Universe Today

Two debris disks resemble the Kuiper Belt. Image credit: UC Berkeley Click to enlarge
A survey by NASA’s Hubble Space Telescope of 22 nearby stars has turned up two with bright debris disks that appear to be the equivalent of our own solar system’s Kuiper Belt, a ring of icy rocks outside the orbit of Neptune and the source of short-period comets.

The debris disks encircling these stars fall into two categories – wide and narrow belts – that appear to describe all nine stars, including the sun, which are known to have debris disks linked to planet formation. In fact, the sharp outer edges of the narrow belts, such as the Kuiper Belt in our solar system, may be a tip-off to the existence of a star-like companion that continually grooms the edge, in the same way that shepherding moons trim the edges of debris rings around Saturn and Uranus.

Research astronomer Paul Kalas, professor of astronomy James Graham and graduate student Michael Fitzgerald of the University of California, Berkeley, along with Mark C. Clampin of Goddard Space Flight Center in Greenbelt, Md., will report their discovery and conclusions in the Jan. 20 issue of Astrophysical Journal Letters.

The newfound stellar disks, each about 60 light years from Earth, bring to nine the number of stars with dusty debris disks observable at visible wavelengths. The new ones are different, however, in that they are old enough – more than 300 million years – to have settled into stable configurations akin to the stable planet and debris orbits in our own solar system, which is 4.6 billion years old. The other seven, except for the sun, range from tens of millions to 200 million years old – young by solar standards.

In addition, the masses of the stars are closer to that of the sun.

“These are the oldest debris disks seen in reflected light, and are important because they show what the Kuiper Belt might look like from the outside,” said Kalas, the lead researcher. “These are the types of stars around which you would expect to find habitable zones and planets that could develop life.”

Most debris disks are lost in the glare of the central star, but the high resolution and sensitivity of the Hubble Space Telescope’s Advanced Camera for Surveys has made it possible to look for these disks after blocking the light from the star. Kalas has discovered debris disks around two other stars (AU Microscopii and Fomalhaut) in the past two years, one of them with the Hubble telescope, and has studied details of most of the other known stars with disks.

“We know of 100-plus stars that have infrared emission in excess of that emitted from the star, and that excess thermal emission comes from circumstellar dust,” Kalas said. “The hard part is obtaining resolved images that give spatial information. Now, two decades after they were first discovered, we are finally beginning to see the dust disks. These recent detections are really a tribute to all the hard work that went into upgrading Hubble’s instruments during the last servicing mission.”

The small sampling already shows that such disks fall into two categories: those with a broad belt, wider than about 50 astronomical units; and narrow ones with a width of between 20 and 30 AU and a sharp outer boundary, probably like our own Kuiper Belt. An astronomical unit, or AU, is the average distance between the Earth and sun, about 93 million miles. Our Kuiper Belt is thought to be narrow, extending from the orbit of Neptune at 30 AU to about 50 AU.

Most of the known debris disks seem to have a central area cleared of debris, perhaps by planets, which are likely responsible for the sharp inner edges of many of these belts.

Kalas and Graham speculate that stars also having sharp outer edges to their debris disks have a companion – a star or brown dwarf, perhaps – that keeps the disk from spreading outward, similar to the way that Saturn’s moons shape the edges of many of the planet’s rings.

“The story of how you make a ring around a planet could be the same as the story of making rings around a star,” Kalas said. Only one of the nine stars is known to have a companion, but then, he said, no one has yet looked at most of these stars to see if they have faint companions at large distances from the primary star.

He suggests that a stray star passing by may have ripped off the edges of the original planetary disk, but a star-sized companion would be necessary to keep the remaining disk material, such as asteroids, comets and dust, from spreading outward.

If true, that would mean that the sun also has a companion keeping the Kuiper Belt confined within a sharp boundary. Though a companion star has been proposed before – most recently by UC Berkeley physics professor Richard Muller, who dubbed the companion Nemesis – no evidence has been found for such a companion.

