# # Program information file # PROGRAM_ID 2024A021 PROGRAM_TITLE A Radial Velocity Analysis of the Classical Cepheid T Vul from Near-IR Spectroscopy PROGRAM_INV1 Kristin Brady PROGRAM_INV2 Catherine Pilachowski PROGRAM_INV3 Mike Pierce PROGRAM_INV4 PROGRAM_INV5 PROGRAM_SCICAT stellar PROGRAM_ABSTRACT_BEG The best-fit cosmological expansion models from measurements of sound waves in the early universe [a.k.a., baryon acoustic oscillations or BAO], imply an expansion rate for the universe that can be directly compared to local, empirical, measurements of the Hubble Constant, Ho. Unfortunately, the inferred and measured values for Ho are in disagreement. This is often referred to as the tension in Ho. Most of the empirical measures of Ho rely on Cepheid variable stars, whose intrinsic luminosity is strongly correlated with their pulsation period. One approach to measuring the geometric distances of Galactic Cepheids is known as the Baade-Wesselink [B-W] method. The method makes use of a Cepheid's radial velocity curve over its pulsation cycle to provide the change in radius. Since the change in brightness results from a change in the angular diameter, the addition of radial velocities allows one to solve for the distance, and thereby determine the Cepheid's luminosity. However, a geometric correction, known as the projection, or P-factor, is needed to account for the fact that one observes a projected velocity over the facing hemisphere, instead of the pure radial, expansion velocity. We propose to undertake a pilot study of Galactic Cepheids in order to investigate whether velocities acquired in the near-IR significantly reduce the uncertainties in the B-W systematics in distances that arise for the P-factor. The first step for this pilot study requires RVs from near-IR spectra to compare to the RVs from optical spectra that have been obtained. For our pilot study, the Galactic Cepheid T Vul was chosen for its measured HST parallax distance and log P > 1.0 in order to test the application of B-W for those systems that will be accessible in extragalactic systems with the ELTs. PROGRAM_ABSTRACT_END