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Aims. We aim to investigate the origin of the discrepant results reported inthe literature about the presence of Na I in the atmosphere of HD209458 b,based on low- and high-resolution transmission spectroscopy. Methods. Wegenerated synthetic planetary atmosphere models and we compared them with thetransmission light curves and spectra observed in previous studies. Our modelsaccount 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 ofplanetary atmospheres that are consistent with previous low- andhigh-resolution spectroscopic observations. Either both datasets areinterpreted as consistent with a total absence of Na I in the planetaryatmosphere (with Hubble Space Telescope data being affected by limb darkening),or the terminator temperature of HD209458 b has to have an upper limit of about1000K. In particular, we find that 1D transmission spectra withlower-than-equilibrium temperatures can also explain the previously reporteddetection of absorption signal at low resolution due to differential transitdepth in adjacent bands, while the cores of the Na I D lines may be masked bythe strong RM signal seen at high resolution. We also rule out high-altitudeclouds, which would otherwise mask the absorption signal at low resolution, asthe source of the discrepancies. Conclusions. This work highlights thesynergies between different observing techniques, specifically low- andhigh-resolution spectroscopy, to fully characterise transiting exoplanetsystems.

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