A key uncertainty, Kalas said, is that while we can see many of the large planetesimals in our Kuiper Belt, we can barely detect the dust.

“Ironically, our own debris disk is the closest, yet we know the least about it,” he said. “We would really like to know if dust in our Kuiper Belt extends significantly beyond the 50 AU edge of the larger objects. When we acquire this information, only then will we be able to place our sun correctly in the wide or narrow disk categories.”

The star survey by Kalas, Graham, Fitzgerald and Clampin was one of the first projects of the Advanced Camera for Surveys aboard the Hubble, which was installed in 2002. The 22 stars were observed over a two year period ending in September 2004. The stars with debris disks detectable in visible light were HD 53143, a K star slightly smaller than the sun but about 1 billion years old, and HD 139664, an F star slightly larger than the sun but only 300 million years old.

“One is a K star and the other is an F star, therefore they are more likely to form planetary systems with life than the massive and short-lived stars such as beta-Pictoris and Fomalhaut,” he noted. “When we look at HD 53143 and HD 139664, we may be looking at astrophysical mirrors to our Kuiper Belt.”

The disk around the oldest of the two stars, HD 53143, is wide like that of beta-Pictoris (beta-Pic), which was the first debris disk ever observed, about 20 years ago, and AU Microscopii (AU Mic), which Kalas discovered last year. Both beta-Pic and AU Mic are about 10 million years old.

The disk around HD 139664, however, is a narrow belt, similar to the disk around the star Fomalhaut, which Kalas imaged for the first time early last year. HD 139664 has a very sharp outer edge at 109 AU, similar to the sharp outer edge of our Kuiper Belt at 50 AU. The belt around HD 139664 starts about 60 AU from the star and peaks in density at 83 AU.

“If we can understand the origin of the sharp outer edge around HD 139664, we might better understand the history of our solar system,” Kalas said.

The research was supported by grants from NASA through the Space Telescope Science Institute.

Original Source: UC Berkeley News Release

A graphic representing NASA’s ACE and Wind and ESA’s Cluster spacecraft encountering solar particle jets. Image credit: UC Berkeley Click to enlarge
A fleet of NASA and European Space Agency space-weather probes observed an immense jet of electrically charged particles in the solar wind between the Sun and Earth. The jet, at least 200 times as wide as the Earth, was powered by clashing magnetic fields in a process called “magnetic reconnection”.
magnetic reconnection in the solar wind

These jets are the result of natural particle accelerators dwarfing anything built on Earth. Scientists build miles-long particle accelerators on Earth to smash atoms together in an effort to understand the fundamental laws of physics.

Similar reconnection-powered jets occur in Earth’s magnetic shield, producing effects that can disable orbiting spacecraft and cause severe magnetic storms on our planet, sometimes disrupting power stations.

The newly discovered interplanetary jets are far larger than those occurring within Earth’s magnetic shield. The new observation is the first direct measurement indicating magnetic reconnection can happen on immense scales.

Understanding magnetic reconnection is fundamental to comprehending explosive phenomena throughout the Universe, such as solar flares (billion-megaton explosions in the Sun’s atmosphere), gamma-ray bursts (intense bursts of radiation from exotic stars), and laboratory nuclear fusion. Just as a rubber band can suddenly snap when twisted too far, magnetic reconnection is a natural process by which the energy in a stressed magnetic field is suddenly released when it changes shape, accelerating particles (ions and electrons).

“Only with coordinated measurements by Sun-Earth connection spacecraft like ACE, Wind, and Cluster can we explore the space environment with unprecedented detail and in three dimensions,” says Dr. Tai Phan, lead author of the results, from the University of California, Berkeley. “The near-Earth space environment is the only natural laboratory where we can make direct measurements of the physics of explosive magnetic phenomena occurring throughout the Universe.” Phan’s article appears as the cover article in Nature on January 12.

