3 Star sample

In order to derive a confident empirical calibration of the D₄₀₀₀, we decided to measure this spectral feature in all the stars belonging to the Lick/IDS Library [7, (Burstein et al. 1984]; [23, Faber et al. 1985]; [8, Burstein et al. 1986]; [27, Gorgas et al. 1993], hereafter G93; and [67, Worthey et al. 1994)]. The suitability of the Lick/IDS library to obtain empirical fitting functions has been widely demonstrated by the works of G83, W94, and [66, Worthey & Ottaviani (1997)], who, in overall, have derived analytical expressions for 25 spectral indices in the 4000-6000 Å region. [57, Trager et al. (1998)], and references therein, have extensively shown the usefulness of the Lick/IDS absorption-line index system in the study of old stellar populations. In brief, the Lick/IDS library contains 460 stars of all spectral types and luminosity classes. Although a large fraction are field stars from the solar neighbourhood, members of open clusters (covering a wide range of ages) and galactic globular clusters (with different metallicities) are also included.

Table 1 lists the final data sample, which comprises 392 stars out of the original set of 424 objects with published atmospheric parameters in W94 (the remaining 32 stars presented high uncertainties in the derived measurements and were not included in order to guarantee the quality of the final dataset). The first two columns list the Henry Draper Catalog number, if any, and other common designation. The spectral type and luminosity class are given in the third column (see references at the end of the table). The following columns list the atmospheric parameters employed in this work. Most of them are those listed in the electronic table of W94. Since we wish to keep the consistency with the previous fitting functions for the Lick indices, we have been reluctant to change any atmospheric parameter. However, we have introduced new parameters for some of the stars which lack them in W94, and, in some cases, the original parameters have been improved using more recent or reliable data from the literature (see the description in the table notes). As a whole, effective temperature, gravity and metallicity data are finally available for 383, 382 and 354 stars, respectively.

To summarize, the Lick/IDS library spans the following ranges in atmospheric parameters: $T_{\rm eff}$ from 2747 to 21860 K, $\log g$ from 0 to 5.12, and [Fe/H] from -2.70 to 0.46. It is important to note that the star sample does not homogeneously cover the parameter space, being more densely populated in the metallicity interval -1.0 dex to +0.5 dex, whereas the gravity range is wider close to solar metallicity. A more important deficiency of the library is the paucity of hot stars. Since we are also interested in applying the derived fitting functions to the modeling of younger stellar populations, we have enlarged the sample with the inclusion of additional hot stars. For this purpose, we have employed flux-calibrated spectra corresponding to representative spectral types and luminosity classes from the compilation of [44, Pickles (1998)]. In particular, 43 spectra of O5-F0 stars with $6720<T_{\rm eff}<39800$ K have been incorporated. It is worth noting that we have checked that no systematic offset is apparent between our 4000 D measurements and those in Pickles' spectra. To display the range of stellar parameters covered by the resulting library, in Fig. 2 we present a $\log g-T_{\rm eff}$diagram of the calibrating stars.