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Science Briefing

Sasha Madronich. (Photo by Bob Bumpas.)
At a remote spot in New Zealand, scientists will be intensively monitoring ultraviolet radiation for one of the world's first in-depth, model-to-measurement intercomparisons. Sasha Madronich (ACD) is heading to the town of Lauder to measure UV and run models this spring in collaboration with Richard McKenzie of the New Zealand National Institute of Water and Atmospheric Research (NIWAR).

Sasha's model, which predicts UV levels reaching the ground based on conditions through the atmosphere, has been used internationally for such tasks as gauging the impact of stratospheric ozone reduction or urban smog. However, a lack of reliable UV monitoring has hindered the model's validation.

One of the main tasks at the Lauder site will be to see how well sky conditions work as a predictor of incoming UV. Sasha's model will be run in near-real time to compare its results with measurements of direct solar UV as well as diffuse sky radiation and reflections from the ground. Also on the agenda is an evaluation of broad-band radiometers used for photodissociation measurements and a check of calibrations for spectral radiometers.

Sasha hopes the visit will help build a framework for continuing collaboration betwen NCAR and NIWAR. "UV radiation is the driving force of atmospheric photochemistry, as well as an important environmental stress in the biosphere. The historical record of UV observations is disappointing, but recent improvements have been made in both modeling and experimental techniques. The New Zealand studies should provide us with a sense of the accuracy and limitations of both models and measurements. Hopefully they will increase our confidence in evaluating current and future UV changes due to stratospheric ozone depletion, local pollution, and other atmospheric changes."

Ted Kennelly. (Photo by Bob Bumpas.)
The Sunlike star, 51 Pegasi, that yielded one recently discovered planet is unlikely to yield another--at least not one as massive as three Jupiters. That conclusion comes from data collected and recently analyzed by HAO scientists Ted Kennelly and Tim Brown, along with several non-NCAR colleagues. "AFOE Observations of 51 Pegasi: Evidence for Planets?" was presented at the winter meeting of the American Astronomical Society in San Antonio, Texas, last month.

The scientists base their findings on observations of variations over time in 51 Pegasi's radial velocity (the star's speed relative to our solar system). They used the advanced fiber-optic Echelle spectrograph (AFOE) built by HAO and the Harvard-Smithsonian Center for Astrophysics and located at the Whipple Observatory at Mt. Hopkins, Arizona.

When a star has one or more planets orbiting around it, the planets induce a compensating motion in the star so that the bodies are all orbiting around a single center of mass, normally located very close to the star. The resulting stellar wobble can be detected to a long-term precision of 20-30 meters per second using the AFOE.

Tim Brown. (Photo by Bob Bumpas.)
Beginning in 1993, the AFOE team regularly observed a set of stars that included 51 Pegasi. About 45 light-years from the Earth, "It is a nearby star as these things go," says Tim, "and it is a very close analog to the Sun in size and age." Many of the 51 Pegasi observations were analyzed by Carrie Rowland, an undergraduate at Stetson University visiting on an HAO fellowship.

In October, Tim and Ted learned of a planet discovered near 51 Pegasi by Michael Mayor and Didier Queloz (Geneva Observatory, Switzerland). The Mayor-Queloz data implied a planet half the mass of Jupiter that took only four days to orbit 51 Pegasi, at a distance of 5 million miles (only 5% of the Earth-Sun distance). Skeptical yet intrigued, Ted and Tim stepped up their observations of 51 Pegasi to twice nightly. The data soon convinced them of the new planet's presence, making them the second group to confirm the results of Mayor and Queloz after a team from San Francisco State University. "The trail of planet detection is littered with corpses, so the fact that three independent groups have confirmed this planet's existence is encouraging," says Tim.

Because the AFOE data on 51 Pegasi extend back to 1993--further back than the Swiss data--the AFOE team was able to analyze nearly 100 observations to separate out the effects of the newly discovered planet and see if other, longer-term signals were present from a second planet. Based on their analyses to date, they conclude that there is probably not an object larger than about three Jupiter masses closer to 51 Pegasi than two astronomical units, or roughly the distance between the Sun and Mars. Continued analysis will narrow the range of uncertainty, says Tim.

A data summary can be found on the World Wide Web at http://www-sgk.harvard.edu:1080/~sylvain/51Peg.html


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