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|Buzzoni, A., Bertone, E., & Chavez, M.:|
|"Snapshot metallicity estimate of resolved stellar systems through Lick Fe5270 diagnostic",|
2009, Astrophysical Journal, 703, L127.
We outline a new method to derive a "snapshot" metallicity estimate of stellar systems (providing one resolves at least the brightest part of the CMD) just on the basis of low-resolution (i.e., 6-8 Å FWHM) spectroscopy of a small stellar sample. Our method relies on the Fe5270 Lick index measurements and takes advantage of the special behavior of this spectral feature, that reaches its maximum strength among the ubiquitous component of K-type giants. This makes the Fe5270(max) estimate a robust and model-independent tracer of cluster [Fe/H], being particularly insensitive to the age of the stellar population. A comparison of the Fe5270(max) distribution derived from globular and open clusters, as well as from the field giant population in the Galaxy disk, confirms a tight correlation of the index maximum vs. cluster [Fe/H] allover the entire metallicity range for stellar population with [Fe/H] ≥ −2.0. Relying on a theoretical calibration of the feature, we trust to effectively infer cluster metallicity within a typical uncertainty of 0.1-0.2 dex, depending on RGB luminosity sampling of the observations. A handful of stars (5-10 objects) is required for the method to be applied, with low-metallicity stellar populations more easily managed, being Fe5270(max) located within the few brightest RGB stars of the system. In any case, we show that even the observation of a coarse stellar set would allow us to place a confident lower limit to cluster metallicity.
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|Figure 1 - |
The Fe5270 index as a function of Teff from BLUERED synthetic stellar spectra of model atmospheres with log g = 3.0 dex and solar metallicity (dots). Square markers trace the raw Fe5270 index trend by neglecting the TiO molecular opacity.
|Figure 2 - |
Fitting-function predictions for Fe5270(max) vs. [Fe/H] relationship according to Buzzoni et al. (1994) (B94, see eq.(1)), Worthey et al. (1994) (W94, eq.(3)) and Gorgas et al. (1993) (G93, eq.(4)). Two relevant cases for stellar gravity are considered, namely log g = 3 and 1 dex. The theoretical values from BLUERED synthetic spectra are also overplotted (dashed line).
|Figure 3 - |
The expected Fe5270 Lick index strength in SSPs for an illustrative set of theoretical CMDs, from Buzzoni (1989, 1995a) models. Left panel explores the index change vs. [Fe/H] (as labeled on the plots) for a set of 15 Gyr populations, while right panel takes into account the index variation for fixed values of metallicity with changing SSP age (from 15 and 2 Gyr). The width of the isochrones is proportional to the local index value. Small square markers locate the maximum value, throughout.
|Figure 4 - |
The observed distribution of Fe5270(max) from the star cluster sample of Gorgas et al. (1993). Big dots mark the strongest value of Fe5270 among the sampled giant stars in each open and globular cluster, as summarized in Table 1. Only bona fide cluster members are considered, according to Gorgas et al. (1993) classification. The two field giants samples studied by Buzzoni et al. (1994) and Buzzoni et al. (2001) (small triangles and squares, respectively) are also overplotted, considering only giant stars (i.e., 1 ≤ log g ≤ 3) with confident fundamental parameters. Observations are compared with stellar fitting functions for fixed gravity, i.e. log g = 3 and 1 as labeled, according to eq.(1). Also theoretical predictions from stellar population synthesis are reported, according to Buzzoni (1989, 1995a) SSP models (thick vertical bars); an age t = 12.5 Gyr is adopted throughout, with the additional cases of 5 Gyr populations for the metal-rich ([Fe/H] ≥ −0.5) regime more pertinent to Galactic open clusters (left "column" at each relevant metallicity).
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