Caltech Question of the Month: How does a high or low water table affect our ability to feel earthquakes?

Submitted by Susan Rogers, Azusa, California, and answered by Dave Wald, visiting associate in geophysics at Caltech, and U.S. Geological Survey geophysicist.

When considering earthquake damage, it is important to distinguish between the effects of ground shaking and ground deformation (landslides, liquefaction, ground settling). In the Northridge earthquake, most of the damage was due to the strong shaking of buildings and their contents, but there was also a substantial amount of damage to foundations of buildings and residences caused by landslides and permanent ground settling.

Now, ground shaking is only marginally affected by the level of the water table. Whether or not the near-surface sediments are saturated with water does not significantly change the passing of seismic waves. So for shaking, for the most part, the level of the water table is not that important.

However, if the shallow sediments are water saturated (shallow water table) there is an increased chance of settling and landslides. If in addition the sediment is very sandy, the possibility of liquefaction is greatly increased and, therefore, the damage in such an area may be more widespread than if the water table had been deeper.

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Robert Tindol
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Black Hole That Periodically Ejects Its Inner Disk As Jets Discovered

WASHINGTON—Astronomers observing a disk of matter spiralling into a black hole in our galaxy have discovered that the black hole periodically hurls the inner portion of the disk into space as jets travelling at near the speed of light.

According to Stephen Eikenberry, an astrophysicist at the California Institute of Technology, the superhot gas in the inner disk shines brightly in X-rays, and dramatic dips in the X-ray emission suggest that the inner disk vanishes every 20 to 40 minutes. Infrared and radio observations at the same time show huge flares which indicate that matter is being thrown out of the system.

Eikenberry and colleagues from the Massachusetts Institute of Technology and NASA's Goddard Space Flight Center will discuss their findings at a 9:30 a.m. press conference on Wednesday, January 7, during the winter meeting of the American Astronomical Society.

The scientists observed the disappearance of the inner portion of the disk, known as an accretion disk, at the same time that glowing plasma is ejected from the black hole system. In August, Eikenberry and his collaborators at Caltech observed infrared flares from the black hole system, known as GRS 1915+105, using the Mt. Palomar 200-inch telescope.

At the same time, Ronald Remillard and his collaborators at MIT monitored X-ray dips from the same black hole using NASA's Rossi X-ray Timing Explorer (RXTE) satellite. Jean Swank and her collaborators at NASA/GSFC observed similar dips, antion between the disappearance of the inner disk and the jet ejection has never been seen until now."

"This work is also exciting because it may help us understand many other types of systems with jets," notes Robert Nelson, who works with Eikenberry at Caltech. "Astronomers have found jets in a wide range of objects, from quasars—incredibly powerful objects seen out to the edge of the observable universe—to young protostars."

The half-hour spacing between the ejections may be telling researchers that what they had thought were smooth, continuous outflows may in fact be intermittent explosions.

"There are many fine details in the X-ray dips that we may now seriously investigate to better understand the ejection mechanism," adds Edward R. Morgan, who works with Remillard at MIT. "In particular, there is a very unusual X-ray flash at the bottom of these dips in which the X-ray spectrum changes significantly. This may be the trigger for the rapid acceleration of the disk material."

The black hole in GRS 1915+105 became known to astronomers in 1992 as an X-ray nova, which is believed to signify the sudden flow of hot gases into a black hole from a companion star in a binary system. The black hole in GRS 1915+105 is thought to have a mass equal to ten Suns or more, all crushed by its own gravity into a tiny sphere contained within an "event horizon," which itself has a radius of about 20 km.

When a black hole pulls gas from the atmosphere of a companion star, the matter spirals in toward the event horizon like water going down a drain, and the swirling disk created by the flow is known to astronomers as an "accretion disk." The gas in the disk heats up dramatically due to the large acceleration and friction. Just before entering the event horizon, the gas reaches temperatures of millions of degrees, causing it to glow in X-rays.

