The abundance gradient in M 33 is studied on the basis of IPCS and CCD data on emission lines in selected HII regions, using OII, OIII, SII, and SIII in the wavelength range λλ3700–9600 Å to refine the oxygen abundances in the inner parts as well as to study the behavior of S/O. Spatially resolved observations in each HII region permit the ionization structure to be studied and enable ionization parameters and stellar effective temperatures to be separately determined. The main results are: (i) the hardness of the ionizing radiation and the ionization parameter increase outwards (with diminishing abundances) along the galactic radius; (ii) the O/H gradient is steep in the inner regions, but much flatter in the outer regions; (iii) N/O is constant over most of the visible disc, but lower in an outer HII region; and (iv) the S/H gradient is shallower than the O/H gradient exemplifying what appears to be a universal trend for S/O to decrease with O/H in HII regions. This latter trend is rather unexpected from the viewpoint of nucleosynthesis theory.
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Vı́lchez et al. (1988) studied this question.