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Index of CfA Almanac stories.

The CfA Almanac

June 1998

PLASMA PASTA--Like wisps of glowing angel hair, thin ropes of broiling plasma shoot out of magnetic hot spots on the surface of the Sun and loop through the lower atmosphere before crashing back onto the surface. This high-resolution extreme- ultraviolet image is typical of the extraordinary fine- scale features seen in the Sun's inner corona by the TRACE spacecraft. (Image from Leon Golub and the SAO TRACE team)


TRACE PRODUCES NEW IMAGES--AND UNDERSTANDING-- OF SUN'S CORONA

"I think they may tell theorists that all their models of coronal heating could be wrong," says the CfA's Leon Golub, speaking of the flood of remarkable high-resolution images of the Sun's inner atmosphere returned by his experiment aboard the TRACE spacecraft. "We are watching things develop and change on time scales that just don't correspond to anything we have ever seen before."

NASA's Transition Region and Coronal Explorer (TRACE) mission, was launched exactly on schedule, at 18:40 PST April 1, aboard a Pegasus-XL rocket dropped from the belly of huge L1011 jet aircraft from Vandenberg Air Force Base, on a mission to improve the understanding of events in the narrow "transition region" of the Sun's atmosphere, between the relatively cool surface where temperatures are about 3,000 degrees Celsius, and the extremely hot upper atmosphere, or corona, where temperatures may reach 8 million degrees Celsius.

Using instruments sensitive to extreme-ultraviolet wavelengths of light, TRACE is studying the detailed connections between surface features and the overlying, changing atmospheric layers of hot, ionized gas, called plasma. The surface features and atmospheric structures are linked by fine-scale solar magnetic fields.

TRACE's telescope--incorporating unusual "normal incidence" mirrors developed at SAO--is unique in its ability to resolve these fine details and to combine rapidly taken images into mini-movies of turbulent activity in the seething plasma just above the solar surface.

"The TRACE telescope has roughly ten times the temporal resolution and five times the spatial resolution of previously launched solar spacecraft," says Alan Title of the Lockheed Martin Advanced Technology Center, TRACE principal investigator.


SOLAR INFERNO--Seen in detail in x-rays by TRACE, the turbulent transition zone between solar surface and lower corona looks more a region of Hell than the heavens.
In fact, the TRACE instrument is really four telescopes in one. Its 30-centimeter (12-inch) primary and 6-centimeter (2-inch) secondary super-polished mirrors are individually coated in four distinct quadrants to allow light from different bandwidths (colors) to be reflected and analyzed. An electronic detector collects images over a 370,000-square-kilometer field of view, or nearly 25 percent of the Sun's disk.

In addition to the optics, other key SAO contributions to the TRACE mission include the primary mirror

mount and the pre-filter assembly that provides for both thermal control and visible-light blocking. A science team at SAO has also contributed to the overall mission development and operations planning, and is deeply involved in data analysis.

"Sometimes it takes months, even years, of data reduction and analysis to know if a space mission has been successful," SAO's Jonathan McDowell noted as he showed a TRACE image to the crowd at May's Observatory Night. "And sometimes you just have to see one photograph!" McDowell's assessment is shared by the scientific community.

TRACE was launched into a polar orbit to enable virtually continuous observations of the Sun, uninterrupted by the Earth's shadow, for months at a time. This orbit gives the mission the greatest chance of observing the random processes which lead to flares and massive eruptions in the Sun's atmosphere. TRACE also joins a fleet of spacecraft studying the Sun during a critical period when solar activity is beginning its rise to a peak after the current quiescent phase of its 11-year cycle. The coming months should provide solar scientists with periods of strong solar activity interspersed with periods when the Sun is relatively passive and quiet.

TRACE, which cost $49 million, is the third in NASA's Small Explorer series of small, quickly developed, relatively low-cost missions. (The CfA's SWAS is another in that series and also will be launched by the same Pegasus-jet combo in January 1999.) It is also the first space science mission with an open data policy. All data obtained by TRACE will be available to other scientists, students, and the general public shortly after the information becomes available to the primary science team.

The team at SAO that that helped make TRACE successful, includes, in addition to Golub, Bob Bond, Jay Bookbinder, David Caldwell, Peter Cheimets, Kathy Daigle, William Davis, Edward DeLuca, Leslie Frazier, Jim Grenier, Larry Knowles, Mike Maresco, Warren Martell, Rebecca McMullen, Dale Noll, and Janice Wilson. Of course, as Jay Bookbinder points out, "The full listing would be quite long, since lots of people made significant contributions to TRACE--engineers, QA support folks, fiscal, and administrators--it was truly a team effort!"