The solar wind is a dilute stream of electrically charged (ionized) gas that blows continually from the Sun. Because the solar wind is electrically charged, it carries solar magnetic fields with it. The solar wind arising from different places on the Sun carries magnetic fields pointing in different directions. Magnetic reconnection in the solar wind takes place when “sheets” of oppositely directed magnetic fields get pressed together. In doing so, the sheets connect to form an X-shaped cross-section that is then annihilated, or broken, to form a new magnetic line geometry. The creation of a different magnetic geometry produces extensive jets of particles streaming away from the reconnection site.

Until recently, magnetic reconnection was mostly reported in Earth’s “magnetosphere”, the natural magnetic shield surrounding Earth. It is composed of magnetic field lines generated by our planet, and defends us from the continuous flow of charged particles that make up the solar wind by deflecting them. However, when the interplanetary magnetic field lines carried by the solar wind happen to be in the opposite orientation to the Earth?s magnetic field lines, reconnection is triggered and solar material can break through Earth’s shield.

Some previous reconnection events measured in Earth?s magnetosphere suggested that the phenomenon was intrinsically random and patchy in nature, extending not more than a few tens of thousands of kilometers (miles). However, “This discovery settles a long-standing debate concerning whether reconnection is intrinsically patchy, or whether instead it can operate across vast regions in space,” said Dr. Jack Gosling of the University of Colorado, a co-author on the paper and a pioneer in research on reconnection in space.

The broader picture of magnetic reconnection emerged when six spacecraft ? the four European Space Agency Cluster spacecraft and NASA’s Advanced Composition Explorer (ACE) and Wind probes ? were flying in the solar wind outside Earth?s magnetosphere on 2 February 2002 and made a chance discovery. During a time span of about two and a half hours, all spacecraft observed in sequence a single huge stream of jetting particles, at least 2.5 million kilometers wide (about 1.5 million miles or nearly 200 Earth diameters), caused by the largest reconnection event ever measured directly.

“If the observed reconnection were patchy, one or more spacecraft most likely would have not encountered an accelerated flow of particles,” said Phan. “Furthermore, patchy and random reconnection events would have resulted in different spacecraft detecting jets directed in different directions, which was not the case.”

Since the spacecraft detected the jet for more than two hours, the reconnection must have been almost steady over at least that timespan. Another 27 large-scale reconnection events ? with the associated jets – were identified by ACE and Wind, four of which extended more than 50 Earth diameters, or 650,000 kilometers (about 400,000 miles). Thanks to these additional data, the team could conclude that reconnection in the solar wind is to be looked at as an extended and steady phenomenon.

The 2 February 2002 event could have been considerably larger, but the spacecraft were separated by no more than 200 Earth diameters, so its true extent is unknown. Two new NASA missions will help gauge the actual size of these events and examine them in more detail. The Solar Terrestrial Relations Observatory (STEREO) mission, scheduled for launch in May or June of 2006, will consist of two spacecraft orbiting the Sun on opposite sides of the Earth, separated by as much as 186 million miles (almost 300 million kilometers). Their primary mission is to observe Coronal Mass Ejections, billion-ton eruptions of electrically charged gas from the Sun, in three dimensions. However, the spacecraft will also be able to detect magnetic reconnection events occurring in the solar wind with instruments that measure magnetic fields and charged particles. The Magnetospheric Multi-Scale mission (MMS), planned for launch in 2013, will use four identical spacecraft in various Earth orbits to perform detailed studies of the cause of magnetic reconnection in the Earth’s magnetosphere.

Original Source: NASA News Release

Haro 11 galaxy closeup view. Image credit: Hubble. Click to enlarge
A tiny galaxy has given astronomers a glimpse of a time when the first bright objects in the universe formed, ending the dark ages that followed the birth of the universe.

Astronomers from Sweden, Spain and the Johns Hopkins University used NASA’s Far Ultraviolet Spectroscopic Explorer (FUSE) satellite to make the first direct measurement of ionizing radiation leaking from a dwarf galaxy undergoing a burst of star formation. The result, which has ramifications for understanding how the early universe evolved, will help astronomers determine whether the first stars ? or some other type of object ? ended the cosmic dark age.

