On the composition of H II regions in southern galaxies. III. NGC 2997 and 7793.

AAT/IPCS spectra have been obtained for a part of the nuclear region and for four H II regions in the SABc galaxy NGC 2997, and for the nucleus and five H II regions in the prototype Sd Sculptor group galaxy NGC 7793. Additional observations of an H II region in the Scd galaxy NGC 300 extend previously reported results. Abundances are deduced from emission-line intensities. The abundance gradient found in NGC 7793 is very similar to that of NGC 300 and characteristic of late-type spirals, while that of NGC 2997 is milder (relative to the isophotal radius). The relationship between abundance gradients and other characteristics of spiral galaxies is discussed; the data suggest that the yield varies systematically according to local conditions, such as the surface density of stars in the disc. Calibration of line strengths in terms of abundance is discussed in an Appendix.