Sciences analysed unique 4 to 4.35 billion-year-old minerals from outback Australia and found evidence that a fringe theory detailing the development of continents during the first 500 million years of Earth history – the Hadean (“hellish”) Eon – is likely to be correct.

The research, published in the latest edition of Science, follows on from results by Professor Harrison and his colleagues published earlier this year that confirmed that our planet was also likely to have had oceans during most of the Hadean.

“A new picture of early Earth is emerging,” Professor Harrison said. “We have evidence that the Earth’s early surface supported water – the key ingredient in making our planet habitable. We have evidence that this water interacted with continent-forming magmas throughout the Hadean.

“And now we have evidence that massive amounts of continental crust were produced almost immediately upon Earth formation. The Hadean Earth may have looked much like it does today rather than our imagined view of a desiccated world devoid of continents.”

Professor Harrison and his team gathered their evidence from zircons, the oldest known minerals on Earth, called zircons. These ancient grains, typically about the width of a human hair, are found only in the Murchison region of Western Australia. The team analysed the isotopic properties of the element hafnium in about 100 tiny zircons that are as old as 4.35 billion years.

Conventionally, it has been believed that the Earth’s continents developed slowly over a long period of time beginning about 4 billion years ago – or 500 million years after the planet formed.

However, hafnium isotope variations produced by the radioactive decay of an isotope of lutetium indicate many of these ancient zircons formed in a continental setting within about 100 million years of Earth formation.

“The evidence points to almost immediate development of continent followed by its rapid recycling back into the mantle via a process akin to modern plate tectonics,” according to Professor Harrison.

The isotopic imprint left on the mantle by early melting shows up again in younger zircons – providing evidence that they have tapped the same source. This suggests that the amount of mantle processed to make continent must have been enormous.

“The results are consistent with the Earth hosting a similar mass of continental crust as the present day at 4.5-4.4 billion years.

“This is a radical departure from conventional wisdom regarding the Hadean Earth,” said Professor Harrison.

“But these ancient zircons represent the only geological record we have for that period of Earth history and thus the stories they tell take precedence over myths that arose in the absence of observational evidence.”

“The simplest explanation of all the evidence is that essentially from its formation, the planet fell into a dynamic regime that has persisted to the present day.”

Original Source: ANU News Release

Posted on by Fraser Cain

More Einstein Rings Discovered

Einstein ring gravitational lens: SDSS J163028.15+452036.2. Image credit: Hubble. Click to enlarge
As Albert Einstein developed his theory of general relativity nearly a century ago, he proposed that the gravitational field from massive objects could dramatically warp space and deflect light.

The optical illusion created by this effect is called gravitational lensing. It is nature’s equivalent of having a giant magnifying lens in space that distorts and amplifies the light of more distant objects. Einstein described gravitational lensing in a paper published in 1936. But he thought the effect was unobservable because the optical distortions produced by foreground stars warping space would be too small to ever be measurable by the largest telescopes of his time.

Now, almost a century later, astronomers have combined two powerful astronomical assets, the Sloan Digital Sky Survey (SDSS) and NASA’s Hubble Space Telescope, to identify 19 new “gravitationally lensed” galaxies, adding significantly to the approximately 100 gravitational lenses previously known. Among these 19, they have found eight new so-called “Einstein rings”, which are perhaps the most elegant manifestation of the lensing phenomenon. Only three such rings had previously been seen in visible light.

In gravitational lensing, light from distant galaxies can be deflected on its way to Earth by the gravitational field of any massive object that lies in the way. Because of this, we see the galaxy distorted into an arc or multiple separate images. When both galaxies are exactly lined up, the light forms a bull’s-eye pattern, called an Einstein ring, around the foreground galaxy.

The newly discovered lenses come from an ongoing project called the Sloan Lens ACS Survey (SLACS). A team of astronomers, led by Adam Bolton of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., and Leon Koopmans of the Kapteyn Astronomical Institute in the Netherlands, selected the candidate lenses from among several hundred thousand optical spectra of elliptical galaxies in the Sloan Digital Sky Survey. They then used the sharp eyes of Hubble’s Advanced Camera for Surveys to make the confirmation.

“The massive scale of the SDSS, together with the imaging quality of the Hubble telescope, has opened up this unprecedented opportunity for the discovery of new gravitational lenses,” Bolton explained. “We’ve succeeded in identifying the one out of every 1,000 galaxies that show these signs of gravitational lensing of another galaxy.”

