Extragalactic chemical abundances: do HII regions and young stars tell the same story? The case of the spiral galaxy NGC 300

Abstract

(Abridged) We have obtained new spectrophotometric data for 28 HII regions in the spiral galaxy NGC 300, a member of the nearby Sculptor Group. The detection of auroral lines, including [OIII]4363, [SIII]6312 and [NII]5755, has allowed us to measure electron temperatures and direct chemical abundances for the whole sample. We determine for the first time in this galaxy a radial gas-phase oxygen abundance gradient based solely on auroral lines, and obtain the following least-square solution: 12+log(O/H)=8.57-0.41 R/R25, where the galactocentric distance is expressed in terms of the isophotal radius R25. The gradient corresponds to -0.077 dex/kpc, and agrees very well with the galactocentric trend in metallicity obtained for 29 B and A supergiants in the same galaxy. The intercept of the regression for the nebular data virtually coincides with the intercept obtained from the stellar data. This allows little room for depletion of nebular oxygen onto dust grains, although in this kind of comparison we are somewhat limited by systematic uncertainties, such as those related to the atomic parameters used to derive the chemical compositions. We discuss the implications of our result with regard to strong-line abundance indicators commonly used to estimate the chemical compositions of star-forming galaxies, such as R23. By applying a few popular calibrations of these indices based on grids of photoionization models on the NGC 300 HII region fluxes we find metallicities that are higher by 0.3 dex (a factor of two) or more relative to our nebular (Te-based) and stellar ones. We confirm a metallicity dependence of the `softness' parameter eta=(O+/O++)/(S+/S++), in the sense that softer stellar continua are found at high metallicity.