Spectra

The changes of the synthetic spectra with gravity are illustrated in Fig. 9. The differences are larger for the cooler model in the sense that at lower $\hbox{$\,T_{\rm eff}$}$, higher gravity model atmospheres emit more optical flux than lower gravity models. This is caused by the enhanced near-IR absorption of water vapor in the models with larger gravity and hence larger gas pressures in the line forming region. For larger effective temperatures the differences diminish and are confined to individual gravity sensitive features in the spectrum.

Changes in the metallicity $[{\rm M/H}]$ result in changes of the synthetic spectra that depend strongly on the effective temperature, as shown in Fig. 10. For high effective temperatures, the changes are small in the $[{\rm M/H}]$ range we have investigated. At lower $\hbox{$\,T_{\rm eff}$}$, however, the changes are dramatic as illustrated in the top panel of Fig. 10. This is due to the increasing importance of molecules at lower temperatures. The concentration of molecules and thus their opacities depend strongly on the metal abundances. Molecules are less important at higher $\hbox{$\,T_{\rm eff}$}$ and thus the spectra are less sensitive to metallicity changes.

The very small sensitivity of the atmospheric structure on the mass of the star is mirrored by only small changes in the resulting low-resolution spectra (Fig. 11). The differences correspond to scaling factors close to unity over a large wavelength range and are thus basically negligible for most purposes. The absolute changes in limited wavelength intervals of the spectra, however, can be equivalent to changes in $\log(g)$, thus the mass of the star needs to be considered as a parameters for applications that use absolute spectra.