'Winking' star may be home to new solar system

Posted: June 19, 2002

A distant star with an unusual cycle of brightening and dimming may have one or more planets forming around it, astronomers announced Wednesday.

Detailed observations of the star KH 15D, carried out over the last year by an international network of observatories, revealed a pattern of brightness changes that astronomers have interpreted to be created by one or more objects churning up material in a protoplanetary disk around the star.

Out In
Images of KH 15D out of eclipse (left) and in eclipse (right). Photos: Wesleyan University
"Essentially, the star winks at us," said William Herbst, the Wesleyan University astronomer who, with graduate student Catrina Hamilton, led the studies of the star.

KH 15D was discovered by Herbst and another student, Kristin Kearns, in 1997, and was the subject of studies by Wesleyan students using the universityís 0.6-meter (24-inch) telescope. Those observations showed an unusual pattern: every 48.3 days the star fades to 4% of its brightness and remains dim for 18 days before returning to its normal brightness.

While variable stars and eclipsing binary stars are quite common, Herbst said he had seen nothing like it among the thousands of stars he and his students monitor. In an effort to obtain more data about the star and be able to monitor it 24 hours a day, he arranged an international monitoring effort using telescopes in the US, Israel, and Uzbekistan. From September 2001 until this April those telescopes observed five cycles of the star, with good data from three of the cycles.

Analysis of those observations have allowed Herbst and Hamilton to construct models to explain the brightness changes of KH 15D. Because the star is Sun-like and young -- approximately three million years old -- they believe the star may have a protoplanetary disk of material around it. This belief is supported by observations of the star during the eclipses: the light becomes slightly bluer, which would be true if the light was being scattered by material in a disk with larger particle sizes than interstellar dust grains -- possibly as large as asteroids.

If the disk is not quite edge-on as seen from the Earth, then the regular series of brightness changes could be explained if there are isolated clumps, or waves, of material in the disk rising above the plane, occasionally blocking the star as the disk rotates around it. Those waves could only exist if there is an object or objects in the disk whose gravitational interactions keep the material from smoothing out.

What astronomers donít know is both the number and size of the objects required to maintain the clumps in the disk. Hamilton pointed out that they get a better fit to the data if they assume there are two clumps in the disk with a period of 96.7 days, rather than one with a period of 48.3 days. "It might be two blobs, it might be one; weíre not sure," she said. The object or objects creating the clumps could be anything from a planet to a low-mass star, based on the limited data.

Other astronomers are cautiously optimistic that this star may provide new insights into planetary formation. "This object may turn out to be something of a Rosetta Stone for deciphering planetary formation," said Alan Boss, an astronomer with the Carnegie Institution of Washington not involved with the work. "This is an extremely promising system."

Given the great distance of the star from the Earth -- 2400 light-years away -- and the fact that the clumps in the disk orbit the star closer than Mercury orbits the Earth, astronomers hold out little hope of directly observing that section of the disk, let along any planets that may be there, using current telescopes. Ray Jayawardhana, a University of California astronomer who has studied protoplanetary disks around other stars, said that it may be possible to detect if the star has a large protoplanetary disk, by either directly observing it or inferring its existence through excess emission of infrared or millimeter-wave radiation.

However, even the best telescopes today could not see anything going on in the interesting inner portion of the disk, which makes these indirect observations all the ore useful. 'We will not likely in our lifetime to be able to see detail like this directly,' said Herbst.

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