The SLACS team scanned the spectra of approximately 200,000 galaxies 2 to 4 billion light-years away. The team was looking for clear evidence of emission from galaxies twice as far from Earth and directly behind the closer galaxies. They then used Hubble’s Advanced Camera for Surveys to snap images of 28 of these candidate lensing galaxies. By studying the arcs and rings produced by 19 of these candidates, the astronomers can precisely measure the mass of the foreground galaxies.

Besides producing odd shapes, gravitational lensing gives astronomers the most direct probe of the distribution of dark matter in elliptical galaxies. Dark matter is an invisible and exotic form of matter that has not yet been directly observed. Astronomers infer its existence by measuring its gravitational influence. Dark matter is pervasive within galaxies and makes up most of the total mass of the universe. By searching for dark matter in galaxies, astronomers hope to gain insight into galaxy formation, which must have started around lumpy concentrations of dark matter in the early universe.

“Our results indicate that, on average, these ‘elliptical lensing galaxies’ have the same special mass-density structure as that observed in spiral galaxies,” Bolton continued. “This corresponds to an increase in the proportion of dark matter relative to stars as one moves away from the center of the lensing galaxy and into its fainter outskirts. And since these lensing gelaxies are relatively bright, we can solidify this result with further ground-based spectroscopic observations of the stellar motions in the lenses.”

“Being able to study these and other gravitational lenses as far back in time as several billion years allows us to see directly whether the distribution of dark [invisible] and visible mass changes with cosmic time,” Dr. Koopmans added. “With this information, we can test the commonly held idea that galaxies form from collision and mergers of smaller galaxies.”

The Sloan Digital Sky Survey, from which the SLACS lens-candidate sample was selected, was begun in 1998 with a custom-built ground-based telescope to measure the colors and brightnesses of more than 100 million objects over a quarter of the sky and map the distances to a million galaxies and quasars. “This type of gravitational-lens survey was not an original goal of the SDSS, but was made possible by the excellent quality of the SDSS data,” said Scott Burles of the Massachusetts Institute of Technology in Cambridge, Mass., a SLACS team member and one of the creators of the SDSS.

“An additional bonus of the large size of the SDSS database is that we can design our search criteria so as to find the lenses that are most suitable for specific science goals,” said SLACS team member Tommaso Treu of the University of California, Santa Barbara. “Whereas until now we have selected the largest galaxies as our targets, in the next stages of the survey we are targeting smaller lens galaxies. There have been suggestions that the structure of galaxies changes with galaxy size. By identifying these rare objects ‘on demand,’ we will soon be able for the first time to test whether this is true.”

Added SLACS team member Leonidas Moustakas of the NASA Jet Propulsion Laboratory and the California Institute of Technology in Pasadena, Calif.: “These Einstein rings also give an unrivaled magnified view of the lensed galaxies, allowing us to study the stars and the formation histories of these distant galaxies.”

The SLACS Survey is continuing, and so far the team has used Hubble to study almost 50 of their candidate lensing galaxies. The eventual total is expected to be more than 100, with many more new lenses among them. The initial findings of the survey will appear in the February 2006 issue of the Astrophysical Journal and in two other papers that have been submitted to that journal.

Original Source: Hubblesite News Release

Posted on by Fraser Cain

Mars Express Radar Data is Coming In

Artist’s impression of MARSIS deployment complete. Image credit: ESA. Click to enlarge
The Mars Express radar, MARSIS, has now been deployed for more than four months. Here we report on the activities so far.

For the operational period up to now, Mars Express has been making its closest approaches to Mars predominantly in the daytime portion of its orbit. The MARSIS radar’s scientists are mainly collecting data about the upper layers of the Martian atmosphere, or “ionosphere”, which is the highly electrically conducting layer that is maintained by sunlight.

They are also continuing the laborious analysis of all data gathered during the first night-time observations last summer, especially in the search for and interpretation of possible signals from subsurface layers. This includes the search for a possible signature of underground water, in frozen or liquid state.

Radar science is a complex business – it is based on the detection of radio waves reflected by boundaries between different materials. By analysis of these “echoes”, it is possible to deduce information about the kind of material causing the reflection, such as estimates of its composition and physical state.

Different materials are characterised by their “dielectric constant”, that is the specific way they interact with electromagnetic radiation, such as radio waves. When a radio wave crosses the boundary of different layers of “material”, an echo is generated and carries a sort of “fingerprint” from the specific materials.

