Theoretical colors for Cepheids: recent model improvements

Improvements in nonlinear stellar pulsation theory coupled with recent improvements in low temperature opacities will result in better atmospheric structures for pulsating stars. The areas we are looking into are: (1) increased resolution of the ionization region using a non-Lagrangian code developed by J. Castor, (2) improvement in the multi-frequency transfer using the multi-frequency grey approach, and (3) use of new opacities (T-4) that treat lines better and include molecular contributions (CH, H2O, CO and N2) at low temperatures