3. Codes and models
All the convective models used in this paper were computed with the ATLAS9 version of June 1995. We will indicate with K95 the models computed with the "overshooting" option switched on, and with NOVER those with the "overshooting" option switched off. The K95 models differ from those stored on the CD-ROM 13 (Kurucz, 1993a) (K93) for the way how the "approximate overshooting" is handled as we described in Sect. 2.3. Furthermore, among the the K93 convective models, only those with solar metallicity were computed for 72 instead of 64 layers. These models are based on an improved solution of transfer equation, which is able to remove the jump between the temperatures of the first and second layers. Many wavelengths are optically thick at the first or second depth in the atmosphere in cool models with 64 optical depths. Increasing the number of layers toward the top makes the calculations for the upper layers much more accurate. If no chromospheric temperature rise is present, fluxes from the upper layers of models with 72 depths are much more reliable.
In the K95 grids the larger number of layers and the use of the improved solution of the transfer equation was extended also to the models with non-solar metallicities.
As far as the Kurucz solar model is concerned, we refer in this paper to that computed on January 1994 (SUNK94), which could be different from the solar model stored on the CD-ROM 13 distributed before 1994. Because the improvements related with the "overshooting" option do not affect the solar model, the SUNK94 model does not change when the 1995 version of the ATLAS9 is used. The SUNK94 model is listed in Table 1.
Table 1. The Kurucz solar model (SUNK94)
In the computation of Balmer profiles we used both the BALMER9 and SYNTHE codes of Kurucz (1993a, 1993b). Both codes have subroutines from D. Peterson which account for Stark, van der Waals, Doppler, natural, and resonance broadening of the hydrogen lines. For , , , and , Stark profiles folded with the Doppler profiles are taken from Vidal, Cooper, and Smith (1973) (VCS) tables. The damping constants of the resonance profiles are computed according to Ali & Griem (1965, 1966), while the damping constants of the natural profiles are computed from the transition probablities . The BALMER9 code computes the first four Balmer lines normalized to the flux at 100 Å from the line center. The SYNTHE code computes a synthetic spectrum including all the known lines lying in the studied region.
Both BALMER9 and SYNTHE codes are consistent with the ATLAS9 code, in the sense that same input data and same numerical approaches for solving the basic equations are used in all the codes. There are some differences in the opacities computations. For instance, no molecular continous opacities have been still inserted in SYNTHE.
© European Southern Observatory (ESO) 1997
Online publication: July 3, 1998