|Figure 1: Coverage of the final combined library in the stellar parameter space. ``B+F'' models are those constructed for M giants from Fluks et al. and BBSW spectra (see LCB97)|
The ``base model grid'' of AH95 atmosphere models (referred to hereafter as the ``Extended'' models) used here present several improvements compared to the previous generation models of Allard (1990). In particular, new molecular opacities have been incorporated and the opacity sampling technique has been introduced in order to improve the treatment of some of the atomic and molecular lines. For solar metallicity, we used an upgraded version (referred to as the ``NextGen'' models version) of the ``base model grid''. In these new models a more extensive list of 12 million TiO lines (Jørgensen 1994) has also been included, with a more rigourous line-by-line treatment instead of the Just Overlapping Line Approximation (JOLA) previously used. The main effect is the reduction of the derived effective temperature by 150 K (Jones et al. 1996). We also found that these new models provide more realistic UBV colors than the previous generations (see Sect. 4.2). Because the ``NextGen'' model grid is still incomplete, only solar-abundance spectra can be included in order to cover the whole range of and provided by the hybrid library. For other metallicities, we still use the ``Extended'' models.
In the combined library, all the original spectra were rebinned on the same K95 wavelength grid, from 9.1 nm to 160 m, with a mean resolution of 10 Å in the ultraviolet and 20 Å in the visible. Recall that the M giant model spectra ``B+F'' are metallicity-independent blueward of 4900 Å owing to the introduction of the Fluks et al. models in this spectral range, and that a black-body tail was attached for 4070 nm (see LCB97). The AH95 spectra stop at 20 m, and for > 3 m the resolution is not sufficient for an accurate description of the synthetic spectrum. In order to cover the whole K95 wavelength range, we then connected a black-body to the synthetic spectra beyond 5 m (M band).
These model spectra - except for the updated NextGen models used in this work for solar metallicity - are available on a CD-ROM collecting various materials for galaxy evolution modelling (Leitherer et al. 1996), or by request on anonymous ftp, and have been used for single stellar populations models (Bruzual 1996; Bruzual et al. 1997; Lejeune 1997).
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