CfA ASTRONOMERS DISCOVER POSSIBLE PLANET-FORMING DISK

Two teams of astronomers searching for signs of other solar systems with telescopes in Chile and Hawaii have independently discovered a disk around a nearby star that may be forming--or may have already formed--planets.

The disk of dust, about three times the diameter of Pluto's orbit around the Sun, surrounds a star roughly 220 light-years from Earth, which, according to some theories, has the right age to be forming planets now. Like the majority of stars in our galaxy, this object is a member of a binary system, suggesting that the presence of a companion star does not necessarily disrupt a disk before it has had enough time to form planets.

The joint discovery was made by one team comprised of Ray Jayawardhana, Lee Hartmann, and Giovanni Fazio of the CfA and Scott Fisher, Charles Telesco, and Robert Pina of the University of Florida in Gainesville, which used the National Science Foundation's 4-meter Blanco Telescope at the Cerro Tololo InterAmerican Observatory in La Serena, Chile; and, by a second team comprised of David Koerner of the University of Pennsylvania, Michael Ressler and Michael Werner of CalTech/Jet Propulsion Laboratory, and Dana Backman of Franklin & Marshall College, which used the 10-meter Keck II Telescope on Mauna Kea, Hawaii.

The newly-discovered disk surrounds a star known to astronomers as HR4796A in the southern constellation Centaurus. HR4796A is roughly 20 times more luminous than the Sun and a few times more massive. It is separated from its faint companion, dubbed HR4796B, by about 500 Astronomical Units. (An Astronomical Unit, or AU, is the average distance between the Sun and the Earth, roughly 150,000,000 kilometers, or 93,000,000 miles.) The disk itself appears to be roughly 250 AU across, and is seen nearly edge-on in images taken at wavelengths in the mid-infrared part of the spectrum. "What's most exciting is that we are looking at a disk just at the time it is forming planets or has recently done so," says Jayawardhana, a Harvard graduate student, who found the disk while making observations for his PhD thesis.

PLANETARY WOMB--A false color image of the disk around star HR4796A. The position of the star A and its companion B are indicated by crosses. The disk is seen at the mid-infrared wavelength of 19.2 micrometers. The emission arises from small solid particles, resembling dust, that are heated by star A's visible and ultraviolet light. The elongated shape of the emission indicates that the disk is seen nearly edge-on. In addition, the disk appears to lie in the orbital plane of the binary star system, since the emission is nearly parallel to the imaginary line connecting A and B. The dust may be in the process of clumping together in the early stages of planet formation. (University of Florida/CfA/NOAO)

This disk may be a close cousin of the disk around the star Beta Pictoris that astronomers have known about for 14 years. Beta Pictoris is estimated to be about 200 million years old whereas the HR4796 pair is only about 10 million years in age. "Unlike the case of Beta Pictoris, we know the age of this star pretty well, and it seems perfect for planets to be forming in its disk," Jayawardhana explains.

"From previous work by Michael Jura at UCLA and colleagues, it was known that the primary star was surrounded by a dust cloud with a hole in it," according to Hartmann. "Our images show that the cloud is indeed a disk, so that the hole could be cleared out by the gravity of one or more inner planets."

Hartmann, author of a soon-to-be-released textbook on star formation, adds: "This discovery could also tell us about how binary companions affect disks. Perhaps this disk is truncated on the outside at a radius of about 125 AU because of the companion star's gravity. A true test of this idea will require better measurements of the companion's orbit."

The HR4796A disk was first seen in images taken at 20 micrometers, a wavelength in the mid- infrared part of the spectrum. (20 micrometers is about 40 times the wavelength of visible light.) The images were obtained with a state-of-the-art mid-infrared array camera, known as OSCIR, built at the University of Florida.

"The new generation of mid-infrared detectors is what made this discovery possible," says Telesco, who built the camera. Advanced detector chips, like the one used in OSCIR, are a recent innovation that has made it possible for astronomers to make observations in this relatively unexplored part of the spectrum. "These sensitive cameras, when used on the world's best telescopes, will lead to an explosion of new results as exciting as the HR4796A disk," Telesco predicts.

The research of both teams was supported in large part by the NASA Origins Program, with additional support to the CfA/Florida team from NSF, NOAO, and the Smithsonian Institution.


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