Refraction in planetary atmospheres: improved analytical expressions and comparison with a new ray-tracing algorithm Article Swipe
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· 2015
· Open Access
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· DOI: https://doi.org/10.1093/mnras/stv1078
· OA: W1924711739
Atmospheric refraction affects to various degrees exoplanet transit, lunar\neclipse, as well as stellar occultation observations. Exoplanet retrieval\nalgorithms often use analytical expressions for the column abundance along a\nray traversing the atmosphere as well as for the deflection of that ray, which\nare first order approximations valid for low densities in a spherically\nsymmetric homogeneous isothermal atmosphere. We derive new analytical formulae\nfor both of these quantities, which are valid for higher densities, and use\nthem to refine and validate a new ray tracing algorithm which can be used for\narbitrary atmospheric temperature-pressure profiles. We illustrate with simple\nisothermal atmospheric profiles the consequences of our model for different\nplanets: temperate Earth-like and Jovian-like planets, as well as HD189733b,\nand GJ1214b. We find that, for both hot exoplanets, our treatment of refraction\ndoes not make much of a difference to pressures as high as 10 atmosphere, but\nthat it is important to consider the variation of gravity with altitude for\nGJ1214b. However, we find that the temperate atmospheres have an apparent scale\nheight significantly smaller than their actual density scale height at\ndensities larger than 1 amagat, thus increasing the difficulty of detecting\nspectral features originating in these regions. These denser atmospheric\nregions form a refractive boundary layer where column abundances and ray\ndeflection increases dramatically with decreasing impact parameter. This\nrefractive boundary layer mimics a surface, and none of the techniques\nmentioned above can probe atmospheric regions denser than about 4 amagat on\nthese temperate planets.\n
