'Counterpart' method for abundance determinations in H ii regions

Abstract We suggest a new way of determining abundances and electron temperatures in H ii regions from strong emission lines. Our approach is based on the standard assumption that H ii regions with similar intensities of strong emission lines have similar physical properties and abundances. A 'counterpart' for a studied H ii region may be chosen among H ii regions with well-measured abundances (reference H ii regions) by comparison of carefully chosen combinations of strong-line intensities. Then the abundances in the investigated H ii region can be assumed to be the same as those in its counterpart. In other words, we suggest to determine the abundances in H ii regions 'by precedent'. To get more reliable abundances for the considered H ii region, a number of reference H ii regions are selected and then the abundances in the target H ii region are estimated through extrapolation/interpolation. We will refer to this method of abundance determination as the counterpart method or, for brevity, the C method. We define a sample of reference H ii regions and verify the validity of the C method. We find that this method produces reliable abundances. Finally, the C method is used to obtain the radial abundance distributions in the extended discs of the spiral galaxies M83, NGC 4625 and NGC 628.