0 Assigned ID: 7785 ----- Attempting Submission 1 (Thu Apr 16 23:15:54 GMT 2026) ----- Classical Cepheids anchor the HST distance ladder, but the precision of HST measurements exceeds the realism of the convection physics in pulsation models. We propose to implement the full nonlocal three-equation Kuhfuss time-dependent convection formalism in MESA, extending current one-equation Cepheid hydrodynamics to include nonlocal turbulent transport. This framework will produce self-consistent nonlinear Cepheid models across the masses, luminosities, and metallicities relevant to Milky Way calibrators, the LMC, NGC 4258, and SN Ia host galaxies. We will compute synthetic HST observables in F555W, F814W, F160W, and F350LP, together with periods, amplitudes, phase lags, radius variations, and instability-strip boundaries, and benchmark the models against archival HST datasets including FGS and WFC3 parallaxes, distance-ladder imaging, and COS phase-resolved spectroscopy. The resulting grid will directly test Cepheid assumptions that matter for local-H0 determinations: phase corrections for sparse cadence, light-curve templates, metallicity dependencies, color terms, and Wesenheit magnitudes in the native HST system. By embedding this program in a public open-source stellar-evolution and hydrodynamics framework, the project will deliver the first community-standard Cepheid models capable of connecting HST observables to phase-dependent convection with predictive power. The outcome will be a new theoretical foundation for interpreting existing HST Cepheid programs, quantifying residual Cepheid systematics in the distance ladder, and sharpening future HST observing strategies. Earl Bellinger is an Assistant Professor in the Department of Astronomy at Yale University and a developer of the MESA stellar evolution code. His research focuses on the theory and observation of pulsating stars, including Cepheids, RR Lyrae, solar-like oscillators, red giants, and white dwarfs. Ebraheem Farag is a YCAA Postdoctoral Research Fellow at Yale University and a developer of the MESA stellar evolution code. He recently led work implementing the one-equation Kuhfuss model in MESA and using it to model the evolution and pulsations of Cepheids. Selim Kalici is a graduate student in the Department of Astronomy at Yale University. Selim's Ph.D. thesis work is focused on nonlinear hydrodynamic models of pulsating stars, and using those models to characterize stars observed by Gaia, OGLE, and HST. Stellar Physics and Stellar Types 0 false false true false false false false SUPPORTED