The use of the NEXTGEN model atmospheres for cool giants in a light curve synthesis code

We have written a light curve synthesis code that makes direct use of model atmosphere specific intensities, in particular the NEXTGEN model atmospher e grid for cool gi ants (T-eff less than or equal to 6800 K and log(g) less than or equal to 3.5, Hauschildt et al. 1999). We point out that these mod els (computed using spherical geometry) predict a limb darkening behaviour that deviates significantly from a simple linear or two-parameter law (ther e is less intensity at the limb of the star). The presence of a significant ly nonlinear limb darkening law has two main consequences. First, the ellip soidal light curve computed for a tidally distorted giant using the NEXTGEN intensities is in general different from the light curve computed using th e same geometry but with the black body approximation and a one- or two-par ameter limb darkening law. In most cases the light curves computed with the NEXTGEN intensities have deeper minima than their black body counterparts. Thus the light curve solutions for binaries with a giant component obtaine d with models with near linear limb darkening (either black body or plane-p arallel model atmosphere intensities) are biased. Observations over a wide wavelength range (i.e. both the optical and infrared) are particularly usef ul in discriminating between models with nearly linear limb darkening and t he NEXTGEN models. Second, we show that rotational broadening kernels for R oche lobe filling (or nearly filling) giants can be significantly different from analytic kernels due to a combination of the nonspherical shape of th e star and the radical departure from a simple limb darkening law. As a res ult, geometrical information inferred from V-rot sin i measurements of cool giants in binary systems are likewise biased.