THE SURFACE GRAVITIES OF COOL DWARF STARS REVISITED

Abstract

On the base of high-resolution spectra and standard model atmosphere analyses we propose to employ the pressure- broadened Mg Ib lines to derive the gravity parameter for F and G stars. These lines are advocated to be a much more robust and reliable tracer compared to the ionization equilibrium of, say, Fe I/Fe II,whichissusceptibletooverionizationeffectsandun- certaintiesinthetemperaturestructureofthemodelatmosphere. It is demonstrated that the strong line method circumvents the long-standing discrepancy (logg 0.5 dex) of the standard F star Procyon, the surface gravity of which is precisely known duetoitsnearnessandbinarynature.Wealsodiscusssimilaref- fects on other, predominantly metal-poor stars. In fact, many of the F and hotter G stars deviate in the LTE metal abundances of neutral and ionized species by up to 0.2 dex. However, once the surfacegravityparameterisxed,veryreliableironabundances from Fe II can be derived as well. As a consequence a number of stars considered to be standards will require revised stellar parameters in future analyses. This will have some impact on stellar distances, ages, and galactic evolution in particular.