New infrared interferometer techniques yield unprecedented detail of dust envelopes around stars both young and old

By Robert Sanders, Public Affairs

AUSTIN, TEXAS -- Given the world's largest telescope, the Keck I, what astronomer would throw away more than 95 percent of the light it collects?

Yet that is what a team of astronomers at the University of California, Berkeley, has done in order to obtain the best interferometer in the infrared region of the spectrum, the invisible wavelengths just beyond red.

They placed a mask over the telescope that reduced the 10-meter (400-inch) diameter telescope to a set of 21 small slits. Interference between the light coming through pairs of holes can paint a detailed picture of distant stars that appear as mere points of light through the unmasked Keck.

"This is a new use for the Keck Telescope and a new technique that makes the Keck still more versatile and spectacular," says Charles H. Townes, a professor of physics in the Graduate School at UC Berkeley and a Nobel Laureate in Physics. Townes will discuss the interferometer results in his Russell Prize Lecture on Jan. 6 at 11:40 a.m. CST at the 193rd national meeting of the American Astronomical Society. The meeting is being held January 5-9 at the Austin Convention Center in Austin, Texas.

In his talk, entitled "The Quest for High Spatial Resolution," Townes also will discuss high-resolution imaging with UC Berkeley's Infrared Spatial Interferometer of stars in the thermal or mid-infrared region of the spectrum. This unique instrument was built by Townes and Senior Space Fellow William C. Danchi of UC Berkeley's Space Sciences Laboratory and is parked on the side of Mt. Wilson in Southern California.

"Both these instruments give unprecedented high resolution results in the infrared," Danchi says. "They are revolutionizing our understanding of stars that hitherto appeared to be point-like, many of which are cooler than our Sun and heavily obscured by dust, making imaging at infrared wavelengths all important."

Graduate student John D. Monnier also is presenting a paper at the meeting on Jan. 8 in which he describes use of the Keck aperture masking technique to detail the structure and composition of dust envelopes surrounding stars both young and old. The work was done by postdoctoral fellow Peter Tuthill, Monnier and Danchi.

Interferometry is a technique more regularly used in radio astronomy, where signals from separate telescopes are combined as if from one very large telescope. An example is the Very Large Array in Socorro, New Mexico, which has 27 telescopes separated by distances as large as tens of kilometers.

Interferometric techniques are much less developed at infrared and optical wavelengths, Danchi says, due in part to the very tight optical tolerances needed and the effects of the atmosphere, which causes the twinkling of stars.

The atmospheric problems can be partly eliminated, though, by combining signals from three separate telescopes to form "closure phases."

In the case of the Keck I Telescope, an aperture mask consisting of 21 slots in an aluminum plate is placed in front of the secondary mirror, blocking all but about three percent of the light. The Keck then behaves like a collection of 21 small 35-centimeter telescopes.

"This mask, designed by Tuthill, provides 210 separate interferometers all at once, yielding a very good angular resolution and a detailed picture in the infrared," Townes says.

Fringe patterns from the interference of these smaller "telescopes" are recorded with the facility's near-infrared camera (NIRC) at the focus of the Keck I telescope and reconstructed into an image using software originally developed for radio interferometry.

So far, complex images with interesting structure were observed for every type of star looked at, Danchi says. Among these are AGB stars, Wolf-Rayet stars and Young Stellar Objects or YSOs.

AGB stars are intermediate mass stars near the end of their lives, losing material to the interstellar medium through stellar winds. Over the course of time this material, along with material from supernovae, is recycled into new stars, planets and other objects.

Using the Infrared Spatial Interferometer, Danchi, Townes, and their associates discovered recently that many of these AGB stars do not lose material continuously as had been previously thought. Instead they emit clouds of gas and dust episodically.

"Some kind of episodic instability causes them to puff off material on a time scale of tens to hundreds of years," Townes says. The physical origin of the time scales is not understood and is still under investigation.

The carbon star CW Leo.

Now, using the mask technique with the Keck telescope, they have seen still more detail. Clumps of dust appear to be shooting off several stars, such as the carbon star CW Leo. (See image at left.) Over time, motion of this material can be traced to understand better the processes in the stars that caused these events.

The star MWC 349A, more massive and hotter than our own Sun.

Young stars, or YSOs, are also of great interest, Danchi says, not just those like our Sun, but more massive, hotter stars like MWC 349A. Astronomers want to understand how they form from clouds of gas and dust, how disks of material form and rotate around the young star, how planets agglomerate from material in the disks, and how the disks later disperse.

MWC 349A is hot, about 30,000 Kelvin, and about 20 solar masses. For a long time it has been believed to have a massive disk of material rotating around it. Because it is so hot, it emits photons that ionize material in the disk. When the ionized atoms recombine, they emit copious photons at radio wavelengths.

In the past, much of the detailed knowledge of this star came from radio interferometry using the VLA. The aperture masking technique provides for the first time an image of the disk itself. (See photo at left.)

"From this image we can learn about the mass of the disk, its lifetime and its orientation," Danchi says. "We can also look for evidence of changes with time and for clumpiness, which could be due to companions forming."

Besides having larger separation between telescopes, the Infrared Spatial Interferometer is uniquely different than the Keck masking technique, Townes says. It uses a laser local oscillator and heterodyne detection to easily match the signals coming from separate telescopes in much the way radio telescopes do. The UC Berkeley team is the only one using so-called "heterodyne interferometers" like this at optical or infrared wavelengths.

This type of detector allows not only high spatial resolution, but also a determination of the spectra of light coming from stellar envelopes. With this advantage, Townes, Danchi and Monnier are studying the composition of dust, primarily graphite (carbon) and silicates (silicon), and the molecules ammonia (NH3) and silane (SiH4) that form on the dust particles.

"These new interferometric techniques are telling us a lot about stellar dynamics, especially how stars vary over relatively short times like months or years," Townes says.

The UC Berkeley team involved with the Infrared Spatial Interferometer also includes senior research physicist Manfred Bester, David Hale and recent doctoral graduate Everett Lipman, as well as Monnier and Tuthill. Collaborative spectroscopic work looking at the dust also has been done by Dr. Thomas Geballe of the Joint Astronomical Center in Hawaii. The UC Berkeley work has been supported by the National Science Foundation and the Office of Naval Research.

Research involving aperture masking with the Keck telescope is supported by a grant from the National Science Foundation.

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