A sensitivity study in two yield metrics over an option space local to a 6m HWO telescope

The Astro2020 Decadal Survey recommended a “future large IR/O/UV telescope optimized for observing habitable exoplanets and general astrophysics” that would “search for biosignatures from a robust number of about ~25 habitable zone [exo]planets,” now dubbed the Habitable Worlds Observatory (HWO). The search for 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. Previous work (Morgan et al. 2022), evaluated the number of Earth-size, habitable zone exoplanets that could be spectrally characterized for a range of spectral resolutions, signal-to-noise ratios, and bandwidths for a 6-m diameter exoplanet direct imaging mission. In this paper, we evaluate yield with two of those metrics for a range of parameters: aperture diameter, inner working angle, contrast, and exozodiacal dust brightness around a 6-m diameter telescope point design that is a blend of scaled HabEx and LUVOIR designs. The first metric is the water line search metric, which has spectral resolution = 70, SNR = 5, bandwidth = 20%, wavelength = 800-1000 nm and which was used to establish the ~25 HZ exoplanet goal of the Astro2020. The second metric is a broadband metric that covers 500 – 1000 nm in four 20% bandwidth sub-bands. The sub-bands are tailored to minimize integration for each sub-band: 500 – 700 nm uses R=7 to look for Rayleigh scattering, 700 – 850 nm uses R=140 for the oxygen line, and 800 – 1000 nm uses R=70 for the water line; all have SNR = 8.5.