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The Case of the Missing Beryllium

This cross-section shows the Sun (right) and a star that is 25 percent bigger (left). The upper layer is the zone where large convective currents occur. For both stars, the level where lithium is destroyed (the dotted line) is well below the bottom of the convection zone, and where beryllium is destroyed (solid line) is even deeper. Both are destroyed in the larger star, which indicates that the surface layers have been mixed way down to its deep layers, so convective currents cannot be the cause of this mixing. Other mechanisms, including stellar rotation and turbulence, must play a crucial mixing role. For stars like the Sun, in which lithium is destroyed while beryllium is unaffected, the mixing is not as deep, but again convection must be augmented by additional mix-master mechanisms. The temperatures of the various levels are indicated in Kelvin degrees (K).

Prof. Ann M. Boesgaard has been studying lithium and beryllium in stars for over two decades. She has found that a star's temperature and age are important factors in determining how much of these fragile elements remain in a star. But her most recent study shows that there must be at least one more factor that decides how much lithium and beryllium a star has.

Lithium and beryllium atoms are destroyed by nuclear fusion in the hot interiors of stars. Lithium atoms "burn" when the temperature is about 2 million degrees Kelvin. Beryllium atoms "burn" deeper in the stars, where the temperature is about 3 million degrees. Strong convective currents and other mixing mechanisms transport the atoms of these elements from the surface of the star to its interior, where it is so hot that they can no longer survive. The amount of these two elements remaining on the star's surface indicates how deeply the surface layers penetrate into the interior.

Prof. Boesgaard and her colleagues have found that the youngest stars still have the lithium and beryllium that they were born with, but older stars have destroyed up to 99 percent of their lithium and up to 85 percent of their beryllium.

The stars in the young Hyades star cluster in the constellation of Taurus have been studied extensively for lithium content. At 700 million years of age, the Hyades cluster has a pronounced deficiency in lithium in stars that are 25 to 40 percent more massive than the Sun. This is known as the "lithium dip."

Recently, a team led by Prof. Boesgaard used the Keck I 10-meter telescope atop Mauna Kea to investigate whether there is also a "beryllium dip." They found one that is not dramatic as the lithium dip because not as much of the surface matter circulates down to the deeper level where beryllium can be destroyed.

Our Sun (5 billion years old) has lost all but one percent of its original lithium but retains most of its beryllium. This means the surface atoms have been mixed with the material in the inside of the Sun down to the region where the temperature is 2 million degrees, but not as deep as 3 million degrees.

Since the destruction of lithium atoms takes time, younger stars have not destroyed as much lithium as the Sun, nor have they destroyed any beryllium. The team has concluded that lithium and beryllium are lost while stars are in the most stable phase of their lives, not in the tumultuous period of formation. The effects of this destruction are evident only after stars attain the age of about 200 million years.

The amount of destruction also depends partly on the mass of the stars, and here the pattern for lithium differs from the pattern for beryllium. Cooler, low-mass dwarf stars destroy lithium but not beryllium. Warmer stars that are between 25 and 40 percent more massive than the Sun destroy both. Stars over 60 percent more massive than the Sun destroy neither. This strange pattern is not predicted by any theory about the circulation of surface material to the interior.

Theorists will need to reexamine their ideas in light of this new beryllium data. The mix-master in the warmer dwarfs seems to be different from the mix-master in the cooler dwarfs. Extra mixing, induced by rotation, appears to be a possible explanation for the warmer stars. Those stars with higher initial rotation destroy more lithium and beryllium than those with slow rotation.

Prof. Boesgaard's team also includes Eric Armengaud, a French student working in Hawaii, and Jeremy King, assistant professor at the University of Nevada, Las Vegas. Prof. King studied under Prof. Boesgaard and received his Ph.D. from UH in 1993.