A Critical UV Legacy: A Hubble Roadmap for HWO Science Readiness

The Habitable Worlds Observatory (HWO) will provide the first opportunity to directly image and spectrally characterize terrestrial exoplanets in the habitable zones of nearby stars. Maximizing its scientific return requires a comprehensive understanding of the high-energy radiation environments of target stars, which shape planetary atmospheres and govern the production, destruction, and detectability of biosignatures. Ultraviolet (UV) radiation plays a particularly critical role in atmospheric chemistry. Far-ultraviolet (FUV) and near-ultraviolet (NUV) photons regulate key photochemical pathways, influence ozone stability, and drive the formation of prebiotic molecules. However, the majority of high-priority HWO target stars lack high-quality UV observations. Existing datasets are sparse, heterogeneous, or limited by calibration uncertainties, and no comparable UV observatory is expected for at least 5-10 years (with UVEX offering more limited spectral resolution, wavelength coverage, and sensitivity). The Hubble Space Telescope (HST) remains the only observatory capable of acquiring high-resolution FUV and NUV spectra for these targets over the next 10-15 years. We therefore advocate for a coordinated HST program to systematically obtain UV spectra of high-priority HWO targets, ideally in conjunction with X-ray observations. This effort is essential for enabling accurate target prioritization, constraining stellar radiation environments, and ensuring robust interpretation of future HWO observations.