In 1994, Mirabel and Luis Rodriguez observed radio emission from jets in GRS 1915+105, and they determined that the speed of the jets was greater than 90 percent of the speed of light, or roughly 600 million miles per hour. Since RXTE began observing the X-ray sky in early 1996, the exceptionally chaotic behavior of GRS 1915+105 in X-rays has been chronicled on many occasions.

The new results gained by Eikenberry's team brings together these phenomena by showing that modest jet ejections and the pattern of X-ray variations are synchronized in an organized way.

"The repeated ejections are really amazing," says Craig Markwardt, a member of the NASA/GSFC team. "The system behaves like a celestial version of Old Faithful. At fairly regular intervals, the accretion disk is disrupted and a fast-moving jet is produced."

"This jet is staggeringly more powerful than a geyser," adds Swank. "Every half hour, the black hole GRS 1915+105 throws off the mass of an asteroid at near the speed of light. This process clearly requires a lot of energy; each cycle is equivalent to 6 trillion times the annual energy consumption of the entire United States."

"Since the disk-jet interaction is so poorly understood, we're hoping that further analysis of these observations will show us more details of what is happening so close to the black hole," Eikenberry says. "We're planning more detailed studies for the coming year which should give us even more clues as to the nature of these incredibly powerful events.

"Right now, we still aren't even sure why these dips and ejections occur every half hour or so—why not every week or every 30 seconds, for instance?

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Robert Tindol
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Biological Activity the Likely Culprit of Atmospheric Nitrous Oxide Increases

PASADENA–Nitrous oxide (N2O) is an atmospheric gas known to contribute both to global warming and ozone depletion. New research suggests that its changing concentration in the atmosphere is largely a result of biological activity.

In the December 5 issue of the journal Science, Yuk Yung of the California Institute of Technology and Charles Miller of Caltech's Jet Propulsion Lab (now at the Haverford College Department of Chemistry) describe their work on isotopes of N2O, a greenhouse gas that is of increasing concern because its concentration in the atmosphere has been rising for several decades.

N2O is a molecule with two atoms of nitrogen and a single atom of oxygen. It is created in the decay of organic material, principally plants, but is also generated in the manufacture of nylon.

Scientists have known for years that N2O enters the nitrogen cycle, but the ultimate sources and sinks of the gas have been unclear. By contrast, carbon dioxide, another greenhouse gas, is known to be a direct consequence of industrial activity.

"Nitrous oxide is less important as a greenhouse molecule than carbon dioxide, and slightly less important than methane," says Yung, a professor of planetary science at Caltech. "But the concentrations have been increasing since good measurements began 20 years ago, and ice core samples in Greenland and Antarctica suggest that it has been increasing since the Industrial Revolution began."

Yung and Miller specifically looked at isotopes of nitrous oxide once it enters the stratosphere. Isotopes are variations of a chemical element that have the same number of protons (and thus the same atomic number), but a different number of neutrons. Thus, a 15N atom of nitrogen has its regular seven protons and seven neutrons, but an additional neutron as well.

A careful analysis of isotopic variations is an effective way of tracing substances to their sources. If the nitrogen-based fertilizer in agriculture has a known isotopic makeup and that same percentage is found in the stratosphere, for example, then it can be concluded that agricultural fertilization is a contributor.

Yung and Miller examined theoretically how isotopes of nitrous oxide interact with ultraviolet light energy. They predict that, as N2O is destroyed by light, heavier isotopes survive preferentially because molecules comprising slightly heavier isotopes require a bit more energy for the atoms to separate.

From their theory and related atmospheric measurements presented in the same issue by researchers at the Scripps Institution of Oceanography and the University of California at San Diego, Yung and Miller conclude that new chemical sources do not need to be introduced to account for the isotopic concentrations that are indeed observed in the stratosphere.

