Townes' team challenges conventional wisdom

A research group at the University of California-Berkeley, led by 85-year-old Nobel Laureate Charles Townes, is using laser-based heterodyne interferometry to challenge conventional wisdom about the size of red giant stars. Graduate student Jonathon Weiner presented the group's most recent work in a June meeting of the American Astronomical Society and said that the new size measurements are about 10% to 20% higher than previous estimates.¹

"This might cause theorists to make some changes to their models, which could give rise to whole new types of physics in our understanding of what's going on in the stars," he said. Weiner used the word "might" because the implications and significance of the Berkeley observations are still a matter of debate. For the last 10 years or so, interferometric and other measurements of stellar diameters have indicated that the actual sizes may have been a bit larger that what theorists have believed.

The Berkeley observations have yielded the largest differences to date because of the unprecedented observational power of the technique, Weiner believes, and may lead to new understandings of the inner workings of stars as they undergo life cycle changes. One former team member described the technique as "mixing starlight and laser light."

What the Townes team has essentially done is use laser technology to combine the power of mid-infrared optics for seeing through dust clouds with the signal-resolving power of the heterodyne interferometry that is commonly used in radio astronomy. They dubbed the resulting device an infrared spatial interferometer (ISI).

The ISI consists of two 65-in. telescopes mounted on trailers that allow them to be moved to different relative separations and orientations (see figure). Originally built about 10 years ago, the ISI relies on the basic interferometric technique of separating two telescopes to yield an observational effect analogous to having one telescope mirror as large as the separation. "If you put your telescopes 10 m apart, then that's basically as good as a 10-m telescope dish, which is about the size of the largest telescope in the world," Weiner said.

Originally the ISI telescopes were placed 10 or 20 m apart to yield a resolving power comparable to the size of the dust shells around dying red giant stars, which are two to three orders-of-magnitude larger than the sun and project dust clouds out into space with diameters that exceed the girth of the stellar disc by another factor of two or three. While the distance between the telescopes allowed accurate resolution of the size of these distant stellar objects, resolving the intensity and phase of the mid-infrared signals (ranging from 9 to 12 µm) themselves required the addition of two CO₂ lasers.

"Lightwaves in the mid-infrared oscillate at trillions of hertz, much too fast to stay put in an electronic detector for measurement as is done with radio waves," Weiner said. So the heterodyne scheme in the ISI mixes starlight from each telescope aperture with a CO₂-laser local oscillator that downconverts each signal into microwave frequencies. Path-length matching between the two signals and fringe detection can then be performed in a correlator.

"Although heterodyne methods form the backbone of almost every modern astronomical interferometer from the submillimeter through the entire radio spectrum, the ISI is unique in its use of heterodyne detection in the infrared," Townes wrote.²

In 1998, the distance between telescopes on Mt. Wilson (east of Los Angeles, CA) was increased to 60 m to resolve the size of the stellar disc normally obscured by dust clouds. Subsequent observations of red giant stars that oscillate periodically in brightness, collectively known as Mira, have indicated that the stars may actually be large enough to have a more complex internal oscillation pattern than was originally theorized. The researchers hope over the next few years that observational improvements enabled by the ISI will help them to provide theorists with more hard data.

"It would help them a lot if we could tell them exactly what the star was doing over the cycle, not only the size of the star at some given point in the cycle, but actually how it changes over the cycle," Weiner said. "For instance, some people think that the stars don't really change in diameter very much, that they only get hotter and cooler, and other theories call for a significant change in diameter."

REFERENCES

1. J. Weiner et al., Astro. J. 544, 1097 (Dec. 1, 2000).
2. D. D. S Hale et al., Astro. J. 537, 998 (Jul. 10, 2000).