The team will present its results Jan. 12 at the American Astronomical Society’s 207th meeting in Washington, D.C.

Considered by many astronomers to be relics from an early stage of the universe, dwarf galaxies are small, very faint galaxies containing a large fraction of gas and relatively few stars. According to one model of galaxy formation, many of these smaller galaxies merged to build up today’s larger ones. If that is true, any dwarf galaxies observed now can be thought of as “fossils” that managed to survive ? without significant changes ? from an earlier period.

Led by Nils Bergvall of the Astronomical Observatory in Uppsala, Sweden, the team observed a small galaxy, known as Haro 11, which is located about 281 million light years away in the southern constellation of Sculptor. The team’s analysis of FUSE data produced an important result: between 4 percent and 10 percent of the ionizing radiation produced by the hot stars in Haro 11 is able to escape into intergalactic space.

Ionization is the process by which atoms and molecules are stripped of electrons and converted to positively charged ions. The history of the ionization level is important to understanding the evolution of structures in the early universe, because it determines how easily stars and galaxies can form, according to B-G Andersson, a research scientist in the Henry A. Rowland Department of Physics and Astronomy at Johns Hopkins, and a member of the FUSE team.

“The more ionized a gas becomes, the less efficiently it can cool. The cooling rate in turn controls the ability of the gas to form denser structures, such as stars and galaxies,” Andersson said. The hotter the gas, the less likely it is for structures to form, he said.

The ionization history of the universe therefore reveals when the first luminous objects formed, and when the first stars began to shine.

The Big Bang occurred about 13.7 billion years ago. At that time, the infant universe was too hot for light to shine. Matter was completely ionized: atoms were broken up into electrons and atomic nuclei, which scatter light like fog. As it expanded and then cooled, matter combined into neutral atoms of some of the lightest elements. The imprint of this transition today is seen as cosmic microwave background radiation.

The present universe is, however, predominantly ionized; astronomers generally agree that this reionization occurred between 12.5 and 13 billion years ago, when the first large-scale galaxies and galaxy clusters were forming. The details of this ionization are still unclear, but are of intense interest to astronomers studying these so-called “dark ages” of the universe.

Astronomers are unsure if the first stars or some other type of object ended those dark ages, but FUSE observations of “Haro 11” provide a clue.

The observations also help increase understanding of how the universe became reionized. According to the team, likely contributors include the intense radiation generated as matter fell into black holes that formed what we now see as quasars and the leakage of radiation from regions of early star formation. But until now, direct evidence for the viability of the latter mechanism has not been available.

“This is the latest example where the FUSE observation of a relatively nearby object holds important ramifications for cosmological questions,” said Dr. George Sonneborn, NASA/FUSE Project Scientist at NASA’s Goddard Space Flight Center, Greenbelt, Md.

This result has been accepted for publication by the European journal Astronomy and Astrophysics.

Original Source: JHU News Release

A heavily cratered lunar surface by bombarding asteroids. Image credit: NASA Click to enlarge
Hit-and-run collisions between embryonic planets during a critical period in the early history of the Solar System may account for some previously unexplained properties of planets, asteroids, and meteorites, according to researchers at the University of California, Santa Cruz, who described their findings in the January 12 issue of the journal Nature.

The four “terrestrial” or rocky planets (Earth, Mars, Venus, and Mercury) are the products of an initial period, lasting tens of millions of years, of violent collisions between planetary bodies of various sizes. Scientists have mostly considered these events in terms of the accretion of new material and other effects on the impacted planet, while little attention has been given to the impactor. (By definition, the impactor is the smaller of the two colliding bodies.)

But when planets collide, they don’t always stick together. About half the time, a planet-sized impactor hitting another planet-sized body will bounce off, and these hit-and-run collisions have drastic consequences for the impactor, said Erik Asphaug, associate professor of Earth sciences at UCSC and first author of the Nature paper.