From the time delay for an echo to be received by the radar instrument, the distance or the depth of the layers of material producing the echo can be deduced.

While the Mars Express point closest approach is in daylight, MARSIS is only operating at higher frequencies within its capability because the lower-frequency radio signals get disturbed. With these higher frequencies, MARSIS can study the ionosphere and the surface, while some shallow subsurface sounding can still be attempted.

During night-time observations, like those performed briefly last summer immediately after deployment, it is possible for MARSIS to use all frequencies for scientific measurements, including the lowest ones, suitable for penetrating under the soil of Mars.

Tuning to different frequencies for different targets in different conditions is not the only secret of MARSIS. The instrument, responding to signals reflected from any direction, requires scientists also do a huge amount of analysis work to remove these interfering signals from the echoes.

A typical example of what they look for is “clutter backscattering”, which are reflections apparently coming from the subsurface, but actually produced by irregularities in the surface terrain that delay the return of the echo. For this “cleaning” work, the team also makes use of “surface echo simulator” computer programs.

In the first months of operations, MARSIS performed its first ionospheric sounding. The data are converted into typical plots, called “ionograms”, where the altitude at which the echo was generated, deduced by the echo time delay, is given for each transmitted frequency. The intensity of the various echo signals detected is indicated in different colours.

In parallel to the analysis of surface and subsurface signals, the scientists are studying all ionograms to draw the first conclusions on the nature and behaviour of the ionosphere of Mars, and of its interaction with the planet and the surrounding environment.

Original Source: ESA Portal

Posted on by Fraser Cain

M-Class Dwarfs Could Be Good For Life After All

The number of HabCat stars, as a function of distance. Image credit: Turnbull, Tarter. Click to enlarge
Scientists have been searching actively for signs of intelligent extraterrestrial civilizations for nearly half a century. Their main approach has been to point radio telescopes toward target stars and to “listen” for electronic transmissions from other worlds. A radio telescope is like a satellite TV dish – only bigger. Just as you can tune your TV to different frequencies, or channels, researchers can use the electronics attached to a radio telescope to monitor different frequencies at which they suspect ET may be transmitting signals out into the galaxy.

So far, no broadcasts have been received. But then, no one knows how many other civilizations with radio transmitters are out there – or, if they exist, where they are likely to be found. It’s only recently that the existence of planets around other stars has been confirmed, and because current planet-finding techniques are limited to detecting relatively large planets, we have yet to find the first Earth-like planet orbiting another star. Most planet hunters believe it’s only a matter of time before we find other Earths, but no one can yet make even a well-founded guess about how many terrestrial planets are in our galactic neighborhood.

With so little information to go on, it has been difficult for scientists involved in SETI (the search for extra-terrestrial intelligence) to decide how to focus their search. So they’ve have had to make some assumptions. One of those assumptions, which may seem a bit odd at first, is that humans are “normal.” That is to say that, because we know for certain that intelligent life evolved on our planet, it stands to reason that other stars like ours may have planets like ours orbiting them, on which other intelligent civilizations have emerged. Based on this terrestrial bias, SETI searches thus far have focused on stars like our sun.

“The observational SETI programs have traditionally confined themselves to looking at stars that are very similar to our own star,” says Jill Tarter, director of the SETI Institute’s Center for SETI Research in Mountain View, California. “Because, after all, that’s the one place where we know that life evolved on a planetary surface and produced a technology that might be detectable across interstellar distances.”

Astronomers classify stars according to their surface temperature. The sun is a G-class star. SETI searches to date have focused on G stars and stars that are either somewhat hotter than the sun (F stars) or somewhat cooler than the sun (K stars). That has yielded a catalog of about a quarter of a million target stars. According to conventional astronomical wisdom, stars hotter than F-class would burn out too quickly for intelligent life to develop on planets that orbit them. Historically, M-dwarf stars, which are dimmer than K stars, also have been dismissed as potential SETI targets.

The two major arguments against M dwarfs have been:

They’re too dim. M dwarfs put out so little solar radiation that a habitable planet would have to be very close-in. Farther-out planets would be frozen solid, too cold for life. A close-in planet would be tidally locked, though, always showing the same face to the star, as the moon does to Earth. The star-facing side would roast, while the opposite side would freeze. Not so good for having lots of liquid water around. And, says Tarter, “Liquid water is essential for life, at least for life as we know it.”

They’re too active. M dwarfs are known to have a lot of solar-flare activity. Solar flares produce UV-B radiation, which can destroy DNA, and X-rays, which in large doses are lethal. Presumably such radiation would be as harmful to extraterrestrial life as it is to life on Earth.

