A sulphur abundance study of NGC 300 by an empirical calibration method

We propose an empirical sulphur abundance determination method based on the strong sulphur emission lines, [S ii] λλ6716, 6731 and [S iii] λλ9069, 9531 in H ii regions. From a compilation of literature data we have made a calibration of sulphur abundance versus ([S ii] + [S iii])/Hβ, similar to what has been widely used for the more easily observable iso-electronic element oxygen. This enables abundance determinations in extragalacticH ii regions withoutmeasurements of weak temperature sensitive lines for use in model calculations. As a first application of the empirical calibration 15 spectra covering the wavelength range 3650–10000 Å have been obtained of H ii regions at various galactocentric distances in the spiral galaxy NGC 300, by employing specially designed multiaperture slitmasks which allow simultaneous observations of several H ii regions. Sulphur and oxygen abundances are determined and an oxygen gradient in agreement with previously published work is found as well as a somewhat steeper 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 confirm the trend demonstrated by Dı́az et al. (1991) of a decrease in log S/O at high metallicities, but still more data are needed before any definite conclusions can be drawn. We find as expected that the radiation softness parameter η decreases with increasing radius and the slope is comparable to what is found for spiral galaxies of similar metallicity.