Thus, sources such as the decay of plant life and the burning of rainforests and other biomass burning can account for the signatures that are seen. Experimental verification of the predictions is now under way in the laser spectroscopy lab of Caltech cosmochemist Geoff Blake.

Understanding the sources can give society a better grip on the possibilities of dealing with the problem, Yung says.

"I think the most reasonable explanation for the increase is that we are accelerating biological activity globally," he says. "Because of global warming, the use of agricultural fertilizers, and nitrogen made from pollution that acts just like a fertilizer, the biosphere has been stimulated. This fosters the growth/decay cycle which leads to N2O release."

The next step for the researchers is to pin down the precise isotopic signatures of various biological and atmospheric processes. But there may be little that can realistically or politically be done if biology on a planetary scale is responsible, Geoff Blake says.

"We may just have to live with it.

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Robert Tindol
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Geophysicists Develop Model to Describe Huge Gravity Anomaly of Hudson Bay Region

PASADENA—While the gravity field of Earth is commonly thought of as constant, in reality there are small variations in the gravitational field as one moves around the surface of the planet.

These variations have typical magnitudes of about one–ten thousandth of the average gravitational attraction, which is approximately 9.8 meters per second per second. A global map of these variations shows large undulations at a variety of length scales. These undulations are known as gravity anomalies.

There are many such anomalies in Earth's gravity field, but one of the largest negative gravity anomalies (implying the attractions of gravity being a little less than average, or in other words, a mass deficit) centered over Hudson Bay, Canada. Using a new approach to analyzing planetary gravity fields, two geophysicists, Mark Simons at the California Institute of Technology and Bradford Hager at M.I.T., have shown that incomplete glacial rebound can account for a substantial portion of the Hudson Bay gravity anomaly.

With this new information, Simons and Hager were able to place new constraints on the variations in strength of the materials that constitute the outer layers of Earth's interior (the crust and mantle). Their work appears in the December 4 issue of the journal Nature.

About 18,000 years ago, Hudson Bay was at the center of a continental–sized glacier. Known as the Laurentide ice sheet, this glacier had a thickness of several kilometers. The weight of the ice bowed the surface of Earth down. The vast majority of the ice eventually melted at the end the Ice Age, leaving a depression in its wake.

While this depression has endured for thousands of years, it has been gradually recovering or "flattening itself out." The term "glacial rebound" refers to this exact behavior, whereby the land in formerly glaciated areas rises after the ice load has disappeared.

Evidence of this is seen in coastlines located near the center of the former ice sheet. These coastlines have already risen several hundred meters and will continue to rebound.

"The rate at which the area rebounds is a function of the viscosity of Earth," says Simons. "By looking at the rate of rebound going on, it's possible to learn about the planet's viscosity."

Simons says that geophysicists have known for some time about the Hudson Bay gravity anomaly, but have hitherto been uncertain how much of the gravity anomaly is a result of glacial rebound and how much is due to mantle convection or other processes.

The gravity anomaly is measured from both the ground and from space. Simons and Hager use a gravity data set developed by researchers at the Goddard Space Flight Center.

However, knowing how much of an anomaly exists at a certain site on Earth is not sufficient to determine the pliability of the materials beneath it. For this, Simons and his former M.I.T. colleague Hager have developed a new mathematical tool that looks at the spatial variations of the spectrum of the gravity field.

In many instances, this approach allows one to separate the signatures of geologic processes that occur at different locations on Earth. In particular, Simons and Hager were able to isolate the glacial rebound signature from signatures of other processes, such as manifestations of plate tectonics, that dominate that gravity field but are concentrated at other geographic locations.

Having an estimate of incomplete postglacial rebound allowed Simons and Hager to derive a model of how the viscosity of the mantle changes with depth. Simons and Hager propose one such model that explains both the gravity anomaly as well as the uplift rates estimated from the coastlines.

