Spectral resolution is one of the key instrument parameters for a future flagship mission aiming to detect and characterize rocky exoplanets. At visible wavelengths, the relatively narrow molecular oxygen A-band absorption feature at 760 nm is one of the driving spectrograph design considerations. We combined numerical models for a segmented space telescope, coronagraph, and lenslet array integral field spectrograph to simulate high-fidelity data products for an Earth-analog exoplanet observed in a 20% bandpass centered on this absorption feature. The simulations were repeated over a set of spectral resolutions, integration times, and noise realizations. The extracted planet spectra were fed into a Bayesian spectral retrieval pipeline. Our nested sampling forward model interpolates over a 6-parameter grid of terrestrial reflectance spectra. For a signal-to-noise ratio per spectral bin of 15, we find modest constraints on the O2 atmosphere mixing ratio are possible at spectral resolutions R<110, with [16th , 84th] percentile confidence interval [∼ 5%, ∼ 40%] given a true input mixing ratio of 21%. At higher spectral resolutions, for a fixed integration time we find no significant improvement in the confidence interval of the O2 mixing ratio.
The exoplanet atmosphere characterization goals of future UV/Optical/Infrared flagship space missions will drive challenging design requirements for instrument wavefront controls, spatial and spectral sampling, spectral bandwidth, and detector performance. The new ExoSpec Project links four previously distinct research efforts at Goddard and Ames for enabling and enhancing the characterization of directly-imaged exoplanets. ExoSpec is comprised of three laboratory subsystem demonstrations: high-contrast integral field spectrographs, p-channel CCDs, and parabolic deformable mirrors. A fourth component, exoplanet spectral retrieval, is an iterative data simulation activity driven by the goal of optimizing the system-level instrument design for atmosphere retrieval metrics. The ExoSpec Project's emphasis on system-level spectroscopy performance complements the objectives of other technology demonstrations supported by NASA.
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