The limb-darkening for spherically symmetric NextGen model atmospheres: A-G main-sequence and sub-giant stars

We compute passband-integrated specific intensities for spherically symmetric models generated with the PHOENIX code. The models cover the range 5000 K < T(eff) < 10 000 K with surface gravities between 3.5 and 5.5, in steps of 0.5. The adopted mixing-length parameter is 2.0 pressure scale heights, while the microturbulent velocity is 2.0 km s -1. The calculations were carried out for four photometric systems: Str?mgren, Johnson, Geneva and Walraven, completing 24 filters. The series introduced by Claret (2000) and a new bi-parametric (exponential) law are shown to be able to describe the intensity distribution of spherical models with acceptable  (alpha). Simple limbdarkening laws do not produce accurate fits to the computed radiation fields. On the other hand, the concept of "quasi" spherical models is introduced and we present the corresponding limb-darkening coefficients for the most commonly adopted limb-darkening laws for the same filters.