Two battling blue stars create unique spiral dust cloud around distant star, UC Berkeley astronomers report

By Robert Sanders, Public Affairs

BERKELEY--Anyone who appreciates the graceful beauty of a spiral galaxy or the lovely precision of a nautilus shell will marvel at a recently discovered spiral star.

Astronomers at the University of California, Berkeley, discovered the shapely star with the help of an advanced imaging system on the Keck I telescope in Hawaii.

In a letter in the April 8 issue of Nature they proudly display the picture and report a likely explanation for its spiral dust tail. They predict the tail is produced by two hot stars engaged in a courtly dance that generates cool dust and sprays it out in a spiral much like a lawn sprinkler spews out water droplets.

The star is called Wolf-Rayet 104, or WR 104, one of a class of peculiar, hot, massive, luminous stars. Wolf-Rayet stars are typically three times the size and 25 times heavier than our sun, says William C. Danchi, a senior space fellow and principal investigator at UC Berkeley's Space Sciences Laboratory.

The star is too far away - 4800 light years, or 28 million billion miles, from Earth, in the direction of the constellation Sagittarius - to obtain detail through conventional imaging techniques on any standard optical telescope, including the world's largest, the Keck. Instead, the astronomers turned the Keck into a large interferometer by masking more than 95 percent of its collecting area. This allowed them to map details of the dust around WR 104 in a feat akin to seeing details on the head of a pin at a distance of two miles, Danchi said.

"This is the first time anyone has gotten any detail of a dusty Wolf-Rayet star," he said.

This star is unique in another way, noted his colleague Peter G. Tuthill, an assistant research physicist at the UC Berkeley laboratory.

"We've discovered a lot about the binary system without ever seeing it," he said. "It's the first time we've gotten all this information - for example, the orbital period and separation of the binary stars, and the fact that the orbit is nearly circular - from an examination of the influence these two stars have on the circumstellar environment."

Wolf-Rayet stars are not only much bigger than the sun, they also are 100,000 times brighter, the same as the ratio of the brightness of the sun to the moon. At this level of brightness, the radiation field around the star takes on a life of its own. It drives off the outer atmosphere of the star by photon pressure.

"Wolf-Rayet stars are so bright that they are literally flying apart," Danchi said.

This results in a dense, high-velocity stellar wind surrounding the star, which first drew the attention of astronomers.

Wolf-Rayet 104 is particularly unusual in that it smokes like a chimney, he said. The stellar wind is extremely dusty, causing it to give off infrared or heat radiation. The dust has puzzled astronomers for two decades because the radiation from the star is so intense it should incinerate the dust as soon as it's born.

How do these snowflakes of dust survive the furnace?

The researchers concluded that WR 104 must have another star lurking in its midst, making it a binary star system. And though the companion is not quite a twin brother to the Wolf-Rayet, it is nevertheless a luminous, blue OB star with a strong stellar wind of its own.

"It would seem, from the point of view of a dust flake caught between the frying pan and the fire, that two hot stars are better than one," Tuthill said. "When the stellar wind from the OB companion meets the wind from the Wolf-Rayet star, a shock front forms, which compresses and cools the material from the stellar winds. It is in this 'cocoon,' shielded from the direct glare of the stars, that dust formation may flourish."

The spiral results from the fact that binary stars, like planets, are in constant motion. Therefore the dust nursery at the collision front between the stellar winds is carried around with the natural orbital motion of the companion OB star, doing a complete rotation once every 220 days.

The spiral shape is a consequence of material being swept radially outward from the rotating dust formation zone by the stellar wind. This is the classic "lawn sprinkler" spiral where the water is always flowing straight from the center, but as the spigot rotates, the water, when seen from above, looks like it's flowing in a spiral.

The image of WR 104 taken in April 1998 at the Keck I telescope by Danchi, Tuthill and physics department graduate student John Monnier shows the spiral to be about 200 astronomical units across (200 times the distance of the Earth from the sun), or 18 billion miles.

The image was reconstructed using the technique of aperture masking interferometry, in which most of the area of the Keck I was masked off, making it equivalent to 36 small circular regions, or holes, through which light could pass and interfere. Fringe patterns from the interference of these smaller "telescopes" are recorded by the facility's near-infrared camera (NIRC) at the focus of the Keck I and reconstructed into an image using software originally developed for radio interferometry. This technique is much less sensitive to atmospheric twinkling or seeing than direct imaging, and allowed the astronomers to achieve a resolution of several tens of milliarcseconds.

Without this technique the astronomers would have seen only a blurry shape about 10 times larger.

"This instrument gives unprecedented high resolution results in the infrared," Danchi said. "It is 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."

One finding is that stars and their dust clouds are not all uniform and symmetric.

"People have always assumed that stars were spherical or disk-shaped, because they couldn't image them," he said. "With interferometric techniques like this, we are seeing stars with unexpected shapes and more complex shapes than we expected."

The images obtained with this technique give astronomers a taste of what they'll see when large optical telescopes are linked successfully into interferometers. This is the goal at the Keck Observatories, where Keck I and II will be linked, as well as at other observatories around the country.

Similar detail also is expected from adaptive optics, where movable mirrors compensate for atmospheric blurring. Danchi and Tuthill both point out, though, that the technique used here is a much cheaper alternative.

The work reported this week in Nature was sponsored by grants from the National Science Foundation.


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