“You end up with planets that leave the scene of the crime looking very different from when they came in–they can lose their atmosphere, crust, even the mantle, or they can be ripped apart into a family of smaller objects,” Asphaug said.

The remnants of these disrupted impactors can be found throughout the asteroid belt and among meteorites, which are fragments of other planetary bodies that have landed on Earth, he said. Even the planet Mercury may have been a hit-and-run impactor that had much of its outer layers stripped away, leaving it with a relatively large core and thin crust and mantle, Asphaug said. That scenario remains speculative, however, and requires additional study, he said.

Asphaug and postdoctoral researcher Craig Agnor used powerful computers to run simulations of a range of scenarios, from grazing encounters to direct hits between planets of comparable sizes. Coauthor Quentin Williams, professor of Earth sciences at UCSC, analyzed the outcomes of these simulations in terms of their effects on the composition and final state of the remnant objects.

The researchers found that even close encounters in which the two objects do not actually collide can severely affect the smaller object.

“As two massive objects pass near each other, gravitational forces induce dramatic physical changes–decompressing, melting, stripping material away, and even annihilating the smaller object,” Williams said. “You can do a lot of physics and chemistry on objects in the Solar System without even touching them.”

A planet exerts enormous pressure on itself through self-gravity, but the gravitational pull of a larger object passing close by can cause that pressure to drop precipitously. The effects of this depressurization can be explosive, Williams said.

“It’s like uncorking the world’s most carbonated beverage,” he said. “What happens when a planet gets decompressed by 50 percent is something we don’t understand very well at this stage, but it can shift the chemistry and physics all over the place, producing a complexity of materials that could very well account for the heterogeneity we see in meteorites.”

The formation of the terrestrial planets is thought to have begun with a phase of gentle accretion within a disk of gas and dust around the Sun. Embryonic planets gobbled up much of the material around them until the inner Solar System hosted around 100 Moon-sized to Mars-sized planets, Asphaug said. Gravitational interactions with each other and with Jupiter then tossed these protoplanets out of their circular orbits, setting off an era of giant impacts that probably lasted 30 to 50 million years, he said.

Scientists have used computers to simulate the formation of the terrestrial planets from hundreds of smaller bodies, but most of those simulations have assumed that when planets collide they stick, Asphaug said.

“We’ve always known that’s an approximation, but it’s actually not easy for planets to merge,” he said. “Our calculations show that they have to be moving fairly slowly and hit almost head-on in order to accrete.”

It is easy for a planet to attract and accrete a much smaller object than itself. In giant impacts between planet-sized bodies, however, the impactor is comparable in size to the target. In the case of a Mars-size impactor hitting an Earth-size target, the impactor would be one-tenth the mass but fully one-half the diameter of the Earth, Asphaug said.

“Imagine two planets colliding, one half as big as the other, at a typical impact angle of 45 degrees. About half of the smaller planet doesn’t really intersect the larger planet, while the other half is stopped dead in its tracks,” Asphaug said. “So there is enormous shearing going on, and then you’ve got incredibly powerful tidal forces acting at close distances. The combination works to pull the smaller planet apart even as it’s leaving, so in the most severe cases the impactor loses a large fraction of its mantle, not to mention its atmosphere and crust.”

According to Agnor, the whole problem of planet formation is highly complex, and unraveling the role played by hit-and-run fragmenting collisions will require further study. By examining planetary collisions from the perspective of the impactor, however, the UCSC researchers have identified physical mechanisms that can explain many puzzling features of asteroids.

Hit-and-run collisions can produce a wide array of different kinds of asteroids, Williams said. “Some asteroids look like small planets, not very disturbed, and at the other end of the spectrum are ones that look like iron-rich dog bones in space,” he said. “This is a mechanism that can strip off different amounts of the rocky material that composes the crust and mantle. What’s left behind can range from just the iron-rich core through a whole suite of mixtures with different amounts of silicates.”

