'Counterpart' method for abundance determinations in Hii regions

We suggest a new way of determining abundances and electron temperatures in Hii regions from strong emission lines. Our approach is based on the standard assumption that Hii regions with similar intensities of strong emission lines have similar physical properties and abundances. A 'counterpart' for a studied Hii region may be chosen among Hii regions with well-measured abundances (reference Hii regions) by comparison of carefully chosen combinations of strong-line intensities. Then the abundances in the investigated Hii region can be assumed to be the same as those in its counterpart. In other words, we suggest to determine the abundances in Hii regions 'by precedent'. To get more reliable abundances for the considered Hii region, a number of reference Hii regions are selected and then the abundances in the target Hii 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 Hii 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, NGC4625 and NGC628. [PUBLICATION ABSTRACT]