These arguments seem reasonable. But there’s a catch. Most of the stars in the galaxy – more than two-thirds of them – are M dwarfs. If M dwarfs can host habitable planets, those planets might well be home to intelligent species. With radio transmitters. So, as scientists have begun to learn more about other solar systems, and as computer models of solar-system formation have gotten more sophisticated, some SETI researchers have begun to question the assumptions that led them to reject M dwarfs as potential SETI targets.

For example, atmospheric modeling has shown that if a planet orbiting an M dwarf close in had a reasonably thick atmosphere, circulation would transfer the sun’s heat around the planet and even out the temperature worldwide.

“If you put a little bit of greenhouse gas into an atmosphere, the circulations can keep that atmosphere at a reasonable temperature and you can dissipate the heat from the star-facing side and bring it around to the farside. And, perhaps, end up with a habitable world,” says Tarter.

Scientists have also learned that most of an M dwarf’s hyperactivity occurs early in its life cycle, during the first billion years or so. After that, the star tends to settle down and burn quietly for many billions of years more. Once the fireworks end, life might be able to take hold.

The question of M-dwarf habitability is a critical one for Tarter. The SETI Institute is in the process of building a new radio telescope, the Allen Telescope Array. Comprised of 350 small antennas, the array will do double duty: it will be used by radio astronomers to survey the skies and it will search for radio transmissions from extraterrestrial civilizations.

“It’s an observatory that will simultaneously and continuously do traditional radio-astronomy observing and SETI observations,” says Tarter. “It’s the first telescope ever that’s being built to optimize both of those strategies.”

For the most part, traditional radio-astronomy research will determine where the telescope gets pointed; the SETI Institute will simply hitch a ride on the incoming signals. The array combines the signals from the many small antennas to make a large virtual antenna. By adjusting the electronics, researchers will be able to form as many as eight virtual antennas, each pointed at a different star.

That’s where the M-dwarf question comes into play. At the highest frequencies that the telescope can receive, the instrument can focus on only a tiny spot in the sky. For the SETI search to be as efficient as possible, wherever the telescope is pointed, the institute’s researchers want to have several target stars to set their sights on. If only F, G and K stars are considered, there aren’t enough stars to go around. But if M dwarfs are included as targets, the number of prospects could increase as much as ten-fold.

“To make the most progress and to do the fastest survey of the largest number of stars in the next decade or so,” Tarter says, “I want a huge catalog of target stars. I want millions of stars.”

There is no way to know for sure whether M dwarfs host habitable planets. But no one has yet found a habitable planet around any star other than the sun, and it’s unlikely that one will be discovered for many years to come. Technology capable of finding Earth-sized planets is still in the development stage.

To do their work, though, SETI researchers don’t need to know whether or not the stars they’re investigating actually have habitable planets. They simply need to know which stars have the potential to host habitable worlds. Any star with potential belongs on their list.

“It’s not the star that I’m interested in,” Tarter says. “It’s the techno-signature from the inhabitants on a planet around the star. I don’t ever have to see the star, as long as I know that it’s in that direction. I don’t ever have to see the planet. But if I can see their radio transmitter – bingo! – I’ve gotten there. I’ve found a habitable world.”

That’s why Tarter and her colleagues want to know whether or not to include M dwarfs on their target list. To help answer that question, Tarter convened a workshop in July of this year that brought together astronomers, planetary scientists, biologists, and even a few geologists, to explore whether it made sense to add M dwarfs to the catalog of SETI target stars. Although workshop participants did identify some areas that require further research, no insurmountable problems turned up. The group plans to publish its preliminary findings for scrutiny by the wider scientific community.

And that means that if we ever do receive a radio signal from an extraterrestrial civilization, the beings who sent it just might be residents of a solar system with a dim, red M dwarf at its center.

Original Source: NASA Astrobiology

Posted on by Fraser Cain

Ariane 5 Lofts Record Payload into Orbit

The heavy-lift Ariane 5 ECA. Image credit: Arienspace. Click to enlarge
During the night of Wednesday, November 16 to Thursday, November 17, Arianespace placed two satellites into geostationary transfer orbit: the SPACEWAY 2 high-definition direct broadcast satellite for the American operator DIRECTV, and the TELKOM 2 communications satellite for the Indonesian operator PT Telekomunikasi Indonesia Tbk.

20th successful Ariane 5 launch, 10th in a row, record payload.

