... G

Based on observations obtained with the Isaac Newton and William Herschel Telescopes, and on INES data from the IUE satellite.

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... metallicity

The measurement of chemical abundance from a spectral line normally provides the number fraction (n) relative to the continuous opacity source which, in most stars, is hydrogen. The metallicity of a star may then be given as the logarithm of the ratio of the iron-to-hydrogen fraction ( $\mbox{~$n_{\rm Fe}$ }/\mbox{~$n_{\rm H}$ }$) relative to that in the Sun: $\mbox{~${\rm [Fe/H]}$ }\equiv \log ((\mbox{~$n_{\rm Fe}$ }/\mbox{~$n_{\rm H}$ }) / (\mbox{~$n_{\rm Fe}$ }/\mbox{~$n_{\rm H}$ })_\odot)$. Thus $\mbox{~${\rm [Fe/H]}$ }=-0.3$ implies one half of the solar metallicity, assuming that the relative abundances of species heavier than helium follow some cosmic norm. These conventions break down for hydrogen-deficient stars, where nucleosynthesis has replaced hydrogen with helium and other heavy elements, and continuous opacity is provided by other species. In some cases, it makes sense to cite fractional abundance by number relative to the total number of ions in the plasma (e.g. $\mbox{~$n_{\rm Fe}$ }$). Where this is done for normal stars, the logarithm is taken and an arbitrary constant c added to make the hydrogen abundance $\epsilon_{\rm H} \equiv \log \mbox{~$n_{\rm H}$ }+ c = 12.00$. This number can be used in stars where hydrogen is replaced by helium, but the constant must be modified to reflect the change in mass fractions of H, He and possibly other species. Mass fractions ($\beta$) provide a more convenient measure because they are conserved when other species transmute, but they are less often used in the literature. For this paper we adopt the convention $\mbox{~${\rm [\beta_{\rm Fe}]}$ }= \log (\beta_{\rm Fe} / \beta_{\rm Fe \odot})$ to refer to the mass fraction of iron relative to that in the Sun. Thus $\mbox{~${\rm [\beta_{\rm Fe}]}$ }=0.0$correctly implies a solar metallicity, regardless of the abundances of transmuted elements. For normal stars, $\mbox{~${\rm [\beta_{\rm Fe}]}$ }\equiv\mbox{~${\rm [Fe/H]}$ }$.

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