0 Assigned ID: 6271 ----- Attempting Submission 1 (Wed Apr 09 20:14:47 GMT 2025) ----- The extragalactic toolkit for resolving the Hubble tension has recently expanded to include the J-AGB method, which uses evolved carbon-rich asymptotic giant branch (AGB) stars as standard candles for distance measurements. For carbon stars within a specific infrared color range, two internal processes are thought to regulate luminosity within a narrow, mass-dependent range, making them reliable distance indicators. However, we have evidence that these regulatory processes break down in metal-rich environments, casting doubt on the assumption that the stars are carbon-rich. This proposal seeks to determine the metallicity threshold at which the J-AGB method becomes unreliable and to quantify the associated uncertainties in distance measurements. We propose to analyze archival medium-band WFC3 imaging data from three observing programs, enabling us to chemically classify AGB stars in 18 fields in the Andromeda (M31) and Triangulum (M33) galaxies, and a sample of 22 nearby dwarf galaxies. The well-defined metallicity gradients in M31 and M33, along with the lower metallicity dwarf galaxies, allow us to probe a transition from metal-poor, carbon-star-dominated regions to metal-rich regions where carbon stars are scarce or absent. This study will refine the J-AGB method to improve distance estimates and reduce uncertainties in Hubble constant measurements. Goldman is an expert in observations of Local Group AGB samples and the chemical classification of AGB stars. Girardi and Pastorelli are experts in modeling the evolution of AGB stars and in building mock samples of their photometry in nearby galaxies. Boyer has extensive experience with HST photometry of AGB populations in nearby galaxies. Groenewegen has worked on the J-AGB method in the Magellanic Clouds and the Milky Way. Gilbert is co-PI of the LUVIT program and an expert in the stellar populations of M31 and M33. Stellar Populations and the Interstellar Medium 0 false false false false false false false SUPPORTED