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

The absolute integrated fluxes that we obtained (Paper I) for 34 H II regio ns, with Galactocentric distances R-G in the 6.6-17.7 kpc range, in the emi ssion 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/lamb da 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 measurement s exist. Combining our new T-e([O III]) results with the radio values, we o btain the following temperature relationship: T-e [K] = (372 +/- 38)R-G + 4 260 +/- 350. The O+/H+ and O++/H+ abundances have been obtained assuming a two-temperatu re 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 lo wer, by a factor of 2, than that previously obtained by Shaver et al. No si gnificant flattening of this relation is obtained out to 15 kpc. At the sol ar Galactocentric distance, 12 + log(O/H) = 8.48, in good agreement with th e 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 t han 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 t he Galaxy by Peimbert et al.