How Much Information Does the Sodium Doublet Encode? Retrieval Analysis of Non-LTE Sodium Lines at Low and High Spectral Resolutions

Fisher, Chloe; Heng, Kevin (2019). How Much Information Does the Sodium Doublet Encode? Retrieval Analysis of Non-LTE Sodium Lines at Low and High Spectral Resolutions. Astrophysical journal, 881(1), p. 25. Institute of Physics Publishing IOP 10.3847/1538-4357/ab29e8

Preview	Text 1906.07035.pdf - Draft Version Available under License Publisher holds Copyright. Download (6MB) | Preview
	Text Fisher_2019_ApJ_881_25.pdf - Published Version Restricted to registered users only Available under License Publisher holds Copyright. Download (2MB)

Motivated by both ground- and space-based detections of the sodium doublet in the transmission spectra of exoplanetary atmospheres, we revisit the theory and interpretation of sodium lines in non-local thermodynamic equilibrium (NLTE), where collisions are not efficient enough to maintain a Boltzmann distribution for the excited and ground states of the sodium atom. We consider non-Boltzmann distributions that account for the ineffectiveness of collisions. We analyze the sodium doublet in transmission spectra measured at low (HAT-P-1b, HAT-P-12b, HD 189733b, WASP-6b, WASP-17b and WASP-39b) and high (WASP-49b) spectral resolutions. Nested-sampling retrievals performed on low-resolution optical/visible transmission spectra are unable to break the normalization degeneracy if the spectral continuum is associated with Rayleigh scattering by small cloud particles. Using mock retrievals, we demonstrate that un-normalized ground-based, high-resolution spectra centered on the sodium doublet alone are unable to precisely inform us about the pressure levels probed by the transit chord and hence to identify the region (i.e., thermosphere, exosphere) of the atmosphere being probed. Retrievals performed on the HARPS transmission spectrum of WASP-49b support this conclusion. Generally, we are unable to distinguish between LTE versus NLTE interpretations of the sodium doublet based on the computed Bayesian evidence with the implication that LTE interpretations tend to under-estimate the temperature probed by the transit chord. With the current low-resolution data, the sodium line shapes are consistent with Voigt profiles without the need for sub-Lorentzian wings. The retrieved sodium abundances are consistent with being sub-solar to solar.

Interest & Impact

Downloads

153 since deposited on 14 Apr 2020
23 in the past 12 months

Citations

5 Citations in Web of Science ®
6 Citations in Scopus

Search

in Google Scholar™

Services

Actions (login required)

Edit item

Actions (login required)

Edit item