THE SURFACE GRAVITIES OF COOL DWARF STARS REVISITED

On the base of high-resolution spectra and standard model atmosphere a nalyses we propose to employ the pressure-broadened Mg Ib lines to der ive the gravity parameter for F and G stars. These lines are advocated to be a much more robust and reliable tracer compared to the ionizati on equilibrium of, say, Fe I/Fe II, which is susceptible to overioniza tion effects and uncertainties in the temperature structure of the mod el atmosphere. It is demonstrated that the strong line method circumve nts the long-standing discrepancy (Delta log g similar to 0.5 dex) of the standard F star Procyon, the surface gravity of which is precisely known due to its nearness and binary nature. We also discuss similar effects on other, predominantly metal-poor stars. In fact, many of the F and hotter G stars deviate in the LTE metal abundances of neutral a nd ionized species by up to 0.2 dex. However, once the surface gravity parameter is fixed, very reliable iron abundances from Fe II can be d erived as well. As a consequence a number of stars considered to be st andards will require revised stellar parameters in future analyses. Th is will have some impact on stellar distances, ages, and galactic evol ution in particular.