A Consistency Test of Spectroscopic Gravities for Late‐Type Stars

Chemical analyses of late-type stars are usually carried out following theclassical recipe: LTE line formation and homogeneous, plane-parallel,flux-constant, and LTE model atmospheres. We review different results in theliterature that have suggested significant inconsistencies in the spectroscopicanalyses, pointing out the difficulties in deriving independent estimates ofthe stellar fundamental parameters and hence,detecting systematic errors. The trigonometric parallaxes measured by the HIPPARCOS mission provideaccurate appraisals of the stellar surface gravity for nearby stars, which areused here to check the gravities obtained from the photospheric iron ionizationbalance. We find an approximate agreement for stars in the metallicity range -1<= [Fe/H] <= 0, but the comparison shows that the differences between thespectroscopic and trigonometric gravities decrease towards lower metallicitiesfor more metal-deficient dwarfs (-2.5 <= [Fe/H] <= -1.0), which casts a shadowupon the abundance analyses for extreme metal-poor stars that make use of theionization equilibrium to constrain the gravity. The comparison with thestrong-line gravities derived by Edvardsson (1988) and Fuhrmann (1998a)confirms that this method provides systematically larger gravities than theionization balance. The strong-line gravities get closer to the physical onesfor the stars analyzed by Fuhrmann, but they are even further away than theiron ionization gravities for the stars of lower gravities in Edvardsson'ssample. The confrontation of the deviations of the iron ionization gravities inmetal-poor stars reported here with departures from the excitation balancefound in the literature, show that they are likely to be induced by the samephysical mechanism(s).Comment: AAS LaTeX v4.0, 35 pages, 10 PostScript files; to appear in The Astrophysical Journa