Information Content Analysis for Selection of Optimal JWST Observing Modes for Transiting Exoplanet Atmospheres

Abstract

The James Webb Space Telescope (JWST) is nearing its launch date of 2018, and is expected to revolutionize our knowledge of exoplanet atmospheres. In order to specifically identify which observing modes will be most useful for characterizing a diverse range of exoplanetary atmospheres, we use an information content based approach commonly used in the studies of Solar System atmospheres. We develop a system based upon these information content methods to trace the instrumental and atmospheric model phase space in order to identify which observing modes are best suited for particular classes of planets, focusing on transmission spectra. Specifically, the atmospheric parameter space we cover is T=600-1800 K, C/O=0.55-1, [M/H]=1-100×Solar for a R=1.39 R J , M=0.59 M J planet orbiting WASP-62-like star. We also explore the influence of a simplified opaque gray cloud on the information content. We find that obtaining broader wavelength coverage over multiple modes is preferred over higher precision in a single mode given the same amount of observing time. Regardless of the planet temperature and composition, the best modes for constraining terminator temperatures, C/O ratios, and metallicity are NIRISS SOSS+NIRSpec G395. If the target's host star is dim enough such that the NIRSpec prism can be used, then it can be used instead of NIRISS SOSS+NIRSpec G395. Lastly, observations that use more than two modes, should be carefully analyzed because sometimes the addition of a third mode results in no gain of information. In these cases, higher precision in the original two modes is favorable.

The Detectability and Constraints of Biosignature Gases in the Near- and Mid-infrared from Transit Transmission Spectroscopy

“…Software: PySynPhot (Lim et al 2015), CHIMERA (Batalha & Line 2017;Line et al 2013;Greene et al 2016;Line & Parmentier 2016)), PyMultiNest (Buchner et al 2014)…”

“…To produce model transmission spectra, we leverage a variant 2 of the CHIMERA 3 transmission spectrum routine (Batalha & Line 2017;Line et al 2013;Greene et al 2016 2016). Specifically, we re-parameterize the code (Table 1) to make it more amenable for temperate worlds by including as free parameters the constant-with-altitude mixing ratios of H 2 O, CO 2 , CO, CH 4 , O 3 , N 2 O and an "unknown" background gas with a free-parameter molecular weight (taken to be earth's N 2 +O 2 value), an isothermal "scale-height" temperature, planetary radius at the surface (or scaling there-of), and an opaque gray cloud-top-pressure (nominal truth values given in Table 1).…”

Information in the Reflected-light Spectra of Widely Separated Giant Exoplanets

“…where S ǫ is the measurement error covariance, and S a is the covariance of the prior. This formula is well established in atmospheric remote sensing (Rodgers 2000), and has been used to study transmission spectroscopy of exoplanets (Batalha & Line 2017). The covariant matrix of the posterior S is a 2 × 2 matrix in our problem, and it defines the probability density function of the posterior.…”

A Framework For Optimizing Exoplanet Target Selection For The James Webb Space Telescope

“…There were a number of factors that influenced this decision. First, according to Batalha & Line (2017), "An observation with both NIRISS [SOSS] and NIRSpec G395M/H always yields the highest information content spectra with the tightest constraints, regardless of temperature, C/O, [M/H], cloud effects or precision." In addition, Bean et al (2018) included the NIRSpec G395H as one of the consensus high priority modes for the Early Release Science Program for JWST.…”

“…The wavelength ranges are different, and the capabilities of the instruments are complementary rather than competing. It has been suggested (Batalha & Line 2017) that these two instruments be used in tandem, since there is little overlap in their wavelength coverage, and both have relatively high precision. We have developed an analytical framework and associated computer code that can assist the community in determining the best exoplanet targets for atmospheric characterization by JWST.…”