Oxygen and helium abundances in Galactic H ii 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 ] λλ 3726 and 3729, [O  iii ] λλ 4363 and 5007, He  i λ 5876, H α and H β , 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 ] λ 4363/ λ 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)×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 Δ Y /Δ(O/H) value (≥180) measured in the Galaxy by Peimbert et al.