Beryllium in the Hyades F and G dwarfs from Keck HIRES spectra.
BOESGAARD A.M. and KING J.R.
Abstract (from CDS):
Although there are extensive observations of Li in field stars of all types and in both open and (recently) globular cluster stars, there are relatively few observations of Be. Because Be is not destroyed as easily as Li, the abundances of Li and Be together can tell us more about the internal physical processes in stars than either element can alone. We have obtained high-resolution (45,000) and high signal-to-noise ratio (typically 90 per pixel) spectra of the Be II resonance lines in 34 Hyades F and G dwarfs with the Keck I telescope and HIRES. In addition we took a spectrum of the daytime sky to use as a surrogate for the solar spectrum so we could determine the value for Be in the Sun, analyzed in the same manner as that for the stars. We have adopted stellar temperatures and some Li abundances for these stars from the literature. For most of the F dwarfs we have rederived Li abundances. The Be abundances have been derived with the spectrum synthesis method. We find that Be is depleted, but detected, in the Li gap in the F stars reaching down to values of A(Be)=0.60 dex, or a factor of nearly 7 below the meteoritic Be abundance (a factor of 3.5 below the solar value of Chmielewski et al.). There is little or no depletion of Be in stars cooler than 6000 K, in spite of the large depletions (0.5-2.5 dex) in Li. The mean value of A(Be) for the 10 coolest stars is 1.33±0.06, not far from the meteoritic value of 1.42. The pattern in the Be abundances–a Be dip and undepleted Be in the cool stars–is well matched by the predictions of slow mixing due to stellar rotation. We have interpolated the calculations of Deliyannis and Pinsonneault for Be depletion due to rotational mixing to the age of the Hyades; we find excellent agreement of the predictions with the observed Be abundances but less good agreement with the observed Li abundances. Some of our Hyades stars have photometrically determined rotation periods, but there is no relation between Be and rotation period. (Generally, the lower mass stars have less Li and longer periods, which may indicate greater spin-down and thus more Li depletion relative to Be.) The Li and Be abundances are correlated for stars in the temperature range of 5850-6680 K, similar to results from earlier work on Li and Be in F and G field stars. This indicates that the depletions are not just correlated–the only claim that can be made for the field stars–but are probably occurring together during main-sequence evolution. The Hyades G dwarfs have more Be than the Sun; their initial Be may have been larger or they may not be old enough to have depleted much Be. For those Hyades stars that seem to have little or no depletion of Li or Be, the Li/Be ratio is found to be 75±30; the meteoritic ratio Li/Be is 78. The Hyades ratio is a representative value for the initial ratio in the material out of which the Hyades cluster was formed.
open clusters and associations: individual (Hyades) - Stars: Abundances - Stars: Interiors - Stars: Late-Type