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3. The synthetic spectra

Kurucz (1993a) model atmospheres are available in a range of temperatures that spans from 3 500 to 50 000K, metallicities, [M/H], from -3.5 to +1.0 and surface gravities (log g) from 0.0 to 5.0dex. They have been computed by adopting a microturbulent velocity tex2html_wrap_inline1063. For the case of solar metallicity, [M/H] = 0.0, models for different microturbulent velocities ranging from 0 to
tex2html_wrap_inline1067 are also available (Kurucz 1993c).

For our purposes, models in the following ranges of parameters have been selected:

The fine steps in metallicity for tex2html_wrap_inline1073 make this grid more suitable for the analysis of the so-called super-metal rich stars than our previous theoretical grids (Gulati et al. 1993; Chavez et al. 1995). It must be noted that, in modelling stars with chemical composition different from solar, all heavy elements vary lock-step. Work is already planned in order to complement the Atlas by including synthetic spectra with non solar partitions in the tex2html_wrap_inline1075-elements once the appropriate models are available.

All the 693 spectra have been computed at the resolving power tex2html_wrap_inline979, rotational velocity of
0km s-1 and microturbulent velocity tex2html_wrap_inline1081km s-1.

The use of a fixed value for tex2html_wrap_inline1085 is somewhat arbitrary (see Kurucz 1996 for a detailed discussion). Among synthetic spectra builders, different approaches have been followed. For instance, Barbuy (1994) considers a value of tex2html_wrap_inline1087km s-1 for stars with gravities log tex2html_wrap_inline1091, and tex2html_wrap_inline1093km s-1 for stars with log tex2html_wrap_inline1097, and Milone et al. (1995) adopt tex2html_wrap_inline1087km s-1 for log tex2html_wrap_inline1103, tex2html_wrap_inline1105km s-1 for giant stars with tex2html_wrap_inline1109 log tex2html_wrap_inline1111, and 2.0km s-1 for supergiants (log tex2html_wrap_inline1115). Tripicco & Bell (1995) have smoothly varied the value of tex2html_wrap_inline1085 from 1.0km s-1 for dwarf stars to 2.0km s-1 for supergiants.

At present Kurucz's models for other microturbulent velocities are available only for solar chemical abundance. In order to get insight into the effects of this parameter we have computed an additional set of 18 synthetic spectra with the following parameters: surface gravity fixed at log tex2html_wrap_inline1123, temperatures ranging from 3500 to 8000K with a step of 500K, and microturbulent velocities tex2html_wrap_inline1125km s-1 and tex2html_wrap_inline1087km s-1. The effects of different tex2html_wrap_inline1085's on spectral indices have been discussed in Chavez et al. (1996): in general, they are negligible at high temperatures, while becoming of some significance at low temperatures. At high resolution, the microturbulent velocity effects are temperature and wavelength dependent. As an example, Fig. 4 (click here) shows the synthetic spectra computed with different microturbulent velocities (tex2html_wrap_inline1087km s-1 and tex2html_wrap_inline1139km s-1) for two temperatures, namely 4000K and 5500K, in the wavelength region dominated by the Mgb triplet. In both panels, the differences between the two spectra are also plotted. The average flux difference is larger for 4000K, while, locally, the differences are higher for 5500K.

Figure 4: Effects of microturbulent velocity in the Mgb region: spectra computed for tex2html_wrap_inline1143, [M/H]=+0.0 and tex2html_wrap_inline1147K (top), tex2html_wrap_inline11495500K (bottom). The thick solid line corresponds to tex2html_wrap_inline1139km/s and the dotted line to tex2html_wrap_inline1087km/s. The differences between the two spectra (thin solid line) are also plotted (a constant value equal to 1 was added for graphical reasons)

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