The strange case of Na I in the atmosphere of HD209458 b: Reconciling low- and high-resolution spectroscopic observations G

TL;DR

This study reconciles conflicting observations of sodium in HD209458 b's atmosphere by modeling various scenarios, emphasizing the importance of combining low- and high-resolution spectroscopy for accurate exoplanet atmospheric characterization.

Contribution

It introduces comprehensive atmospheric models that explain discrepancies between low- and high-resolution spectroscopic data, clarifying the presence or absence of Na I in HD209458 b.

Findings

Both datasets can be explained by a sodium-free atmosphere or a terminator temperature below 1000K.

Low-resolution signals may result from differential transit depths, not actual sodium presence.

High-resolution data's Rossiter-McLaughlin effect can mask sodium absorption cores.

Abstract

Aims. We aim to investigate the origin of the discrepant results reported in the literature about the presence of Na I in the atmosphere of HD209458 b, based on low- and high-resolution transmission spectroscopy. Methods. We generated synthetic planetary atmosphere models and we compared them with the transmission light curves and spectra observed in previous studies. Our models account for the stellar limb-darkening and Rossiter-McLaughlin (RM) effects, and contemplate various possible scenarios for the planetary atmosphere. Results. We reconciled the discrepant results by identifying a range of planetary atmospheres that are consistent with previous low- and high-resolution spectroscopic observations. Either both datasets are interpreted as consistent with a total absence of Na I in the planetary atmosphere (with Hubble Space Telescope data being affected by limb darkening), or the terminator temperature of HD209458 b has to have an upper limit of about 1000K. In particular, we find that 1D transmission spectra with lower-than-equilibrium temperatures can also explain the previously reported detection of absorption signal at low resolution due to differential transit depth in adjacent bands, while the cores of the Na I D lines may be masked by the strong RM signal seen at high resolution. We also rule out high-altitude clouds, which would otherwise mask the absorption signal at low resolution, as the source of the discrepancies. Conclusions. This work highlights the synergies between different observing techniques, specifically low- and high-resolution spectroscopy, to fully characterise transiting exoplanet systems.