2MASS Sky Survey
Space Telescope Science Institute
John Bahcall's Web Page
NASA's Education Site - Life Stories of Stars
Stellar Evolution: Images, spectra, and explanation of the life cycle of stars.
Argonne National Laboratory Site with links
John Bahcall's Web Page
Helsinki Neutrino Pages
Super-Kamiokande Site at UC Irvine Discusses Neutrino Mass
Sudbury Neutrino Observatory SNO
Gallium Neutrino Observatory (formerly Gallex)
URLs for solar neutrino experiments
John Bahcall's Neutrino References
The Neutrino Oscillation Industry
A Fermilab scientist's explanation of why neutrinos are interesting
Super-K US Collaboration
Vanderbilt University press release, December 14, 2002
NOTE: A multimedia version of this story, including an animation and additional background, is available on Exploration, Vanderbilt's online research magazine, at http://exploration.vanderbilt.edu.
Classical T Tauri stars - those less than 3 million years old - are invariably accompanied by a thick disk of dust and gas, which is often called a protoplanetary disk because it is a breeding ground for planet formation. Most older T Tauri stars show no signs of encircling disks. Because they are not old enough for planets to form, astronomers have concluded that most of these stars must loose their disk material before planetary systems can develop.
Weintraub and Bary are pursuing an alternative theory. They propose that most older T Tauri stars haven't lost their disks at all: The disk material has simply changed into a form that is nearly invisible to Earth-based telescopes. They published a key observation supporting their hypothesis in the September 1 issue of the Astrophysical Journal Letter and the article was highlighted by the editors of Science magazine as particularly noteworthy. The two researchers currently are preparing to publish additional evidence in support of their hypothesis. The dense disks of dust and gas surrounding classical T Tauri stars are easily visible because dust glows brightly in the infrared region of the spectrum. Although infrared light is invisible to the naked eye, it is readily detectable with specially equipped telescopes. The second group of T Tauri stars that are somewhat older - between three to six billion years - and show no evidence of disks have been labeled as "naked" or "weak line" T Tauri stars.
Because there is no visible evidence that naked T Tauri stars possess protoplanetary disks. So astronomers have concluded that the material must have been absorbed by the star or blown out into interplanetary space or pulled away by the gravitational attraction of a nearby star in the first few million years. According to current theories, it takes about 10 million years to form a Jupiter-type planet and even longer to form a planet like Earth. If the models are correct and if most Sun-like stars loose their protoplanetary disks in the T Tauri stage, then very few stars like the Sun are likely to possess planetary systems.
This picture doesn't sit well with Weintraub, however. "Approaching it from a planetary evolution point of view, I have not been comfortable with some of the underlying assumptions," he says.
Current models do not take the evolution of protoplanetary disks into account. Over time, the disk material should begin agglomerating into solid objects called planetesimals. As the planetesimals grow, an increasing amount of the mass in the disk becomes trapped inside these solid objects where it cannot emit light directly into space. The constituents of the disk that astronomers knew how to detect - small grains of dust and carbon monoxide molecules - should quickly disappear during the first steps of planet building.
"Rather than the disk material dissipating," says Bary, "It may simply become invisible to our instruments." So Weintraub and Bary began searching for ways to determine if such "invisible protoplanetary disks" actually exist.
They decided that their best bet was to search for evidence of molecular hydrogen, the main constituent of the protoplanetary disk, which should persist much longer than the dust grains and carbon monoxide. Unfortunately, molecular hydrogen is notoriously difficult to stimulate into emitting light: It must be heated to a fairly high temperature before it will give off infrared light.
The fact that T Tauri stars are also strong X-ray sources gave them an idea. Perhaps the X-rays coming from the star could act as an energy source capable of stimulating the molecular hydrogen. To produce enough light to be seen from earth, however, the molecular hydrogen could not b mixed with dust and had to be at an adequate density. Studying various theories of planet formation, they determined that the proper conditions should hold in a "flare region" near the outer edge of the protoplanetary disk.
The next step was to get observation time on a big telescope to put their out-of-the-mainstream theory to the test. After repeated rejections, they were finally allocated viewing time on the four-meter telescope at the National Optical Astronomical Observatory in Kitt Peak, Arizona. When they finally took control of the telescope and pointed it toward one of their prime targets - a naked, apparently diskless T Tauri star named DoAr21 - they found the faint signal for which they were searching.
