Challenges in Cepheid Evolution and Pulsation Modeling*

Cepheids have long been used as standard candles to determine distances around the Milky Way and to nearby galaxies. A discrepancy still remains for Hubble Constant determinations using Cepheids vs. the cosmic microwave background or calibrations to the tip of the red-giant branch. Therefore, refinement of Cepheid period-luminosity relations continues to be an active topic of research.

Beyond their utility for the cosmic distance scale, Cepheids are also important ‘laboratories’ for stellar physics. This talk explores outstanding questions and current areas of research in Cepheid evolution and pulsation modeling. We examine the discrepancy between Cepheid masses determined from pulsation properties and binary orbital dynamics with those determined using stellar evolution models. We review attempts to resolve the discrepancy by including rotation, convective overshooting, and mass loss. We also review the impact of uncertainties in the ¹²C(α,γ)¹⁶O and triple-α nuclear reaction rates on Cepheid evolution and the extent of ‘blue loops’ in the Hertzsprung-Russell diagram during which Cepheids are undergoing core helium burning. We consider implications for Cepheids of stellar opacity revisions suggested in light of findings for the Sun and other types of variable stars.

We highlight the opportunity to use the 1-D open-source MESA stellar evolution code, the GYRE linear non-adiabatic pulsation code, and the MESA radial stellar pulsation (RSP) nonlinear hydrodynamics code to investigate changes in input physics for Cepheid models. We touch on forays into 2-D and 3-D stellar modeling applied to Cepheids. Additional areas in which Cepheid models are being tested against observations include: predicting the edges of the Cepheid pulsation instability strip; predictions for period-change rates and implications for instability strip crossings; explaining period and amplitude modulations similar to the Blazhko effect of RR Lyr stars and the origins of additional periodicities that may be non-radial pulsation modes; discovering what can be learned from Cepheid observations in x-ray, ultraviolet, and radio wavelengths.