The library of comparison stars

The library is made of 211 well-studied stars in the temperature range [4000 K; 6300 K]. This temperature interval has been chosen because it covers the full span of stars born from the beginning of the Galaxy, with the elimination of stars too cool for having a reliable detailed analysis. The metallicity of these stars is supposed to reflect the metallicity of the interstellar material from which they were formed, tracing the galactic evolution. In hotter stars, the metallicity might be altered by physical processes inside the stars, and spectra of cooler stars are complicated and might lead to erroneous results.

The choice of the reference stars and their atmospheric parameters are fully described in paper II. A large part of this paper is devoted to homogenizing the parameters because of scattered determinations found in the literature.

The success of our method relies on the existence of a library of high quality, which covers as well as possible the space of the parameters in the considered temperature interval. Figure 7 shows how dwarfs ($\log g \geq 4.2$), turn-off ($4.2 \gt \log g \geq 3.7$) and giants stars ($\log g < 3.7$) of the library span the plane ($T_\mathrm{eff}$,[Fe/H]). As can be seen, the library is not homogeneously filled. The holes in the library reflect the lack of metal deficient stars and cool dwarfs, which are more difficult to observe and to analyse. Of course, in the densest regions of the library the results might be statistically better than in sparse parts. All the spectra of the library have a signal to noise ratio greater than 35, with a mean value of 100. We had to keep in the library a few stars with a low S/N to have enough metal deficient stars, which have fainter magnitudes.

As explained in Sect. 3.5, all the spectra of the library have been convolved to several resolutions. The lowest one correponds to a FWHM of 13.5 km.s^-1 and is adequate for most of the F5 to K7 stars. For younger stars with higher rotational velocities and broadened lines, TGMET uses versions of the library degraded at lower resolutions.