Improved Masses for Critical Cepheid Binaries

Cepheids play an important part in the Hubble Constant tension, and in evolutionary calculation benchmarks, which include comparisons with LMC Cepheids. In some cases, they lead to exotic end-stage objects. An important HST UV legacy is the measurement of the Cepheid masses, the fundamental parameter in stellar evolution. The HST high resolution E140H spectra requested in this proposal (a unique capability of HST) will measure the orbital velocity amplitude of hot companions of three Cepheids, V1334 Cyg, S Mus, and SU Cyg. This can be combined with recent infrared interferometry with the Very Large Telescope Interferometer (VLTI) and CHARA which has resolved the systems, providing visual orbits to augment the spectroscopic orbits. From this combination both masses and distances for the Cepheids have been derived, which already challenge current evolutionary tracks, even those incorporating rotation and core convective overshoot on the main sequence. The observations detailed in this proposal will improve the orbital solution through more accurate orbital velocity amplitudes of the hot companions. In addition for SU Cyg the phasing of the short period orbit of the companion will be more precise. The requested observations are predicted to yield mass solutions accurate to a few percent. These will be the most accurate Cepheid mass determinations available to quantitatively test the underlying physics at this stage.