Today’s mission sets a new record for commercial launches: with over 8,000 kg. injected into orbit, the SPACEWAY 2 and TELKOM 2 satellites represent the heaviest dual payload ever launched.

Today, Ariane 5 is the only commercial launcher in service capable of simultaneously launching two payloads. Ariane 5 ECA offers a payload capacity of nearly 10,000 kg. into geostationary transfer orbit, giving Arianespace’s customers enhanced performance, flexibility and competitiveness through the best launch service in the world.

Today’s mission was the 20th successful launch of an Ariane 5, and the 10th successful launch in a row. One week after the successful launch of the Venus Express spacecraft by a Soyuz rocket from the Baikonur Cosmodrome, this confirms that Arianespace, with its complete family of launchers, offers the best launch solution for operators from around the world.

Original Source: Arienspace News Release

Posted on by Fraser Cain

Gravity Probe B Will Tell Us If Einstein Was Right

An artist’s concept of twisted space-time around Earth. Image credit: NASA. Click to enlarge
Is Earth in a vortex of space-time?

We’ll soon know the answer: A NASA/Stanford physics experiment called Gravity Probe B (GP-B) recently finished a year of gathering science data in Earth orbit. The results, which will take another year to analyze, should reveal the shape of space-time around Earth–and, possibly, the vortex.

Time and space, according to Einstein’s theories of relativity, are woven together, forming a four-dimensional fabric called “space-time.” The tremendous mass of Earth dimples this fabric, much like a heavy person sitting in the middle of a trampoline. Gravity, says Einstein, is simply the motion of objects following the curvaceous lines of the dimple.

If Earth were stationary, that would be the end of the story. But Earth is not stationary. Our planet spins, and the spin should twist the dimple, slightly, pulling it around into a 4-dimensional swirl. This is what GP-B went to space to check

The idea behind the experiment is simple:

Put a spinning gyroscope into orbit around the Earth, with the spin axis pointed toward some distant star as a fixed reference point. Free from external forces, the gyroscope’s axis should continue pointing at the star–forever. But if space is twisted, the direction of the gyroscope’s axis should drift over time. By noting this change in direction relative to the star, the twists of space-time could be measured.

In practice, the experiment is tremendously difficult.

The four gyroscopes in GP-B are the most perfect spheres ever made by humans. These ping pong-sized balls of fused quartz and silicon are 1.5 inches across and never vary from a perfect sphere by more than 40 atomic layers. If the gyroscopes weren’t so spherical, their spin axes would wobble even without the effects of relativity.

According to calculations, the twisted space-time around Earth should cause the axes of the gyros to drift merely 0.041 arcseconds over a year. An arcsecond is 1/3600th of a degree. To measure this angle reasonably well, GP-B needed a fantastic precision of 0.0005 arcseconds. It’s like measuring the thickness of a sheet of paper held edge-on 100 miles away.

GP-B researchers invented whole new technologies to make this possible. They developed a “drag free” satellite that could brush against the outer layers of Earth’s atmosphere without disturbing the gyros. They figured out how to keep Earth’s penetrating magnetic field out of the spacecraft. And they concocted a device to measure the spin of a gyro–without touching the gyro.

Pulling off the experiment was an exceptional challenge. A lot of time and money was on the line, but the GP-B scientists appear to have done it.

“There were not any major surprises” in the experiment’s performance, says physics professor Francis Everitt, the Principal Investigator for GP-B at Stanford University. Now that data-taking is complete, he says the mood among the GP-B scientists is “a lot of enthusiasm, and a realization also that a lot of grinding hard work is ahead of us.”

A careful, thorough analysis of the data is underway. The scientists will do it in three stages, Everitt explains. First, they will look at the data from each day of the year-long experiment, checking for irregularities. Next they’ll break the data into roughly month-long chunks, and finally they’ll look at the whole year. By doing it this way, the scientists should be able to find any problems that a more simple analysis might miss.

Eventually scientists around the world will scrutinize the data. Says Everitt, “we want our sternest critics to be us.”

The stakes are high. If they detect the vortex, precisely as expected, it simply means that Einstein was right, again. But what if they don’t? There might be a flaw in Einstein’s theory, a tiny discrepancy that heralds a revolution in physics.

First, though, there are a lot of data to analyze. Stay tuned.

