Probing a lithium vanishing act.

Probing a lithium vanishing act 

One of the few ways to probe beneath a star's surface is to look for traces of the element lithium in the spectrum of visible light emitted by the star. By working out the amount of lithium present at a star's surface, researchers indirectly obtain information about circulation patterns within the star itself. Recent meaasurements by atronomer Ann M. Boesgaard, a visiting professor at the California Institute of Technology in Pasadena, show dramatic variations in lithium content that appear to depend on a star's mass and age.

Using sophisticated detectors, Boesgaard and her students determined the lithium content of stars in several clusters, including the Hyades and the Pleiades, both found in the constellation Taurus. Stars in young clusters such as the Pleiades (70 million years old) generally show a normal lithium level -- about 1 lithium atom for every billion hydrogen atoms. In contrast, certain stars in older clusters such as the Hyades (800 million years old) show lithium amounts as low as 1 percent of the normal value. Such deficiencies tend to occur in stars 10 to 40 percent more massive than the sun and whose surface temperatures range between 6,800 and 7,1000[deg.]C.

"This was a totally unexpected finding," says Boesgaard. The sharp drop in lithium levels may be caused by flows that carry lithium atoms from the surface deep into the star. When the lithium reaches depths at which the temperature is near 2.5 million[deg.]C, thermonuclear reactions destroy the element.

What drives such deep flows isn't clear yet, says Boesgaard. Researchers have

proposed three possible mechanisms. Lithium atoms, pulled by gravity, may slowly shink into the star's interior until they are consumed. Alternatively, the star may have a turbulent layer just below its convection zone to produce the necessary mixing. The phenomenon may also be connected with the rate at which star rotates. Recent observattions indicate that higher rotation rates seem to correlate with lower lithium levels. Whatever the cause, the observation that lithium deficits are not seen in young stars implies that the circulation process is slow and the effects don't show up for at least 100 million years.

Boesgaard has also looked at the levels of the element beryllium in various types of stars. In this case, even stars in the Hyades show no beryllium depletion. Because beryllium is destroyed in thermonuclear reactions occurring at 3.5 million[deg.]C, the results means that circulation patterns within a star don't go deep enough to burn up beryllium.

The abundance of lithium and beryllium at star surfaces serves as a probe of the internal structure of stars, says Boesgaard. These observations are one of the few direct checks on theoretical models of the way starswork. In addition, they provide clues about what may be happening inside the sun, which, like stars in the Hyades, has normal beryllium levels and a lithium deficiency.