Alkaline Exospheres of Exoplanet Systems: Evaporative Transmission Spectra

Abstract

Hydrostatic equilibrium is an excellent approximation for the dense layers of planetary atmospheres where it has been canonically used to interpret transmission spectra of exoplanets. Here we exploit the ability of high-resolution spectrographs to probe tenuous layers of sodium and potassium gas due to their formidable absorption crosssections. We present an atmosphere-exosphere degeneracy between optically thick and optically thin mediums, raising the question of whether hydrostatic equilibrium is appropriate for Na I lines observed at exoplanets. To this end we simulate three nonhydrostatic, evaporative, density profiles: (i) escaping, (ii) exomoon, and (iii) torus to examine their imprint on an alkaline exosphere in transmission. By analyzing an evaporative curve of growth we find that equivalent widths of W NaD2 ∼ 1 − 10 m Å are naturally driven by evaporation rates ∼ 10 3 − 10 5 kg/s of pure atomic Na. To break the degeneracy between atmospheric and exospheric absorption, we suggest that if the line ratio is D2/D1 1.2 the gas is optically thin on average and roughly indicating a non-hydrostatic structure of the atmosphere/exosphere. We show this is the case for Na I observations at hot Jupiters WASP-49b and HD189733b and also simulate their K I spectra. Lastly, motivated by the slew of metal detections at ultra-hot Jupiters, we suggest a toroidal atmosphere at WASP-76b and WASP-121b is consistent with the Na I data at present.

Hot Exoplanet Atmospheres Resolved with Transit Spectroscopy (HEARTS) IV. A spectral inventory of atoms and molecules in the high-resolution transmission spectrum of WASP-121 b

“…This could indicate that Na i forms an optically thin envelope around the planet (Oza et al 2019). An analytical description of a homogeneous optically thin cloud with an integrated sodium mass of (3.6 ± 0.6) × 10 10 g reproduces the transmission spectrum well, as does a model by Wyttenbach (2020) of a hydrodynamic outflow, and a model by Gebek & Oza (2020) that assumes an optically thin toroidal Na i envelope (see Section 3.1.2, Fig. 6 and Appendix A).…”

“…Recently, Gebek & Oza (2020) showed that the Na-D line ratio in the transmission spectrum approaches a value of 2 when the absorbing Na i gas is optically thin (τ 1) across both absorption line cores: In the optically thin limit, the line depth ratio is proportional to the the ratio of oscillator strengths (Draine 2011), i.e. ∼ 2 for the Na i D-doublet.…”

“…Steinrueck et al 2019), (Showman et al 2019) or data driven exploration (e.g. Waldmann et al 2015;Fisher et al 2019;Oza et al 2019;Seidel et al 2020;Gebek & Oza 2020).…”

Investigation of the upper atmosphere in ultra-hot Jupiter WASP-76 b with high-resolution spectroscopy

“…There is also possibility that there is an instrumental difference. The sodium velocity is up to ∼ 100 km/s considering charge exchange in the sodium exosphere at Jupiter or pickup ions in the Io plasma torus (Gebek & Oza 2020). There is thus a possibility that this spread of the width can be explained by the plasma-driven escape of Na at exo-Io or exo-torus.…”

Characterization of exoplanetary atmospheres through a model-unbiased spectral survey methodology

“…Even sub-Neptunes have already been characterized using this technique (Benneke et al 2019). When transit spectroscopy is performed through high resolution spectrographs, individual atomic species can be resolved (e.g., Na, K, Mg Welbanks & Madhusudhan 2019a; Wyttenbach et al 2017;Gebek & Oza 2020;Hoeijmakers et al 2020a)). Recently, the sample has been extended to include He I (Nortmann et al 2018;Allart et al 2019;Welbanks & Madhusudhan 2019b,b) and Fe I (Hoeijmakers et al (2018), Cubillos et al (2020), Stangret et al (2020)).…”