Original Source: NASA News Release

Posted on by Fraser Cain

Simulation Casts Doubts on One Theory of Star Formation

A slice through a 3-D simulation of a turbulent clump of molecular hydrogen. Image credit: Mark Krumholz. Click to enlarge
Astrophysicists at the University of California, Berkeley, and Lawrence Livermore National Laboratory (LLNL) have exploded one of two competing theories about how stars form inside immense clouds of interstellar gas.

That model, which is less than 10 years old and is championed by some British astronomers, predicts that interstellar hydrogen clouds develop clumps in which several small cores – the seeds of future stars – form. These cores, less than a light year across, collapse under their own gravity and compete for gas in the surrounding clump, often gaining 10 to 100 times their original mass from the clump.

The alternative model, often termed the “gravitational collapse and fragmentation” theory, also presumes that clouds develop clumps in which proto-stellar cores form. But in this theory, the cores are large and, though they may fragment into smaller pieces to form binary or multiple star systems, contain nearly all the mass they ever will.

“In competitive accretion, the cores are seeds that grow to become stars; in our picture, the cores turn into the stars,” explained Chris McKee, professor of physics and of astronomy at UC Berkeley. “The observations to date, which focus primarily on regions of low-mass star formation, like the sun, are consistent with our model and inconsistent with theirs.”

“Competitive accretion is the big theory of star formation in Europe, and we now think it’s a dead theory,” added Richard Klein, an adjunct professor of astronomy at UC Berkeley and a researcher at LLNL.

Mark R. Krumholz, now a post-doctoral fellow at Princeton University, McKee and Klein report their findings in the Nov. 17 issue of Nature.

Both theories try to explain how stars form in cold clouds of molecular hydrogen, perhaps 100 light years across and containing 100,000 times the mass of our sun. Such clouds have been photographed in brilliant color by the Hubble and Spitzer space telescopes, yet the dynamics of a cloud’s collapse into one or many stars is far from clear. A theory of star formation is critical to understanding how galaxies and clusters of galaxies form, McKee said.

“Star formation is a very rich problem, involving questions such as how stars like the sun formed, why a very large number of stars are in binary star systems, and how stars ten to a hundred times the mass of the sun form,” he said. “The more massive stars are important because, when they explode in a supernova, they produce most of the heavy elements we see in the material around us.”

The competitive accretion model was hatched in the late 1990s in response to problems with the gravitational collapse model, which seemed to have trouble explaining how large stars form. In particular, the theory couldn’t explain why the intense radiation from a large protostar doesn’t just blow off the star’s outer layers and prevent it from growing larger, even though astronomers have discovered stars that are 100 times the mass of the sun.

While theorists, among them McKee, Klein and Krumholz, have advanced the gravitational collapse theory farther toward explaining this problem, the competitive accretion theory has come increasingly into conflict with observations. For example, the accretion theory predicts that brown dwarfs, which are failed stars, are thrown out of clumps and lose their encircling disks of gas and dust. In the past year, however, numerous brown dwarfs have been found with planetary disks.

“Competitive accretion theorists have ignored these observations,” Klein said. “The ultimate test of any theory is how well it agrees with observation, and here the gravitational collapse theory appears to be the clear winner.”

The model used by Krumholz, McKee and Klein is a supercomputer simulation of the complicated dynamics of gas inside a swirling, turbulent cloud of molecular hydrogen as it accretes onto a star. Theirs is the first study of the effects of turbulence on the rate at which a star accretes matter as it moves through a gas cloud, and it demolishes the “competitive accretion” theory.

Employing 256 parallel processors at the San Diego Supercomputer Center at UC San Diego, they ran their model for nearly two weeks to show that it accurately represented star formation dynamics.

“For six months, we worked on very, very detailed, high-resolution simulations to develop that theory,” Klein said. “Then, having that theory in hand, we applied it to star forming regions with the properties that one could glean from a star forming region.”

The models, which also were run on supercomputers at Lawrence Berkeley National Laboratory and LLNL, showed that turbulence in the core and surrounding clump would prevent accretion from adding much mass to a protostar.

“We have shown that, because of turbulence, a star cannot efficiently accrete much more mass from the surrounding clump,” Klein said. “In our theory, once a core collapses and fragments, that star basically has all the mass it is ever going to have. If it was born in a low-mass core, it will end up being a low-mass star. If it’s born in a high mass core, it may become a high-mass star.”

McKee noted that the researchers’ supercomputer simulation indicates competitive accretion may work well for small clouds with very little turbulence, but these rarely, if ever, occur and have not been observed to date. Real star formation regions have much more turbulence than assumed in the accretion model, and the turbulence does not quickly decay, as that model presumes. Some unknown processes, perhaps matter flowing out of protostars, keep the gases roiled up so that the core does not collapse quickly.

