THEORETICAL LIMB DARKENING FOR CLASSICAL CEPHEIDS. II. CORRECTIONS FOR THE GEOMETRIC BAADE-WESSELINK METHOD

The geometric Baade-Wesselink method is one of the most promising techniques for obtaining a better calibration of the Cepheid period-luminosity relation by means of interferometric measurements of accurate diameters. In this paper we present new wavelength- and phase-dependent limb-darkening (LD) corrections based on our time-dependent hydrodynamic models of the classical Cepheid Gem. We show that a model simulation of a Cepheid atmosphere, taking into account the hydrodynamic effects associated with the pulsa-tion, shows strong departures from the LD otherwise predicted by a static model. For most of its pulsational cycle the hydrodynamic model predicts a larger LD than the equivalent static model. The hydrodynamics affects the LD mainly at UV and optical wavelengths. Most of these effects evolve slowly as the star pulsates, but there are phases, associated with shocks propagating into the photosphere, in which significant changes in the LD take place on timescales of the order of less than a day. We assess the implication of our model LD corrections fitting the geometric Baade-Wesselink distance of Gem for the available near-IR PTI data. We discuss the effects of our model LD on the best-fit result and analyze the requirements needed to test the time dependence of the LDwith future interferometric measurements. Subject headings:Cepheids — stars: atmospheres — stars: individual ( Geminorum) — stars: oscillations — techniques: interferometri