Boron Abundances and Internal Mixing in Stars. I. The Hyades Giants

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Abstract
The boron abundances of two giants and one turnoff star in the Hyades, and the field K0 III giant β Gem, have been measured from spectra obtained with the Hubble Space Telescope in the vicinity of the B I lines near 2500 Å. This provides a test of the dilution of giant envelopes predicted by standard stellar evolutionary models: since B is destroyed by (p, α) reactions at a temperature near 5 × 106 K, it is expected to be preserved only in an outer shell on the main sequence and diluted by B-free interior material on the giant branch. We find that the B abundance in each giant is reduced below solar by the factor of 10 predicted by standard stellar evolution models. Li, which is more fragile than B, is reduced in the same stars by a factor of 160; the prediction is 60. However, when non-LTE effects are accounted for, the observed Li dilution factor becomes 80, equaling the predicted value within measurement error and resolving a long-standing discrepancy. No unusual mass loss or nonstandard internal mixing is required to match the observations.

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