Their favored model suggests that underneath the oldest parts of continents (some of which are over 4 billion years old) the viscosity of the outer 400 kilometers of Earth is much stiffer than under the oceans. Therefore, these continental keels can resist the erosion by the convective flow that drives plate tectonics.

 

 

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Robert Tindol
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Caltech Question of the Month

Question of the Month Submitted by John Propst, Fullerton, California.

Answered by Ken Libbrecht, Caltech professor of physics.

Light travels at the speed of light, and is created traveling at light speed. When Einstein invented the theory of special relativity, he postulated that the speed of light was a constant.

If you carry your flashlight on a moving train, the photons travel out from it at a constant speed, whether you measure their speed from the ground or measure their speed from the train. You can't derive this fact from anything, so it becomes a fundamental law of physics.

We don't know why nature chooses to operate this way, but many measurements have shown that Einstein's postulate is very accurately followed.

 

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RT

Caltech Biologists Pin Down Chain of Reactions That Turn On the Duplication of DNA

PASADENA—Caltech biologists have pinpointed the sequence of reactions that triggers the duplication of DNA in cells.

In companion papers appearing in recent issues of the journals Science and Cell, Assistant Professor of Biology Raymond Deshaies and his colleagues describe the chain of events that lead to the copying of chromosomes in a baker's yeast cell. Baker's yeast is often used as a model for human cells, so the research could have future implications for technology aimed at controlling cell reproduction, such as cancer treatments.

"We've provided a bird's-eye view of how a cell switches on the machinery that copies DNA," says Deshaies. "These principles can now be translated into a better understanding of how human cells proliferate."

The group's research keys primarily on how cells copy and segregate their chromosomes during the process of duplicating one cell into two. The new papers are concerned with how cells enter the DNA synthesis phase, during which the chromosomes are copied.

A question that cell biologists have sought for years to answer is that of which precise chemical events set off these reactions. The cell cycle is fundamental to the growth and division of all cells, but the process is somehow ramped down once the organism reaches maturity.

The paper appearing in Science describes how DNA synthesis is turned on. In the preceding stage (known as G1), proteins named G1 cyclins trigger the destruction of an inhibitor that keeps DNA synthesis from beginning.

This inhibitor sequesters an enzyme referred to as S-CDK (for DNA synthesis-promoting cyclin-dependent kinase), thereby blocking its action. Once the S-CDK is released, it switches on DNA synthesis. The S-CDK is present before the copying of DNA begins, but the DNA copying is not turned on until the S-CDK is freed of its inhibitor. The Deshaies group has shown that several phosphates are attached to the S-CDK inhibitor. These phosphates act as a molecular Velcro, sticking the inhibitor to yet another set of proteins called SCF.

The Cell paper essentially picks up on the description of the cell cycle at this point. The SCF, which acts like a molecular "hit man," promotes the attachment of another protein, ubiquitin. Ubiquitin in turn attracts the cellular garbage pail, proteasome. The inhibitor is disposed of in the proteasome, thereby freeing the S-CDK, which goes on to stimulate DNA duplication.

The process described above is quite complicated even in this condensed form, and actually is considerably more complicated in its technical details. But the detailed description that Deshaies and his colleagues have achieved is important fundamental science that could have technological implications in the future, Deshaies says.

"This traces the ignition of DNA synthesis down to a relatively small set of proteins," he says. "Any time you figure out how a part of the cell division machinery works, you can start thinking about devising new strategies to turn it on and off."

It is a precise turning on and off of DNA replication, many researchers think, that will someday be the key to better and more specific cancer-fighting drugs. Because a tumor is a group of cells that literally never stops the cell duplication cycle, a greater understanding of the cycle itself is almost certain to be a factor in further medical advances in cancer treatment.

"It could be five to 10 years, but this work could point the way to new cancer-fighting drugs," Deshaies says. "It is much easier to begin a rational approach to developing new treatments for cancer if you are armed with fundamental insights into how the cellular machinery works."