One of the puzzles of the asteroid belt is the evidence of widespread global melting of asteroids. Impact heating is inefficient because it deposits heat locally. It is not clear what could turn an asteroid into a big molten blob, but depressurization in a hit-and-run collision might do the trick, Asphaug said.

“If the pressure drops by a factor of two, you can go from something that is merely hot to something molten,” he said.

Depressurization can also boil off water and release gases, which would explain why many differentiated meteorites tend to be free of water and other volatile substances. These and other processes involved in hit-and-run collisions should be studied in more detail, Asphaug said.

“It’s a new mechanism for planetary evolution and asteroid formation, and it suggests a lot of interesting scenarios that warrant further study,” he said.

Original Source: NASA Astrobiology

Optical image of the galaxy merger NGC 2782. Image credit: UA Steward Observatory. Click to enlarge
Arizona astronomers have discovered a population of what appear to be young star clusters where they aren’t supposed to be. The newborn stars appear to have formed in the debris of the NGC 2782 galaxy collision — debris that lacks what astronomers believe are some important ingredients needed to form stars.

A large, Milky Way-type galaxy collided with a much smaller galaxy in the NGC 2782 collision. It’s an example of the most common type of galaxy collision in the universe. Scientists believe that such collisions played an important role in the buildup of large galaxies in the early universe.

If confirmed, these newly discovered young star clusters and their environment could help shed light on the process of star formation, especially in the early universe in regions far from the crowded, active centers of galaxies.

Karen Knierman, a graduate student and Arizona/NASA Space Grant Fellow at The University of Arizona, and Patricia Knezek of the WIYN Consortium in Tucson, Ariz., are reporting the research at the American Astronomical Society meeting in Washington, D.C., today.

The astronomers found the star clusters by taking deep images of the galaxy collision with the 4 Megapixel CCD camera of the 1.8 meter (71-inch) Vatican Advanced Technology Telescope (VATT) at Mount Graham International Observatory in Arizona.

NGC 2782 lies about 111 million light years away toward the Lynx constellation. When the two galaxies of unequal mass collided about 200 million years ago, their gravitational pull ripped out two tails of debris with very different properties.

Beverly Smith of Eastern Tennessee University and collaborators studied the optical and gas properties of these two tails and published their results in 1994 and 1999. Studying the gas properties tells astronomers about neutral hydrogen gas and molecular gas — both important ingredients in star formation. Smith and collaborators found that the optically bright eastern tail has some neutral hydrogen gas and molecular gas at the base of the tail, and an optically bright, but gas-poor concentration at the end of the tail. The optically faint western tail is rich in neutral hydrogen gas, but has no molecular gas.

Knierman and Knezek found blue star clusters younger than 100 million years along both tails, indicating that those stars formed within the tails after the galaxy collision began.

“That’s surprising because the western tail lacks molecular gas, one of the key ingredients for star formation,” Knierman said.

Star clusters are thought to form from the collapse of giant molecular gas clouds. If this is the case, astronomers would expect to see remnants of the molecular gas which helped give birth to the stars.

Given Smith’s earlier observations of gas in the debris tails, Knierman and Knezek expected they might see star formation in the eastern tail, where molecular gas is clearly present. But they didn’t expect to see star formation in the western tail, where no molecular gas was detected. Finding young star clusters in the western tail should prompt astronomers to question their current models of star formation, the Arizona team said.

“Do we still need a model of giant molecular gas clouds?” Knierman asked. “Or do we need a different model – perhaps one with smaller clumps of molecular gas that might have been destroyed or blown away when these energetic young stars formed?”

Finding unexpected young star clusters in the western tail could help explain why stars form in other places where there may be little molecular gas, like the outer edges of the Milky Way galaxy or in the debris of other galaxy collisions, Knierman and Knezek noted.

“This has important implications in how star formation proceeded when our universe was young and galaxy collisions were much more common than they are today,” Knierman said.

“Only recently have we become aware of the importance of the merging of small galaxies with larger systems in creating galaxies like our own Milky Way,” Knezek added.

Original Source: UA News Release