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, [SII]λλ6716, 6731 and [SIII]λλ9069, 9531 in HII regions. From a compilation of literature data we have made a calibration of sulphur abundance versus ([SII]+[SIII])/Hβ, similar to what has been widely used for the more easily observable iso-electronic element oxygen. This enables abundance determinations in extragalactic HII regions without measurements of weak temperature sensitive lines for use in model calculations. As a first application of the empirical calibration 15 spectra covering the wavelength range 3650-10000Å have been obtained of HII regions at various galactocentric distances in the spiral galaxy NGC 300, by employing specially designed multiaperture slit-masks which allow simultaneous observations of several HII 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 logS/O with increasing radius. The quite low values of S/O seem to confirm the trend demonstrated by Diaz et al. (1991MNRAS.253..245D) of a decrease in logS/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.