Resolving stellar atmospheres I. The Hα line and comparisons to microlensing observations

Context. We present work on Hα spectral line characteristics in PHOENIX stellar model atmospheres and their comparison to microlensing observations. Aims. We examine in detail the Ha equivalent width (EW) and the line shape characteristics for effective temperatures of 4500 K < T _eff < 5600 K where Ha is a strong spectral feature. Methods. We find that Hα EW in models calculated under the assumption of local thermodynamic equilibrium (LTE) is up to 15% smaller than in models without this assumption, non-LTE models (NLTE) and that line shapes vary significantly for the two model types. A comparison with available high quality microlensing data, capable of tracing Hα absorption across the face of one G5III giant, shows that the LTE model that fits the EW best is about 100 K hotter than and the best-fitting NLTE model has a similar T_eff as predicted by the spectral type analysis of the observed star but agree within the uncertainties of the observationally derived temperature. Results. Neither LTE nor NLTE models fit the line shape well. We suspect unmodelled chromospheric emission. Line shape diagnostics suggest lower gravities than derived for the star and are unacceptable low in the case of the LTE models. We show that EW alone is insufficient for comparison to stellar model atmospheres, but combined with a new shape parameter we define is promising. In stellar parameter ranges where the Ha line is strong, a NLTE approach of modeling stellar atmospheres is not only beneficial but mandatory.