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

Abstract

The absolute integrated fluxes that we obtained (Paper I) for 34 H ii regions, with Galactocentric distances RG in the 6.6–17.7 kpc range, in the emission lines [O ii] λλ3726 and 3729, [O iii] λλ4363 and 5007, He iλ5876, 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 [O iii] λ4363/λ5007 ratio in six H ii regions with RG between 6.6 and 14.8 kpc. These new measurements of Te, which are in good agreement with those from radio recombination lines, widen the RG range for which reliable Te measurements exist. Combining our new Te([O iii]) results with the radio values, we obtain the following temperature relationship: TeK]=(372±38)RG+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<RG<15 kpc is 12+log O/H= (−3.95±0.49)×10−2RG+(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 ΔY/Δ(O/H) value (≥180) measured in the Galaxy by Peimbert et al.

An estimate of the time variation of the O/H radial gradient from planetary nebulae

“…Several elements have been investigated, especially oxygen, sulphur, neon and argon in photoionized nebulae (Maciel & Quireza 1999;Deharveng et al 2000) and iron in open cluster stars and cepheids (Friel 1999;Andrievsky et al 2002aAndrievsky et al , 2002b. In the case of the O/H ratio, recent determinations from HII regions, PN and B stars point to an average gradient in the range −0.04 to −0.07 dex/kpc, taking into account an average uncertainty of 0.02 dex/kpc for most determinations.…”

The Oxygen Abundance in Our Galaxy

“…Let us compare this value with recent determinations by other investigators. Caplan et al (2000) and Deharveng et al (2000) have analysed Galactic H ii regions and have obtained the slope -0.0395 dex/kpc with a central oxygen abundance 12+log(O/H) = 8.82 and 12+log(O/H) = 8.48 at the solar galactocentric distance. Rodríguez (1999) considered seven bright Galactic H ii regions with galactocentric distances in the range 6 -10 kpc and has found that all the H ii regions studied are characterized by similar abundances, 12+log(O/H) ∼ 8.45±0.1.…”

“…The name of H ii region is given in column 1. The galactocentric distance taken from Deharveng et al (2000) is reported in column 2. The original oxygen abundance (O/H)SH from Shaver et al (1983) is listed in column 3.…”

“…The linear best fit to those data is presented by the dashed line in Fig.2. The galactocentric distances are from Deharveng et al (2000). The The galactocentric distances are from Deharveng et al (2000).…”

“…The galactocentric distances are from Deharveng et al (2000). The The galactocentric distances are from Deharveng et al (2000). The open squares are original oxygen abundances (O/H)SH from Shaver et al (1983), the dashed line is the best fit to those data.…”