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 
.
For the case of solar metallicity, [M/H] = 0.0, models for different
microturbulent velocities ranging from 0 to
are also available
(Kurucz 1993c).
For our purposes, models in the following ranges of parameters have been selected:
The fine steps in metallicity for 
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 
-elements once
the appropriate models are available.
All the 693 spectra have been computed at the resolving power
, rotational velocity of
0km s-1
and microturbulent velocity 
km s-1.
The use of a fixed value for 
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 
km s-1
for stars with gravities log 
, and
km s-1 for stars with log 
, and Milone et al.
(1995) adopt km s-1
for log 
, 
km s-1 for giant stars with
log 
, and 2.0km s-1 for supergiants
(log 
).
Tripicco & Bell (1995) have smoothly varied the value
of 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 
, temperatures ranging from 3500 to 8000K with a step of
500K, and microturbulent velocities 
km s-1 and
km s-1.
The effects of different '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 (km s-1 and
km 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
, [M/H]=+0.0 and 
K (top),
5500K (bottom). The thick solid line
corresponds to km/s and the dotted line to km/s.
The differences between the two spectra
(thin solid line) are also plotted (a constant
value equal to 1 was added for graphical reasons)