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For refcode 1988MNRAS.235..633V:
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Copyright by Royal Astronomical Society. 1988MNRAS.235..633V J.M. Vilchez, B.E.J. Pagel, Angeles I. Diaz, Elena Terlevich and M.G. Edmunds Instituto de Astrofisica de Canarias, 38200-La Laguna, Tenerife, Spain Royal Greenwich Observatory, Herstmonceux BN27 1RP Dpto. de Fisica Teorica, C-XI, Universidad Autonoma de Madrid, 28049-Madrid, Spain Astronomy Centre, Sussex University, Falmer, Brighton BN1 9QH University College, P.O. Box 78, Cardiff CF1 1XL Accepted 1988 May 26. Received 1988 May 25; in original form 1988 March 11 Summary. 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 {lambda}{lambda}3700-9600 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.
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