A survey of sodium absorption in 10 giant exoplanets with high-resolution transmission spectroscopy

Abstract

ABSTRACT The alkali metal sodium (Na) is one of the most commonly detected chemical species in the upper atmospheres of giant exoplanets. In this work, we conducted a homogeneous survey of Na in a diverse sample of 10 highly irradiated giant exoplanets using high-resolution transmission spectroscopy. Our sample includes nine planets with previous Na detections and one new detection. We confirm previous detections and assess multiple approaches for deriving Na line properties from high-resolution transmission spectra. The homogeneously measured sodium line depths were used to constrain the atmospheric heights (HNa) with respect to the planetary radii (Rp). We assess an empirical trend describing the relative atmospheric height (HNa/Rp) as a function of planetary equilibrium temperature (Teq) and surface gravity (g), in which HNa/Rp decreases exponentially with ξ ∝ gTeq, approaching a constant at large ξ. We also report the sodium D2/D1 line ratios across our sample and find that seven targets have line ratios that are consistent with unity. Finally, we measured net blueshifted offsets of the sodium absorption lines from their rest frame wavelengths for all 10 planets, corresponding to day–night wind velocities of a few km s−1. This suggests that the broad sample of exoplanets share common underlying processes that govern atmospheric dynamics. Our study highlights a promising avenue for using high-resolution transmission spectroscopy to further our understanding of how atmospheric characteristics vary over a diverse sample of exoplanets.