``Rewriting the Chemical History of the Galaxy: Keck Results on the Evolution
of Beryllium and Oxygen.''

by Ann Merchant Boesgaard

ABSTRACT:

The determination of the abundance of oxygen is important in our understanding
of mass-spectrum of previous generations of stars, the evolution of the
Galaxy, stellar evolution, and the age-metallicity relation.  We have measured
O in 24 unevolved stars with Keck HIRES observations of the OH lines in the
ultraviolet spectral region.  We find that O is enhanced relative to Fe with a
straight-line relation between [O/H] and [Fe/H] over three orders of
magnitude in [Fe/H] with very little scatter.  The relation between [O/Fe] and
[Fe/H] is also robustly linear and shows no sign of a break at metallicities
between -1.0 and -2.0, as has been discussed by others.  The absence of a
break in [O/Fe] vs [Fe/H] runs counter to traditional galactic evolution
models, and the interplay of supernovae Type II's and Type Ia's in the
production of O and Fe should be re-examined.

Beryllium is produced by spallation reactions, primarily by collisions between
protons and O atoms.  We have determined Be abundances for this same set of
stars and can look at the trends of Be with Fe and O.  The relationship
between Be and Fe is linear with a slope = 0.96, while the straight-line fit
between B and O has a slope of 1.45.  Traditional models with energetic cosmic
rays impacting CNO nuclei in the ambient interstellar medium are consistent
with this as long as chemical evolution effects are taken into account.  We
find no evidence of a plateau in the Be abundance, indicating the presence of
primordial Be, down to log N(Be/H) = -13.5.