The Case of the Missing Beryllium

University of Hawaiʻi
Dr. Ann M. Boesgaard, (808) 956-8756
Institute for Astronomy
Karen Rehbock, (808) 956-6829
Institute for Astronomy
Posted: Jun 3, 2002

Astronomers have used the world's largest telescope to gain new insights into the hidden cauldron that lies beneath the foggy surface of stars. A team led by Professor Ann M. Boesgaard of the University of Hawaii Institute for Astronomy has found large deficits of lithium and beryllium. These two light elements act as probes into the deeper reaches of stars because they are so fragile.

The team 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. Theoretical models of stars cannot account for this wholesale destructive behavior.

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

The stars in the young Hyades star cluster in the constellation of Taurus, the Bull, have been studied extensively for the 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."

The new observations investigated the beryllium content of these stars. Is there a "beryllium dip"? These are far more challenging observations to make, and so the team used the Keck I 10-meter telescope atop Mauna Kea in Hawaii for its research. The twin Keck telescopes are the world's largest optical telescopes.

The team, which also includes Eric Armengaud, a French student working in Hawaii, and Jeremy King, assistant professor at the University of Nevada, Las Vegas, looked for—and found—a beryllium dip in the Hyades and other young clusters. The beryllium dip 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.

The temperature of the surface of our Sun is about 6,000 K, and its core is about 15 million K. Measurements show that the Sun 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 K, but not as deep as 3 million K. Studying both elements lets us see the degree of mixing between the surface and the interior of a star.

The destruction of lithium atoms takes time. Stars younger than the Sun have not destroyed as much lithium as the Sun. Like the Sun, they have not destroyed any beryllium.

The amount of destruction also depends on the mass of the stars and here the pattern for lithium differs from the pattern for beryllium. For the cooler, low-mass dwarf stars there are huge lithium deficiencies, but beryllium is unscathed. For the warmer stars that are between 25 and 40 percent more massive than the sun, both lithium and beryllium are destroyed. For stars in the middle of that range there is no lithium to be found at all, while beryllium is less affected, but is deficient. Stars that are more than 60 percent more massive than the sun have the full complement of both lithium and beryllium.

This strange pattern is not predicted by any theory about the circulation of surface material to the interior.

The new research shows that the beryllium dip is present in the intermediate-age clusters, Hyades, Coma, and Ursa Majoris. It mimics the lithium dip, but it is not as deep. For the Pleiades and Alpha Per clusters, which are one-tenth the age of the Hyades, there is no beryllium dip and only a minor lithium dip. From this, the team concludes that lithium and beryllium are "burned" while stars are in the most stable phase of their lives, not in the tumultuous period of formation. The effects of the "burning" are evident only after stars attain the age of about 200 million years.

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.

This study was funded by the National Science Foundation.

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

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