In the Wink of a Star
By Anna Edmonds
At about 2,400 light-years distance and located in a cluster of young stars called NGC 2264 in the winter constellation of Monoceros, the star KH 15D shines at full brightness for 48.36 days and then “winks” at astronomers (their whimsical term) 18 days. The star’s eclipse is remarkable, not in its regularity, but in its duration. A single object like another star, a planet, or a moon could not be big enough or move slowly enough to act as the intervening body. Therefore astronomers are positing a collection of smaller objects such as dust grains, rocks, and/or asteroids that could be strung together in an orbiting band or disk. Perhaps it would be like an interrupted ring around Saturn.
Mac Gardiner drew our attention to the article by John Noble Wilford in the New York Times (June 20, 2002) concerning KH 15D. A wealth of similar material can also be found on Internet. This recent spate of interest is because astronomers are debating seriously if this “young” star (it’s only 3 billion years old) and its occluding dust can lead us to discoveries about how our “middle-aged” solar system (4.5 billion years old) was formed.
In order to concentrate the studies of KH 15D, the research team led by graduate student Catrina
Hamilton and Professor William Herbst of Wesleyan University (Middletown, Conn.) organized an international observing campaign for the fall, winter, and spring of 20012002 of astronomers in Tashkent, Munich, Tautenberg, Heidelberg and Tel Aviv, plus five additional US locations. This geographic spread permitted observations around the world as fully as possible. After the five-year preliminary study at Wesleyan, the effort resulted in the announcement in June about KH 15D at a meeting of extra solar planet astronomers at Carnegie Institute in Washington, DC.
According to Hamilton and Herbst, the data gathered this past year not only confirms its basic pattern of eclipse, but also suggests that there may be more than one clump of dust. Even more amazing, they believe that in the last several months they have seen the effects of increasing clumping. Two clumps seem to have slightly different shapes, suggesting that the orbital period may be
96.72 days rather than the originally calculated 48.36 days. Besides these clumps, there is an unusual color difference: When KH 15D is faint it is bluer than when it is bright. On Earth when the light gets dim it generally turns reddish because dust particles scatter blue light better than red. What may be happening with KH 15D is that, rather than the star itself, astronomers may be observing light reflected off the solid objects of the band. Astronomers are studying the sizes of the clumps trying to find what gravitational force could be keeping the clumps organized. Another question related to the sizes is whether or not they are large enough to make the main star wobble. One possibility suggested is that the orbiting objects might cause “density wave” ripples in the band, some of which would extend high enough above the band to block the light from the star at the regular intervals.
The interaction of the orbiting band and its clumps is central to the study of extra solar planetary systems. The mass of KH 15D is too far away for astronomers to know yet how big it or its clumps are. However, they believe the clumps are closer to the star than Mercury is to our Sun. This situation is typical of many massive planets orbiting other stars that have been found since 1995. The puzzle with this proximity is how the building blocks of planets—the elements—could be condensed or trapped in the intense heat so near a star. Could the clumps have “migrated” in from the outer regions, propelled by the density waves? The astronomers acknowledge that all this needs much more study.
“In summary,” writes Professor Herbst, “it appears that nature has provided us with a unique opportunity to study the early evolution of KH 15D will evolve with time…. Time will tell---but, a disk by orienting KH 15D in just the right way to show of course, only if we keep watching.”us its clumpy disk. If we looked at any other angle, we For the winking of a star. would either see no eclipse at all, or, perhaps, a very faint star all the time. No one can predict exactly how