The other authors on the paper in the October 17 issue of Cell are R. M. Renny Feldman, a Caltech graduate student in biology; Craig C. Correll, a Caltech postdoctoral scholar in biology; and Kenneth B. Kaplan, a postdoctoral researcher at MIT.

The other authors of the Science paper from the October 17 issue are Rati Verma, a senior research fellow at Caltech; Gregory Reynard, a Caltech technician; and R. S. Annan, M. J. Huddleston, and S. A. Carr, all of the Research Mass Spectrometry Laboratory at SmithKline Beecham Pharmaceuticals in King of Prussia, Pennsylvania.

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Robert Tindol
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Caltech Scientists Devise First Neurochip

NEW ORLEANS—Caltech researchers have invented a "neurochip" that connects a network of living brain cells wired together to electrodes incorporated into a silicon chip.

The neurochips are being unveiled today at the annual meeting of the Society for Neurobiology, which is being held in New Orleans the week of October 25-30. According to Dr. Jerome Pine, one of the five coinventors of the neurochip, the technology is a major step forward for studying the development of neural networks.

The neurons used in the network are harvested from the hippocampus of rat embryos. Once the cells have been separated out by a protein-eating enzyme, each is individually inserted into a well in the silicon chip that is about half the diameter of a human hair. The cell is spherical in shape when it is inserted and is slightly smaller in diameter than the silicon chip well. When it is set in place and fed nutrients, it grows dendrites and an axon that spread out of the well.

In doing so, each neuron remains close to a single recording and stimulating electrode within the well, and also links up with other dendrites and axons attached to other neurons in other nearby wells.

According to Michael Maher, one of the coinventors, the neurochip currently has room for 16 neurons, which appear to develop normal connections with each other. "When the axons meet dendrites, they make an electrical connection," says Maher, who left Caltech in September to assume a postdoctoral appointment at UC San Diego. "So when one neuron fires, information is transmitted to the next neuron."

The neurochip network will be useful in studying the ways in which neurons maintain and change the strengths of their connections, Maher adds. "It's believed that memory in the brain is stored in the strength of these connections.

"This is pretty much a small brain connected to a computer, so it will be useful in finding out how a neural network develops and what its properties are. It will also be useful for studying chemical reactions at the synapses for weeks at a time. With conventional technology, you can record directly from at most a few neurons for at most a couple of hours."

There are two challenges facing the researchers as they attempt to improve the neurochips. One is providing the set of growth factors and nutrients to keep the cells alive for long periods of time. At present, two weeks is the limit.

The second challenge is finding a way to insert the cells in the silicon wells in a less time-consuming way. At present, the technique is quite labor intensive and requires a highly skilled technician with considerable patience and dexterity.

Other than the sheer effort involved, however, there is no reason that millions of cells could not be linked together at present, Maher says.

The other Caltech coinventors of the neurochip are Hanna Dvorak-Carbone, a graduate student in biology; Yu-Chong Tai, an associate professor of electrical engineering; and Tai's student, John Wright. The latter two are responsible for the silicon fabrication.

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Robert Tindol
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3-D Images of Martian Terrain To Be Shown To the Tune of Holst

PASADENA—Dramatic 3-D pictures of Mars will be shown during a performance of Gustav Holst's "Mars" at the first Caltech-Occidental Concert Band performance of the season.

The Concert Band is conducted by William Bing.

The concert begins at 8 p.m. Saturday, November 15, in Caltech's Beckman Auditorium. The concert is free and open to the public, with seating available on a "first come" basis.

The pictures are those returned to Earth from the Mars Pathfinder since the July 4 landing. Because pictures have been taken from slightly different angles, team scientists have been able to supply a number of high-quality 3-D pictures for public viewing.

The pictures are supplied by Rob Manning, who was Pathfinder chief engineer on the mission. Manning is also a trumpet player with the Caltech Jazz Band and a Caltech alumnus.