“Turbulence opposes gravity; without it, a molecular cloud would collapse far more rapidly than observed,” Klein said. “Both theories assume turbulence is there. The key is (that) there are processes going on as stars begin to form that keep turbulence alive and prevent it from decaying. The competitive accretion model doesn’t have any way to put this into the calculations, which means they’re not modeling real star forming regions.”

Klein, McKee and Krumholz continue to refine their model to explain how radiation from large protostars escapes without blowing away all the infalling gas. For example, they have shown that some of the radiation can escape through cavities created by the jets observed to come out the poles of many stars in formation. Many predictions of the theory may be answered by new and larger telescopes now under construction, in particular the sensitive, high-resolution ALMA telescope being constructed in Chile by a consortium of United States, European and Japanese astronomers, McKee said.

The work was supported by the National Aeronautics and Space Administration, the National Science Foundation and the Department of Energy.

Original Source: UC Berkeley News Release

Posted on by Fraser Cain

Spirit Sees a Martian Lunar Eclipse

Phobos. Image credit: NASA. Click to enlarge
NASA’s Mars Exploration Rover Spirit continues to take advantage of favorable solar power conditions to conduct occasional nighttime astronomical observations from the summit region of “Husband Hill.”

Spirit has been observing the martian moons Phobos and Deimos to learn more about their orbits and surface properties. This has included observing eclipses. On Earth, a solar eclipse occurs when the Moon’s orbit takes it exactly between the Sun and Earth, casting parts of Earth into shadow. A lunar eclipse occurs when the Earth is exactly between the Sun and the Moon, casting the Moon into shadow and often giving it a ghostly orange-reddish color. This color is created by sunlight reflected through Earth’s atmosphere into the shadowed region. The primary difference between terrestrial and martian eclipses is that Mars’ moons are too small to completely block the Sun from view during solar eclipses.

Recently, Spirit observed a “lunar” eclipse on Mars. Phobos, the larger of the two martian moons, was photographed while slipping into the shadow of Mars. Jim Bell, the astronomer in charge of the rover’s panoramic camera (Pancam), suggested calling it a “Phobal” eclipse rather than a lunar eclipse as a way of identifying which of the dozens of moons in our solar system was being cast into shadow.

With the help of the Jet Propulsion Laboratory’s navigation team, the Pancam team planned instructions to Spirit for acquiring the views shown here of Phobos as it entered into a lunar eclipse on the evening of the rover’s 639th martian day, or sol (Oct. 20, 2005) on Mars. This image is a time-lapse composite of eight Pancam images of Phobos moving across the martian sky. The entire eclipse lasted more than 26 minutes, but Spirit was able to observe only in the first 15 minutes. During the time closest to the shadow crossing, Spirit’s cameras were programmed to take images every 10 seconds.

In the first three images, Phobos was in sunlight, moving toward the upper right. After a 100-second delay while Spirit’s computer processed the first three images, the rover then took the fourth image, showing Phobos just starting to enter the darkness of the martian shadow. At that point, an observer sitting on Phobos and looking back toward the Sun would have seen a spectacular sunset! In the fifth image, Phobos appeared like a crescent, almost completely shrouded in darkness.

In the last three images, Phobos had slipped entirely into the shadow of Mars. However, as with our own Moon during lunar eclipses on Earth, it was not entirely dark. The small amount of light still visible from Phobos is a kind of “Mars-shine” — sunlight reflected through Mars’ atmosphere and into the shadowed region.

Rover scientists took some images later in the sequence to try to figure out if this “Mars-shine” made Phobos colorful while in eclipse, but they’ll need more time to complete the analysis because the signal levels are so low. Meanwhile, they will use the information on the timing of the eclipse to refine the orbital path of Phobos. The precise position of Phobos will be important to any future spacecraft taking detailed pictures of the moon or landing on its surface. In the near future it might be possible for one of the rovers to take images of a “Deimal” eclipse to learn more about Mars’ other enigmatic satellite, Deimos, as well.

Original Source: NASA News Release

Posted on by Fraser Cain

Close-Up on Pandora

Cassini’s best close-up view of Pandora. Image credit: NASA/JPL/SSI Click to enlarge
Cassini’s best close-up view of Saturn’s F ring shepherd moon, Pandora, shows that this small ring-moon is coated in fine dust-sized icy material.

