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

Abstract

We propose an empirical sulphur abundance determination method based on the strong sulphur emission lines, [S II] 6716, 6731 and [S III] 9069, 9531 in H II regions. From a compilation of literature data we have made a calibration of sulphur abundance versus ([S II] + [S III])/Hbeta, similar to what has been widely used for the more easily observable iso-electronic element oxygen. This enables abundance determinations in extragalactic H II 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 3650-10000 AA 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 Diaz 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 eta decreases with increasing radius and the slope is comparable to what is found for spiral galaxies of similar metallicity.