The chemical composition gradient across M 33.

The abundance gradient in M33 is studied on the basis of IPCs and CCD data on emission lines in selected n n regions, using O II, O III, S II and S III in the wavelength range λλ3700-9600 A to refine the oxygen abundances in the inner parts as well as to study the behaviour of S/O. Spatially resolved observations in each H II 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 H II 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 decease with O/H in H II regions. This latter trend is rather unexpected from the viewpoint of nucleosynthesis theory.