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 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: Strömgren, 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 σ. Simple limb-darkening 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.