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 atmosphere grid for cool giants (T_eff <= 6800 K and log(g) <= 3.5, Hauschildt et al. 1999). We point out that these models (computed using spherical geometry) predict a limb darkening behaviour that deviates significantly from a simple linear or two-parameter law (there is less intensity at the limb of the star). The presence of a significantly nonlinear limb darkening law has two main consequences. First, the ellipsoidal light curve computed for a tidally distorted giant using the NEXTGEN intensities is in general different from the light curve computed using the same geometry but with the black body approximation and a one- or two-parameter 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 obtained with models with near linear limb darkening (either black body or plane-parallel model atmosphere intensities) are biased. Observations over a wide wavelength range (i.e. both the optical and infrared) are particularly useful in discriminating between models with nearly linear limb darkening and the NEXTGEN models. Second, we show that rotational broadening kernels for Roche lobe filling (or nearly filling) giants can be significantly different from analytic kernels due to a combination of the nonspherical shape of the star and the radical departure from a simple limb darkening law. As a result, geometrical information inferred from V_rot*sin(i) measurements of cool giants in binary systems are likewise biased.