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 HD 209458 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 HD 209458 b has to have an upper limit of about 1000 K. 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.