Sensitivity of exo-Earth yield of a 6 m IR/O/UV telescope to bandwidth, SNR, and spectral resolution

The Astro2020 Decadal Survey recommended a “future large IR/O/UV telescope optimized for observing habitable exoplanets and general astrophysics” to be ready by end of the decade and that mission “to search for biosignatures from a robust number of about ~25 habitable zone [exo]planets.” The search for multiple biosignatures requires high quality spectra of broad bandwidth and sufficient signal-to-noise. The combination of spectral resolution, bandwidth, and signal-to-noise-ratio impacts the number of exo-Earths that can be spectrally characterized. Broader bandwidths and higher signal-to-noise lead to increased integration time and is likely to reduce the number of targets around which an Earth-size, habitable zone exoplanet could be sufficiently characterized. An increased integration will also consume a larger share of mission time dedicated to exoplanet direct imaging, possibly resulting in fewer exoplanets with higher quality spectra. In this paper, we evaluate the number of Earth-size, habitable zone exoplanets that can be spectrally characterized for a range of spectral resolutions, signal-to-noise ratios, and bandwidths for a 6-m diameter exoplanet direct imaging mission. We evaluate the spectral characterization yield for coronagraph-only, starshade-only, and hybrid coronagraph-starshade architectures by scaling down the LUVOIR B 8-m diameter (6.7-m inscribed diameter) architecture and scaling up the HabEx 4m architectures. We evaluate the nominal case of a blind-search survey which uses broadband photometric detections for discovery and orbit determination and we evaluate the bounding case of perfect-prior knowledge which is useful to determine if target depletion occurs.