The JWST Resolved Stellar Populations Early Release Science Program. IX. The RR Lyrae Population in WLM with HST and JWST

RR Lyrae stars are a common, dependable Population II distance indicator and provide an independent tracer of early star formation. Here, we utilize archival Hubble Space Telescope (HST) Advanced Camera for Surveys and JWST/NIRCam observations of the nearby dwarf star-forming galaxy Wolf–Lundmark–Melotte (WLM) to study RR Lyrae stars in JWST filters. We independently identify RR Lyrae stars in HST and JWST imaging in order to evaluate JWST’s efficacy at characterizing RR Lyrae stars in the near-IR. We use a Markov Chain Monte Carlo (MCMC) template-fitting technique to obtain periods, amplitudes, and mean magnitudes from the RR Lyrae time-series data. The spatially overlapping HST and JWST observations allow us to directly compare the same sources observed with the instruments and to calibrate the NIRCam F090W and F150W RR Lyrae period–Wesenheit–metallicity (PWZ) relation to the Gaia-consistent HST PWZ. We additionally assess the epoch-to-epoch consistency of the NIRCam photometry and find evidence of burn-in. We conclude that the zero-point offset is negligible compared to the uncertainties from the template fitting. We conduct an MCMC fit of the PWZ with both HST and JWST data. Our results are threefold. First, we find that we can reliably identify RR Lyrae stars in NIRCam data, but light-curve template fitting proves difficult on short-baseline observations. Second, the HST PWZ fit yields a distance modulus to WLM of μ = 24.85 ± 0.05 (0.93 ± 0.02 Mpc). This is closer than previous measurements, primarily attributed to consistency with the Gaia scale. Last, although the JWST PWZ fit has large uncertainties and a poorly constrained slope, it represents a first-of-its-kind PWZ calibration in NIRCam filters.