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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. | |||
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Summary:
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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Pick up the paper at Astro-ph/0909.3102 | |
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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.
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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).
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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.
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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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