Oxygen and helium abundances in Galactic HII regions -: II.: Abundance gradients

The absolute integrated fluxes that we obtained (Paper I) for 34 H II regions, with Galactocentric distances R-G in the 6.6-17.7 kpc range, in the emission lines [O II] lambda lambda 3726 and 3729, [O III] lambda lambda 4363 and 5007, He I lambda 5876, H alpha and H beta, are analysed to derive the extinctions, the electron densities and temperatures, the ionic abundances O+/H+, O++/H+ and He+/H+ and the O/H abundances. The electron temperature has been derived from the [O III] lambda 4363/lambda 5007 ratio in six H II regions with R-G between 6.6 and 14.8 kpc. These new measurements of T-e, which are in good agreement with those from radio recombination lines, widen the R-G range for which reliable T-e measurements exist. Combining our new T-e([O III]) results with the radio values, we obtain the following temperature relationship: T-e [K] = (372 +/- 38)R-G + 4260 +/- 350. The O+/H+ and O++/H+ abundances have been obtained assuming a two-temperature H II region model. Our O/H relationship for 5 kpc < R-G < 15 kpc is 12 + log O/H = (-3.95 +/- 0.49) x 10(-2) R-G + (8.82 +/- 0.05). The slope is lower, by a factor of 2, than that previously obtained by Shaver et al. No significant flattening of this relation is obtained out to 15 kpc. At the solar Galactocentric distance, 12 + log(O/H) = 8.48, in good agreement with the O/H abundance measured in the local interstellar medium by Meyer et al., and a factor of 2 lower than the solar abundance. Our helium observations show that even a region ionized by a star earlier than O6.5 may contain a significant amount of neutral helium. We confirm the high Delta Y/Delta(O/H) value (greater than or equal to 180) measured in the Galaxy by Peimbert et al.