Ultraviolet Reflected-Light Spectroscopy of Modern Earth Analogs with Starshade Observations

Characterizing rocky, habitable zone planets in reflected light is a key science objective for future direct imaging missions, and the ultraviolet (UV) is uniquely suited for supporting the characterization of Earth-like atmospheres. The Hartley-Huggins ozone band at $0.25\mu \mathrm{m}$ remains detectable at low spectral resolution down to $0.1\mathrm{\%}$ PAL ${\mathrm{O}}_2$, providing a sensitive proxy for oxygen-bearing chemistry even for Proterozoic-like atmospheres where the visible and near-infrared ${\mathrm{O}}_2$ features become inaccessible. This work uses an end-to-end framework for UV starshade spectroscopy of modern Earth analogs that integrates high-fidelity observation simulation, conservative post-processing, and Bayesian spectral retrieval to quantify ozone detection significance. The framework is applied across 108 simulated scenes spanning a range of planet phase angles, system inclinations, and exozodiacal dust densities. The results establish that robust ozone detection with UV-only, $R=7$ reflected light spectroscopy requires a photometric $\mathrm{S}\mathrm{N}\mathrm{R}\sim 30$. UV starshade observations with the Habitable Worlds Observatory achieve this threshold within 3-day integrations for solar-type systems at 10 parsecs with inclinations up to ${60}^{\circ }$ at favorable phases ($\le {90}^{\circ }$). Above this SNR threshold, ozone detections are robust to systematic bias from imperfect exozodi calibration within $\sim 30\mathrm{\%}$, and the median retrieved ozone abundance remains accurate to within one order of magnitude of the true value.