1997A&A...321..859O

A new metallicity scale (accurate to within ±0.3dex) for old stellar systems is derived and discussed. It is particularly useful in cool objects with [Fe/H]>-1.3 and is based on medium resolution (R∼2000) near infrared spectra of a deep and relatively isolated feature at 1.62µm which is mostly due to the CO(6,3) second overtone band-head. The value of [Fe/H] can be determined using handy interpolation formulae (Eqs. 1a, 1b and Eq. 2) and is little influenced by anomalies of the carbon abundance, [C/Fe], since the growth of W_λ(1.62) with metallicity primarily tracks variations of stellar temperature and gravity, rather than the increase of the total carbon abundance. The method is tested using IRSPEC observations of 13 well known globular clusters where the derived [Fe/H] values are in good agreement with the results of Zinn (1985ApJ...293..424Z) for all but two objects, namely NGC 6624 and Terzan 5, which we find much less (≃0.9dex) metallic (first two columns of Table 1 and Fig. 5). Ellipticals are found to have [Fe/H]≃-0.4 and ∼0.5dex below estimates based on the Mg₂ index, this indicates that Mg may be overabundant by a similar amount. A ∼0.5dex enhancement of Si (another α element) is also suggested by the strength of the 1.59µm line in our spectra. The only object with [Fe/H]>0.0 is the globular cluster Terzan 1.