A SULFUR ABUNDANCE STUDY OF NGC-300 BY AN EMPIRICAL CALIBRATION METHOD

We propose an empirical sulphur abundance determination method based o n the strong sulphur emission lines, [S II] lambda lambda 6716, 6731 a nd [S III] lambda lambda 9069, 9531 in H II regions. From a compilatio n of literature data we have made a calibration of sulphur abundance v ersus ([S II] + [S III])/H beta, similar to what has been widely used for the more easily observable iso-electronic element oxygen. This ena bles abundance determinations in extragalactic H II regions without me asurements of weak temperature sensitive lines for use in model calcul ations. As a first application of the empirical calibration 15 spectra covering the wavelength range 3650-10000 Angstrom have been obtained of H II regions at various galactocentric distances in the spiral gala xy NGC 300, by employing specially designed multiaperture slitmasks wh ich allow simultaneous observations of several H II regions. Sulphur a nd oxygen abundances are determined and an oxygen gradient in agreemen t with previously published work is found as well as a somewhat steepe r sulphur abundance gradient. This results in a slight decrease of log S/O with increasing radius. The quite low values of S/O seem to confi rm the trend demonstrated by Diaz et al. (1991) of a decrease in log S /O at high metallicities, but still more data are needed before any de finite conclusions can be drawn. We find as expected that the radiatio n softness parameter eta decreases with increasing radius and the slop e is comparable to what is found for spiral galaxies of similar metall icity.