On the ionization balance of cool stars: the role of surface gravity from Gaia

High-resolution spectroscopic studies of G and K-type dwarf stars have revealed higher iron abundances derived from singly ionized species compared to neutral species; violating the ionization equilibrium under the assumption of local thermodynamic equilibrium. The "ionization balance problem" has troubled astronomers in several works of open clusters (Hyades; Pleiades; and M34) but also for field stars.

Several hypotheses have been proposed to explain the differences between FeII and FeI abundances but with no fully satisfactory answer yet. In this work; we investigate the overabundances of FeII lines reported in our previous work for a sample of 451 solar-type HARPS stars in the solar neighborhood. The spectroscopic surface gravities of this sample which emerge from the ionization balance; appear underestimated for the K-type stars. In order to understand this behavior; we search our FeII line list for unresolved blends and outliers.

For our sample we use a set of reference parameters (effective temperature and metallicity) and derive surface gravities using the Gaia DR2 parallaxes (trigonometric gravities); to calculate the FeI and FeII abundances. We exclude the FeII lines which produce overabundances above 0.10 dex. The derived surface gravities from the clean line list are now in agreement with the trigonometric. Moreover; the difference between FeI and FeII abundance does not show now a correlation with the effective temperature. Finally; we show that the ionization balance of Ti can provide better estimates of surface gravities than iron.

The more accurate gravities based on the Gaia parallaxes are used to improve the spectroscopic values. In this work; we use these gravities to provide a solution to the ionization balance problem observed in the atmospheres of cool dwarfs.