EXTRAGALACTIC CHEMICAL ABUNDANCES: DO HII REGIONS AND YOUNG STARS TELL THE SAME STORY? THE CASE OF THE SPIRAL GALAXY NGC 300

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 several auroral lines, including [OIII] lambda 4363, [SIII] lambda 6312, and [NII] lambda 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(+/- 0.02) - 0.41(+/- 0.03)R/R-25, where the galactocentric distance is expressed in terms of the isophotal radius R-25. The characteristic oxygen abundance, measured at 0.4 x R-25, is 12 + log(O/H) = 8.41. The gradient corresponds to -0.077 +/- 0.006 dex kpc(-1), and agrees very well with the galactocentric trend in metallicity obtained for 29 B and A supergiants in the same galaxy, -0.081 +/- 0.011 dex kpc(-1). The intercept of the regression for the nebular data virtually coincides with the intercept obtained from the stellar data, which is 8.59(+/- 0.05). 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 R-23. 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 2) or more relative to our nebular (T-e based) and stellar ones. We detect Wolf-Rayet stellar emission features in similar to 1/3 of our HII region spectra, and find that in one of the nebulae hosting these hot stars the ionizing field has a particularly hard spectrum, as gauged by the softness parameter eta =(O+/O++)/(S+/S++). We suggest that this is related to the presence of an early WN star. By considering a larger sample of extragalactic HII regions we confirm, using direct abundance measurements, previous findings of a metallicity dependence of eta, in the sense that softer stellar continua are found at high metallicity.