Author: Fraser Cain

Image credit: NASA/JPL

New research from the University of Colorado shows how recycling of material can extend the lifetime of a ring system, such as those around Jupiter, Saturn, Neptune and Uranus. The small moons near the gas giants have long been known to sculpt the shape of the rings. It’s now believed that they’re piles of loosely-collected rubble which pull material out of the rings and then feed it back in when they collide with another object. NASA’s Cassini spacecraft is on its way to Saturn now and should provide more details when it arrives in July 2004.

Although rings around planets like Jupiter, Saturn, Uranus and Neptune are relatively short-lived, new evidence implies that the recycling of orbiting debris can lengthen the lifetime of such rings, according to University of Colorado researchers.

Strong evidence now implies small moons near the giant planets like Saturn and Jupiter are essentially piles of rubble, said Larry Esposito, a professor at CU-Boulder’s Laboratory for Atmospheric and Space Physics. These re-constituted small bodies are the source of material for planetary rings.

Previous calculations by Esposito and LASP Research Associate Joshua Colwell showed the short lifetimes for such moons imply that the solar system is nearly at the end of the age of rings. “These philosophically unappealing results may not truly describe our solar system and the rings that may surround giant extra-solar planets,” said Esposito. “Our new calculations of models explain how inclusion of recycling can lengthen the lifetime of rings and moons.”

The observations from the Voyager and Galileo space missions showed a variety of rings surrounding each of the giant planets, including Jupiter, Saturn, Uranus and Neptune. The rings are mixed in each case with small moons.

“It is clear that the small moons not only sculpt the rings through their gravity, but are also the parents of the ring material,” said Esposito. “In each ring system, destructive processes like grinding, darkening and spreading are acting so rapidly that the rings must be much younger than the planets they circle.”

Numerical models by Esposito and Colwell from the 1990’s showed a “collisional cascade,” where a planet’s moons are broken into smaller moons when struck by asteroids or comets. The fragments then are shattered to form the particles in new rings. The rings themselves are subsequently ground to dust, which is swept away.

But according to Colwell, “Some of the fragments that make up the rings may be re-accreted instead of being ground to dust. New evidence shows some debris has accumulated into moons or moonlets rather than disappearing through collisional erosion.”

“This process has proceeded rapidly,” said Esposito. “The typical ring is younger than a few hundred million years, the blink of an eye compared to the planets, which are 4.5 billion years old. The question naturally arises why rings still exist, to be photographed in such glory by visiting human spacecraft that have arrived lately on the scene,” he said.

“The answer now likely seems to be cosmic recycling,” said Esposito. Each time a moon is destroyed by a cosmic impact, much of the material released is captured by other nearby moons. These recycled moons are essentially collections of rubble, but by recycling material through a series of small moons, the lifetime of the ring system may be longer than we initially thought.”

Esposito and former LASP Research Associate Robin Canup, now with the Southwest Research Institute’s Boulder branch, showed through computer modeling that smaller fragments can be recaptured by other moons in the system. “Without this recycling, the rings and moons are soon gone,” said Esposito.

But with more recycling, the lifetime is longer, Esposito said. With most of the material recycled, as now appears to be the case in most rings, the lifetime is extended by a large factor.

“Although the individual rings and moons we now see are ephemeral, the phenomenon persists for billions of years around Saturn,” said Esposito. “Previous calculations ignored the collective effects of the other moons in extending the persistence of rings by recapturing and recycling ring material.”

Esposito, the principal investigator on a $12 million spectrograph on the Cassini spacecraft slated to arrive at Saturn in July 2004, will look closely at the competing processes of destruction and re-capture in Saturn’s F ring to confirm and quantify this explanation. Esposito discovered the F Ring using data from NASA’s Voyager 2 mission to the outer planets launched in 1978.

Original Source: University of Colorado News Release

Image credit: JACH

A young, hot star has been found nestled inside a cocoon of molecular hydrogen gas half a light-year across. The star is called IRAS 07427-2400, and it’s sending out a solar wind so fast (360,000 km/h) that the shock waves are heating up the gas so it’s visible to Earth-based telescopes. Astronomers believe that these massive stars have so much energy that they blast their environment so that planets aren’t able to form the way they do around more “normal” stars, like our own Sun.

Astronomers have discovered a giant envelope or disk of glowing gas more than half a light year across, illuminated by shockwaves caused by winds travelling at up to 360,000 km/hour (220,000 miles/hour). The disk is orbiting a massive star 20,000 light years from Earth. This is the first time such a disk has been found emitting its own light. The discovery is reported today (8 December 2003) in the journal “Astronomy and Astrophysics”.

The work, led by Dr Nanda Kumar of the Centre for Astrophysics of the University of Porto (CAUP), Portugal, used the United Kingdom Infrared Telescope (UKIRT) in Hawaii, and other telescopes. The team used the new UKIRT Imager Spectrometer (UIST) on UKIRT, to study the young stellar object (YSO) known as IRAS 07427-2400. Their results show that the envelope or disk around the young star is glowing in the light of molecular hydrogen and ionised iron.

Dr Stan Kurtz of the National Autonomous University of Mexico (UNAM), who is an expert in studies of solar system sized disks around massive stars, said “Protostellar disks are known to exist around Sun-like stars, but they are usually seen in silhouette against background light from nebulae. In this case, however, the molecules in the disk are hot enough to shine brightly themselves.”

Dr Kumar adds “This is the first time an envelope like this has been seen in molecular hydrogen emission. It tells us that massive stars form with very different conditions and physical aspects when compared to Sun-like stars.”

