The metallicity gradient of M 33:: chemical abundances of HII regions

Context. We present spectroscopic observations of a sample of 72 emission-line objects, including mainly HII regions, in the spiral galaxy M 33. Spectra were obtained with the multi-object, wide field spectrograph AF2/WYFFOS at the 4.2 m WHT telescope. Line intensities, extinction, and electron density were determined for the whole sample of objects. Aims. The aim of the present work was to derive chemical and physical parameters of a set of HII regions, and from them the metallicity gradient. Methods. Electron temperatures and chemical abundances were derived for the 14 HII regions where both [OII] and [OIII] emission line fluxes were measured, including the electron temperature sensitive emission line [OIII] 436.3 nm and in a few cases [NII] 575.5 nm. The ionization correction factor (ICF) method was used to derive the total chemical abundances. Results. The presence of abundance gradients was inferred from the radial behaviour of several emission-line ratios, and accurately measured from chemical abundances directly derived in 14 HII regions. The oxygen abundances of our HII regions, located in the radial region from similar to 2 to similar to 7.2 kpc, gave an oxygen gradient -0.054 +/- 0.011 dex kpc(-1) Conclusions. The overall oxygen gradient for M 33 obtained using ours and previous oxygen determinations in a large number of HII regions with direct electron temperature determination as well as abundance in young stars presented a two slope shape: -0.19 dex kpc-1 for the central regions (R < 3 kpc), and -0.038 dex kpc-1 for the outer regions (R >= 3 kpc).