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Astron. Astrophys. 321, 859-866 (1997)

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Infrared stellar absorption lines around 1.6µm: a new metallicity scale for old stellar populations *

L. Origlia 1, F.R. Ferraro 2, F. Fusi Pecci 2 and E. Oliva 3

1 Osservatorio Astronomico di Torino Strada Osservatorio 20, I-10125 Pino Torinese, Italy
2 Osservatorio Astronomico di Bologna Via Zamboni 33, I-40126 Bologna, Italy
3 Osservatorio Astrofisico di Arcetri, Largo E.Fermi 5, I-50125 Firenze, Italy

Received 4 June 1996 / Accepted 5 November 1996


A new metallicity scale (accurate to within [FORMULA] 0.3 dex) for old stellar systems is derived and discussed. It is particularly useful in cool objects with [Fe/H] [FORMULA] -1.3 and is based on medium resolution ([FORMULA]) near infrared spectra of a deep and relatively isolated feature at 1.62 µm 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 [FORMULA] (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 (1985) for all but two objects, namely NGC6624 and Terzan 5, which we find much less ([FORMULA] 0.9 dex) metallic (first two columns of Table 1 and Fig. 5). Ellipticals are found to have [Fe/H] [FORMULA] -0.4 and [FORMULA] 0.5 dex below estimates based on the Mg2 index, this indicates that Mg may be overabundant by a similar amount. A [FORMULA] 0.5 dex enhancement of Si (another [FORMULA] element) is also suggested by the strength of the 1.59 µm line in our spectra. The only object with [Fe/H] [FORMULA] 0.0 is the globular cluster Terzan 1.

Key words: stars: late type – stars: population II – globular clusters: general – infrared: stars – stars: abundances

* Based on observations collected at the European Southern Observatory, La Silla, Chile

Send offprint requests to: E. Oliva

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© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998