The central star itself is very young, at roughly 100,000 years old. By comparison, our middle-aged Sun is about 5 billion years old. The surrounding gas disk is huge – its diameter is one thousand times larger than Pluto’s orbit in our own Solar System. The young star is rapidly changing as gas and dust spiral down onto its surface through the disk, a process called “accretion”. The star is already more than one thousand times more luminous than our Sun.

Dr Amadeu Fernandes of CAUP, Porto states “The UKIRT results show that the glow from the disk is not due to the intense light from the central star, but is instead caused by powerful shock waves”. Dr Chris Davis of the Joint Astronomy Centre in Hawaii explains “The disk is possibly being shocked by supersonic winds driven by the central star. These winds, travelling at hundreds of thousands of kilometres per hour, crash into the disk and heat the gas to thousands of degrees.”

Dr Kumar adds “It is also possible that the shocks are powered by large amounts of gas and dust collapsing through the disk onto the young star. Further investigation is required to understand their origin.”

Disks around young, Sun-like stars are known to be the birth places of planets, which can condense out of the gas and dust after the star has formed. This disk has about 150 times the mass of our Sun – enough gas and dust to make a hundred Sun-like stars, or many thousands of planets. However, the results suggest that it will not produce new planets or stars in the future. The intense shock waves have made the gas far too hot to condense. Dr Davis says “This tells us that massive stars like this may not be able to form planets, as their surrounding gas is too hot.”

Instead of forming a cluster of stars, or a family of orbiting planets, the disk will ultimately be destroyed by the intense ultraviolet radiation from the central star. The radiation is already at work, gnawing at the inner edges of the disk and evaporating the gas. Dr Kumar says “We’ve seen open rings of gas around similar stars, also with UKIRT. We think they may be the remnants of large disks that have been almost completely evaporated.”

The complete destruction of the disk will take many thousands of years. Before this happens, the size and brightness of the disk allow researchers to study it with powerful ground-based telescopes such as UKIRT, without the need for a space telescope.

Dr Davis says “We now have the task of searching for other hot, molecular disks around massive young stars, and of fitting the existence of this super-disk into our theories on the birth of massive stars.”

The disk was first discovered in January 2001 by UKIRT, but further observations were needed to confirm its nature. The team used the Caltech Submillimeter Observatory in Hawaii to provide supporting evidence to prove the rotating nature of the disk. Stan Kurtz used the Very Large Array radio telescope in New Mexico to image the central massive star at radio wavelengths. The team returned to use UKIRT in December 2002.

The work described is published on 8th December 2003 in “Astronomy and Astrophysics” volume 412.

Original Source: JACH News Release

Image credit: NRAO

One of the most distant galaxies ever seen seems to be in the midst of extremely active star formation. The galaxy has been dubbed the Cloverleaf, and it’s 11 billion light-years away, so astronomers are seeing it when the Universe was less than 3 billion years old. It has a rate of star formation 300 times greater than our own Milky Way – 1,000 new stars are being formed each year. The discovery was made using the National Science Foundation’s Very Large Array radio telescope.

Astronomers have discovered a key signpost of rapid star formation in a galaxy 11 billion light-years from Earth, seen as it was when the Universe was only 20 percent of its current age. Using the National Science Foundation’s Very Large Array (VLA) radio telescope, the scientists found a huge quantity of dense interstellar gas — the environment required for active star formation — at the greatest distance yet detected.

A furious spawning of the equivalent of 1,000 Suns per year in a distant galaxy dubbed the Cloverleaf may be typical of galaxies in the early Universe, the scientists say.

“This is a rate of star formation more than 300 times greater than that in our own Milky Way and similar spiral galaxies, and our discovery may provide important information about the formation and evolution of galaxies throughout the Universe,” said Philip Solomon, of Stony Brook University in New York.

While the raw material for star formation has been found in galaxies at even greater distances, the Cloverleaf is by far the most distant galaxy showing this essential signature of star formation. That essential signature comes in the form of a specific frequency of radio waves emitted by molecules of the gas hydrogen cyanide (HCN).

“If you see HCN, you are seeing gas with the high density required to form stars,” said Paul Vanden Bout of the National Radio Astronomy Observatory (NRAO).

Solomon and Vanden Bout worked with Chris Carilli of NRAO and Michel Guelin of the Institute for Millimeter Astronomy in France. They reported their results in the December 11 issue of the scientific journal Nature.

In galaxies like the Milky Way, dense gas traced by HCN but composed mainly of hydrogen molecules is always associated with regions of active star formation. What is different about the Cloverleaf is the huge quantity of dense gas along with very powerful infrared radiation from the star formation. Ten billion times the mass of the Sun is contained in dense, star-forming gas clouds.

“At the rate this galaxy is seen to be forming stars, that dense gas will be used up in only about 10 million years,” Solomon said.

In addition to giving astronomers a fascinating glimpse of a huge burst of star formation in the early Universe, the new information about the Cloverleaf helps answer a longstanding question about bright galaxies of that era. Many distant galaxies have super-massive black holes at their cores, and those black holes power “central engines” that produce bright emission. Astronomers have wondered specifically about those distant galaxies that emit large amounts of infrared light, galaxies like the Cloverleaf which has a black hole and central engine.

“Is this bright infrared light caused by the black-hole-powered core of the galaxy or by a huge burst of star formation? That has been the question. Now we know that, in at least one case, much of the infrared light is produced by intense star formation,” Carilli said.

The rapid star formation, called a starburst, and the black hole are both generating the bright infrared light in the Cloverleaf. The starburst is a major event in the formation and evolution of this galaxy.

“This detection of HCN gives us a unique new window through which we can study star formation in the early Universe,” Carilli said.

The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc.

Original Source: NRAO News Release