Craters formed on this object by impacts appear to be covered by debris, a process that probably happens rapidly in a geologic sense. The grooves and small ridges on Pandora (84 kilometers, or 52 miles across) suggest that fractures affect the overlying smooth material.

The crisp craters on another Saturn moon, Hyperion, provide a contrasting example of craters on a small object (see Odd World).

Cassini acquired infrared, green and ultraviolet images on Sept. 5, 2005, which were combined to create this false-color view. The image was taken with the Cassini spacecraft narrow-angle camera at a distance of approximately 52,000 kilometers (32,000 miles) from Pandora and at a Sun-Pandora-spacecraft, or phase, angle of 54 degrees. Resolution in the original image was about 300 meters (1,000 feet) per pixel. The image has been magnified by a factor of two to aid visibility.

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the mission for NASA’s Science Mission Directorate, Washington, D.C. The Cassini orbiter and its two onboard cameras were designed, developed and assembled at JPL. The imaging operations center is based at the Space Science Institute in Boulder, Colo.

For more information about the Cassini-Huygens mission visit http://saturn.jpl.nasa.gov . The Cassini imaging team homepage is at http://ciclops.org .

Original Source: NASA/JPL/SSI News Release

Posted on by Fraser Cain

New Class of Supergiant Binary Stars Discovered

Artist’s impression of a ‘supergiant fast X-ray transient’. Image credit: ESA Click to enlarge
ESA’s Integral gamma-ray observatory has discovered a new, highly populated class of X-ray fast “transient” binary stars, undetected in previous observations.

With this discovery, Integral confirms how much it is contributing to revealing a whole hidden Universe.

The new class of double star systems is characterised by a very compact object that produces highly energetic, recurrent and fast-growing X-ray outbursts, and a very luminous “supergiant” companion.

The compact object can be an accreting body such as a black hole, a neutron star or a pulsar. Scientists have called such class of objects “supergiant fast X-ray transients”. “Transients” are systems which display periods of enhanced X-ray emission.

Before the launch of Integral, only a dozen X-ray binary stars containing supergiants had been detected. Actually, scientists thought that such high-mass X-ray systems were very rare, assuming that only a few of them would exist at once since stars in supergiant phase have a very short lifetime.

However, Integral’s data combined with other X-ray satellite observations indicate that transient supergiant X-ray binary systems are probably much more abundant in our Galaxy than previously thought.

In particular, Integral is showing that such “supergiant fast X-ray transients”, characterised by fast outbursts and supergiant companions, form a wide class that lies hidden throughout the Galaxy.

Due to the transitory nature, in most cases these systems were not detected by other observatories because they lacked the combination of sensitivity, continuous coverage and wide field of view of Integral.

They show short outbursts with very fast rising times – reaching the peak of the flare in only a few tens of minutes – and typically lasting a few hours only. This makes the main difference with most other observed transient X-ray binary systems, which display longer outbursts, lasting typically a few weeks up to months.

In the latter case, the long duration of the outburst is consistent with a “viscous” mass exchange between the star and an accreting compact object.

In “supergiant fast X-ray transients”, associated with highly luminous supergiant stars, the short duration of the outburst seems to point to a different and peculiar mass exchange mechanism between the two bodies.

This may have something to do with the way the strong radiative winds, typical of highly massive stars, feed the compact object with stellar material.

Scientists are now thinking about the reasons for such short outbursts. It could be due to the supergiant donor ejecting material in a non-continuous way. For example, a clumpy and intrinsically variable nature of a supergiant”s radiative winds may give rise to sudden episodes of increased accretion rate, leading to the fast X-ray flares.

Alternatively, the flow of material transported by the wind may become, for reasons not very well understood, very turbulent and irregular when falling into the enormous gravitational potential of the compact object.

“In any case, we are pretty confident that the fast outbursts are associated to the mass transfer mode from the supergiant star to the compact object,” says Ignacio Negueruela, lead author of the results, from the University of Alicante, Spain.

“We believe that the short outbursts cannot be related to the nature of the compact companion, as we observed fast outbursts in cases where the compact objects were very different – black holes, slow X-ray pulsars or fast X-ray pulsars.”

Studying sources such as “supergiant fast X-ray transients”, and understanding the reasons for their behaviour, is very important to increase our knowledge of accretion processes of compact stellar objects. Furthermore, it is providing valuable insight into the evolution paths that lead to the formation of high-mass X-ray binary systems.

Original Source: ESA Portal