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

The absolute integrated fluxes that we obtained (Paper I) for 34 Hii regions, with Galactocentric distances R_G in the 6.6-17.7kpc range, in the emission lines [Oii] λλ3726 and 3729, [Oiii] λλ4363 and 5007, Hei λ5876, Hα and Hβ, 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 [Oiii] λ4363/λ5007 ratio in six Hii regions with R_G between 6.6 and 14.8kpc. 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([Oiii]) 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 Hii region model. Our O/H relationship for 5kpc<R_G<15kpc is 12+logO/H= (-3.95+/-0.49)x10^-2R_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 15kpc. 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 ΔY/Δ(O/H) value (>=180) measured in the Galaxy by Peimbert et al.