In addition to the performance of "Mars" from Holst's suite The Planets, the program will include music by George Gershwin and excerpts from the musical Ragtime.

Guest conductor Daniel Kessner will conduct one of his own compositions. This special work was recently commissioned by a consortium of colleges, including Caltech.

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Robert Tindol
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First Fully Automatic Design of a Protein Achieved by Caltech Scientists

PASADENA—Caltech scientists have found the Holy Grail of protein design. In fact, they've snatched it out of a giant pile of 1.9 x 1027 other chalices.

In the October 3 issue of the journal Science, Stephen L. Mayo, an Assistant Professor of Biology and a Howard Hughes Medical Institute Assistant Investigator, and chemistry graduate student Bassil I. Dahiyat report on their success in constructing a protein of their choice from scratch.

Researchers for some time have been able to create proteins in the lab by stringing together amino acids, but this has been a very hit-and-miss process because of the vast number of ways that the 20 amino acids found in nature can go together.

The number 1.9 x 1027, in fact, is the number of slightly different chains that 28 amino acids can form. And because slight differences in the geometry of protein chains are responsible for biological functions, the total control of formation is necessary to create new biological materials of choice.

By using a Silicon Graphics supercomputer to sort through all possible combinations for a selected protein, Mayo and Dahiyat have identified the target protein's best possible amino acid sequence. Then they have managed to take this knowledge and create the protein in the lab with existing technical processes.

This is a first, says Mayo. "Our goal has been to design brand-new proteins that do what we want them to do. This new result is the first major step in that direction. "Moreover, it shows that a computer program is the way to go in creating biological materials."

The technique they use, automated protein design, combines experimental synthesis of molecules with supercomputer-powered computational chemistry.

Proteins are the molecular building blocks of all living organisms. Composed of various combinations of the 20 amino acids, protein molecules can each comprise just a few hundred atoms, or literally millions of atoms. Most proteins involved in life processes have at least 100 amino acids, Mayo says.

Mayo and Dahiyat, who have been working on this research for five years, have developed a system that automatically determines the string of amino acids that will fold to most nearly duplicate the 3-D shape of a target structure. The system calculates a sequence's 3-D shape and evaluates how closely this matches the 3-D structure of the target protein.

One problem the researchers face is the sheer number of combinations needed to design a protein of choice. The protein that is the subject of this week's Science paper is a fragment of a fairly inconspicuous molecule involved in gene expression, and as such has only 28 amino acids. Even this small number takes a prodigious amount of computational power. A more desirable protein might involve 100 amino acids, which could make the staggering number of 10130 possible amino acid sequences.

Because this number is larger than the number of atoms in the universe, the researchers have had to find clever computational strategies to circumvent the impossible task of grinding out all the calculations.

In this case, the fastest way to the answer is by working backward. Starting with all the amino acid sequences possible for the protein, the computer program finds arrangements of amino acids that are a bad fit to the target structure. By repeatedly searching for, and eliminating, poorly matching amino acid combinations, the system rapidly converges on the best possible sequence for the target.

Subsequently, the simulation can be used to find other sequences that are nearly as good a fit as the best one.

This process has been honed by designing sequences for several different proteins, synthesizing them in the laboratory, and testing their actual properties.

With their innovative strategy, Mayo and Dahiyat are now reproducing proteins that are very similar to the target molecules. (The accompanying illustration shows how closely the protein they have formulated matches the target protein.)

But the goal is not just to create the proteins that already exist in nature. The researchers can actually improve on nature in certain circumstances. By making subtle changes in the amino acid sequence of a protein, for example, they are able to make a molecule more stable in harsh chemicals or hot environments (proteins tend to change irreversibly with a bit of heat, as anyone who has cooked an egg can attest).

"Our technology can actually change the proteins so that they behave a lot better," said Dahiyat, who recently finished his Caltech doctorate in chemistry and will now head Xencor, a start-up company established to commercialize the technology. The ability to create new proteins, and to adapt existing proteins to different environments and functions, could have profound implications for a number of emerging fields in biotechnology.

And, of course, it could help further the understanding of living processes.

"Paraphrasing Richard Feynman, if you can build it, you can understand it," says Mayo. "We think we can soon achieve a better understanding of proteins by going into a little dark room and building them to do exactly what we want them to do."

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Robert Tindol
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Possible Planet-Forming Disk imaged by Caltech Radio Astronomers

PASADENA—A giant disk of gas and dust over 10 times the size of our own solar system has been detected rotating around a young star in the constellation of Auriga. The star is both more massive and brighter than our sun, and appears to be a young version of another star called Beta Pictoris, where astronomers have long suspected the presence of planets.

The new discovery was made by radio astronomers at the California Institute of Technology using the millimeter-wave array at Caltech's Owens Valley Radio Observatory in central California. The results appear in the current issue of the journal Nature, and concern a relatively massive star known as MWC480, which is about 450 light-years from Earth.

How prevalent is planet formation around young stars? Past work had shown that stars similar to our own sun possess protoplanetary disks in their youth, disks we believe will form planets, perhaps as our own solar system did. However, little was known about the propensity of disks to form planets around stars that are more massive than our sun.

According to Vince Mannings, the paper's first author, the new results provide unprecedentedly clear evidence for the presence of a rotating disk of gas surrounding MWC480, and support earlier indications of rotating disks encircling some less massive and young sunlike stars. Not only is the gas around MWC480 clearly discernible at radio wavelengths, he says, but the orbital rotation of the entire disklike cloud is also unambiguously observed.

The presence of rotation suggests that, as for the disks around the young sunlike stars, the disk structure around MWC480 is long-lived. Indeed, this massive reservoir of orbiting material could last long enough to form new planets. "Families of planets, perhaps resembling our own solar system, are thought to originate in such disks," says Mannings. "Our sun, when very young, possibly had a disk similar to that around MWC480."

The star in the middle of the MWC480 disk resembles a much older star called Beta Pictoris, which is surrounded by a comparatively lightweight "debris disk," probably composed in part of dust-grain remnants from processes connected with an earlier phase of planet building. The new results imply that, in its youth, Beta Pictoris may have possessed a massive disk comparable to that now identified around MWC480. Beta Pictoris might have been, effectively, a "planetary construction site," says Mannings.

Other members of the research team are David Koerner, an astronomer at the Caltech/NASA Jet Propulsion Lab, and Anneila Sargent, who is executive director of Caltech's Owens Valley Radio Observatory.

Mannings says, "We believe that the amount of material in this disk is sufficient to produce a system of planets. We detect enough gas and dust to build planets with the same total mass as that of the nine planets in our own solar system. But we emphasize that the possibility of planet building within this particular disk is speculation only."

The radio image is sufficiently detailed to show that the large disk of gas and dust is tilted about 30 degrees from face-on. A tantalizing aspect of the image is that the rotation of the disk can be detected by measuring the velocities of the gas, most of which is in the form of molecular hydrogen. About 1 percent of the disk is dust grains, and just a trace amount of the material is carbon monoxide. The hydrogen is not detected directly, but the gas velocities can be probed using spectral-line radio waves emitted by the carbon monoxide. The Caltech measurements demonstrate that gas south of the star travels approximately toward us, and away from us when north of the star. From our vantage point, the disk is inferred to be rotating roughly from south to north.

For the first time, astronomers have identified clearly a young massive disk that could gradually evolve into a debris disk such as that surrounding the older star Beta Pictoris, perhaps building planets along the way. By studying stars like MWC480, say Mannings, Koerner and Sargent, we can hope to learn not only about the origins of the Beta Pictoris debris disk, but perhaps about the beginnings of our own solar system too. Astronomers have targeted nearby sunlike stars for searches for new planets, but this discovery shows that brighter stars should also be included.

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