Gala: An Automatic Tool for the Abundance Analysis of Stellar Spectra

Abstract

GALA is a freely distributed Fortran code to derive automatically the atmospheric parameters (temperature, gravity, microturbulent velocity and overall metallicity) and abundances for individual species of stellar spectra using the classical method based on the equivalent widths of metallic lines. The abundances of individual spectral lines are derived by using the WIDTH9 code developed by R. L. Kurucz. GALA is designed to obtain the best model atmosphere, by optimizing temperature, surface gravity, microturbulent velocity and metallicity, after rejecting the discrepant lines. Finally, it computes accurate internal errors for each atmospheric parameter and abundance. The code permits to obtain chemical abundances and atmospheric parameters for large stellar samples in a very short time, thus making GALA an useful tool in the epoch of the multi-object spectrographs and large surveys. An extensive set of tests with both synthetic and observed spectra is performed and discussed to explore the capabilities and robustness of the code.

NGC 6362: The Least Massive Globular Cluster With Chemically Distinct Multiple Populations*

“…The chemical abundances of Fe and Na have been derived with the package GALA (Mucciarelli et al 2013b) by matching measured and theoretical equivalent widths (EWs) of a set of unblended lines. EWs and RVs have been measured with the code DAOSPEC (Stetson & Pancino 2008), automatically launched by using the package 4DAO (Mucciarelli 2013), which allows a visual inspection of all the performed Gaussian fits.…”

The GIRAFFE Inner Bulge Survey (GIBS)

“…This parametrisation value is calculated by GALA by varying the best value found around the local minimum. This method and their respective equations are described in detail in Mucciarelli et al (2013). In our sample, the resulting errors are found to be σ T eff = 245 ± 99 K and σ ξ = 0.29 ± 0.11 km s −1 .…”

“…Most likely, the observed uncertainty associated with the initial stellar parameters is not caused by GALA itself, but by the inability to optimise the entire grid of parameters simultaneously in the bulge. Were a larger number of FeI and Fe II lines available in our spectra, the final scatter on stellar parameters would converge to less than 50 K in T eff and 0.09 in log g as described in Mucciarelli et al (2013) thus decreasing the relatively large scatter in [Fe/H] observed in Fig. 3.…”

“…As pointed out in Mucciarelli et al (2013), the errors on the stellar parameters are dominated by the number of FeI lines used and their distribution in both χ ex range and transition values. For each stellar parameter, an optimisation parameter is calculated by applying a jackknife bootstrapping technique.…”

“…This manual procedure was found to produce some systematics, particularly in the high metallicity regime (Hill et al 2011). To correct for this issue and to bring our datasets into a common baseline, we have now changed our method into an automatic procedure, similar to the one used in Hill et al (2011) but using the GALA code (Mucciarelli et al 2013) to spectroscopically refine the first guess photometric parameters. We note that another difference with respect to the aforementioned studies is that for this work we have used ATLAS9 model atmospheres (Castelli & Kurucz 2004).…”