"We found evidence for hydrogen molecules where no hydrogen molecules were thought to exist," says Weintraub. When Bary calculated the amount of hydrogen involved in producing this signal, however, he came up with about a billionth of the mass of the Sun, not even enough to make the Moon. As they argued in their Astrophysical Journal Letter article, they believe that they have detected only the proverbial tip of the iceberg, since most of the hydrogen gas will not radiate in the infrared. But the calculation raises the question of whether the molecular hydrogen that they detected is part of a complete protoplanetary disk or just its shadowy remains. Although they do not completely answer the question, additional observations that the two are readying for publication provides additional support for their contention that DoAr21 contains a sizeable but invisible disk.The new observations are the detection of the same molecular hydrogen emission line around three classical T Tauri stars with visible protoplanetary disks. The strength of the hydrogen emission lines in the three is comparable to that measured at DoAr21. In addition, they have calculated the ratio between the mass of hydrogen molecules that are producing the infrared emissions and the mass of the entire disk in the three systems. For all three they calculate that the ratio is about one in 100 million.
"If the ratio between the amount of hydrogen emitting in the infrared and the total amount of hydrogen in the disk is about the same in the two types of T Tauri stars, which is not an unreasonable assumption, this suggests the naked T Tauri star has a sizable but hard-to-detect disk," says Bary.
Weintraub and Bary admit that they have more work to do to in order to convince their colleagues to adopt their theory. They have been allocated time on a larger telescope, the eight-meter Gemini South in Chile and plan to survey 50 more naked T Tauri stars to see how many of them produce the same molecular hydrogen emissions. If a large number of them do, it will indicate that they have discovered a general mechanism involved in the planetary formation process. They also intend to search for a second, fainter hydrogen emission line. If they find it, it will provide additional insights into the excitation process.
Currently, the number of naked T Tauri stars that have been discovered is much greater than the number of known classical T Tauri stars. If Weintraub and Bary are proven right, however, and a significant percentage of the naked T Tauri stars develop planetary systems, it means that solar systems similar to our own are a common sight in the universe.
PPARK Press Release, April 4, 2002 (PPARK is the UK equivalent of NSF)
It is not only teenagers who like to congregate in intimate groups and disturb their neighbours and surroundings.
As Matthew Bate (University of Exeter), will be explaining to the UK National Astronomy Meeting in Bristol on Friday 12 April, young stars also like to hang around in crowds and undergo chaotic close encounters with each other during their formative years.
After performing one of the largest and most complex simulations of star formation to date, Matthew Bate, Ian Bonnell (University of St Andrews) and Volker Bromm (Harvard-Smithsonian Center for Astrophysics) have found that these cosmic furnaces form in a much more chaotic manner than is generally believed.
To perform the calculation, the astronomers used the supercomputer at the United Kingdom Astrophysical Fluid Facility (UKAFF), a national computing facility for astronomy sited at the University of Leicester. The calculation was so enormous that it required 100,000 hours, roughly 10% of the time available on the supercomputer during 2001.
The simulation followed the collapse of an interstellar gas cloud which was over one light year across and 50 times the mass of the Sun, eventually resulting in the formation of a cluster of 50 stars and brown dwarfs.
One of the big surprises found by the astronomers was how chaotic and dynamic the process of star formation is. The results showed that stars form so close together that they often interact with each other well before they have grown to full size.
In the small, new-born stellar groups, the stars compete with each other for the remaining gas. This process is inherently unfair, with the more massive stars tending to gather more gas than the lower mass stars, while the lowest mass stars are kicked out of the group.
About half of the objects are ejected so quickly that they don't manage to gather enough gas to become stars at all. Rather, they become brown dwarfs, objects with less than 1/13 the mass of the Sun. Unable to generate energy by fusing hydrogen into helium, they cannot continue to shine like the Sun and quickly fade away.
The new calculation supports recent astronomical surveys suggesting that there may be as many brown dwarfs as stars in our Galaxy, and indicates that the high frequency of brown dwarfs is a natural consequence of the competition between stars during their formation.
Another surprise is that many of the encounters between the stars and brown dwarfs in such clusters are close enough to strip off the outer parts of the dusty discs surrounding the young stars. Although many of the discs are initially very large, by the end of the calculation the majority of them have been truncated to less than the size of our Solar System.
Since most stars are believed to form in large star clusters, this suggests that planetary systems like our own may be the exception rather than the rule.
A paper discussing the first analysis of the simulation has been accepted for publication in the Monthly Notices of the Royal Astronomical Society.
ANIMATIONS AND STILL IMAGES (BOTH HIGH AND LOW RESOLUTION) ARE AVAILABLE ON
THE WEB AT:
A major accident in the water tank of Super-Kamiokande, the Japanese neutrino experiment, destroyed 7000 or their 11,000 photomultipliers on November 10, 2001. It is a major setback, will cost perhaps $20 million or $30 million to fix, and will knock out the experiment for a year or so.
The water tanks, containing 12.5 million gallons, were being refilled; they had been drained routinely. People heard a popping, akin to corn popping, and it was, most unfortunately, the 20-inch photomultipliers imploding one after the other. The director is quoted as saying it "surely must have had something to do with the pressure."
JPL Press Release
For the first time ever, a star spinning so fast its mid-section is stretched out has been directly measured by an ultra-high-resolution NASA telescope system on Palomar Mountain near San Diego.
"Measuring the shape of this star, Altair, was as difficult as standing in Los Angeles, looking at a hen's egg in New York, and trying to prove that it's oval-shaped and not circular," said Dr. Charles Beichman, chief scientist for astronomy and physics at NASA's Jet Propulsion Laboratory (JPL), Pasadena, CA.
Altair is a well-known member of the Summer Triangle, clearly visible in the summer night sky across the United States. Scientists using the Palomar Testbed Interferometer, which links multiple telescopes, measured the star's radius at different angles on the sky. They noticed the size of the star varied with changing angles, which was the first tip-off that Altair is not perfectly round.
"This surprising observation led to a bit of challenging detective work to properly interpret the data," said principal investigator Dr. Gerard van Belle of JPL. "We measured the size of another star, Vega, at the same time, which didn't change with angle, so we knew this wasn't just a fluke of the telescope."
Previous studies of Altair raised the prospect that the star might have midriff bulge, but never before had the shape been measured directly. Earlier measurements of the star's spectrum, or light-wave pattern, had hinted that Altair was rotating very fast. When a gaseous orb, like a star, spins fast enough, it tends to expand at the middle, like a beach ball that is squeezed at the top and bottom.
Altair is a perfect example -- it rotates at least once every 10.4 hours, and the new Palomar observations reveal the diameter at its equator is at least 14 percent greater than at its poles. For a star that spins slowly, this effect is minuscule. For example, our Sun rotates once every 30 days and has an equator only .001 percent greater in diameter than its poles.
By measuring Altair's size at separate positions along its edge, van Belle and his colleagues determined that Altair rotates at a speed of at least 210 kilometers per second (470,000 mph) at the equator. Future studies may pin down the speed more precisely.
"Determining the shape of another star helps us learn about the forces that control the shape and structure of all stars, including our star, the Sun," Beichman said. "This tells us more about the Sun's behavior and ultimate fate."
The Palomar Testbed Interferometer has three 50-centimeter (20-inch) telescopes. To study Altair, the telescopes were used two at a time. The combined light from the telescope pairs provided sharpness comparable to a telescope as large as a football field.
"Altair is the twelfth brightest star in the sky -- you'd think that everything there is to know about this star would have been discovered already," said co-investigator Dr. David Ciardi of the University of Florida, Gainesville. "It's a good example of the surprises you're going to encounter when you are able to look at even familiar stars with unprecedented resolution."
The Palomar Testbed Interferometer is paving the way for the Keck Interferometer, Space Interferometry Mission and Terrestrial Planet Finder, all part of NASA's Origins program. The program will hunt for Earthlike planets that might harbor life around other stars. "In the long run, we'll use these interferometric capabilities to search for planets around nearby stars. This is an important first step," said Beichman.
Van Belle and Ciardi co-authored the Altair paper, scheduled to appear in the October 1, 2001, issue of the Astrophysical Journal, with Robert Thompson of JPL and the University of Wyoming, Laramie; Dr. Rachel Akeson of the JPL/Caltech Infrared Processing and Analysis Center, Pasadena, CA; and Dr. Elizabeth Lada of the University of Florida, Gainesville.
Their research was funded by NASA's Office of Space Science, Washington, along with the National Science Foundation. Palomar Observatory is owned and operated by the California Institute of Technology in Pasadena, which manages JPL for NASA. The Palomar Testbed Interferometer was designed and built by a team of JPL researchers led by Drs. Mark Colavita and Michael Shao. Funded by NASA and managed by JPL, the interferometer is located at the Palomar Observatory near the historic 200-inch Hale Telescope.
Images and animation of Altair are available at:
Information on the Palomar Testbed Interferometer is available at:
Information on NASA's Origins Program is available at: