Using the CTIO 4-m telescope , we have obtained optical spectra of HII regions in five Sculptor Group dwarf irregular galaxies .
We derive oxygen , nitrogen , and sulfur abundances from the HII region spectra .
Oxygen abundances are derived via three different methods ( the “ direct ” method , the empirical method guided by photoionization modeling of McGaugh ( 1991 ) , and the purely empirical method of Pilyugin ( 2000 ) ) and are compared .
Significant systematic differences are found between the three methods , and we suggest that a recalibration of the empirical abundance scale is required .
Until differences between these three methods are better understood , the issue of the degree of uniformity of the ISM abundances in a dwarf galaxy can not be properly addressed .
The N/O ratio for the metal-poor dI ESO 473-G24 of log ( N/O ) = - 1.43 \pm 0.03 lies well above the plateau of log ( N/O ) = - 1.60 \pm 0.02 found by Izotov & Thuan ( 1999 ) for a collection of metal-poor blue compact galaxies .
This shows that not all galaxies with 12 + log ( O/H ) \leq 7.6 have identical elemental abundance ratios , and this implies that the Izotov & Thuan scenario for low metallicity galaxies is not universal .
Measurements of the HII regions in NGC 625 yield log ( N/O ) \approx - 1.25 .
Assuming N production by intermediate mass stars , this relatively high N/O ratio may be indicative of a long quiescent period prior to the recent active burst of star formation .
The oxygen abundances in the Sculptor Group dIs are in good agreement with the relationship between metallicity and luminosity observed in the Local Group dIs .
Taken together the observations show a better relationship between metallicity and luminosity than between metallicity and galaxy central surface brightness .
The Sculptor Group dIs , in general , lie closer to the simple closed box model evolutionary path than the Local Group dIs .
The higher gas contents , lower average star formation rates , and closer resemblance to closed box evolution could all be indicative of evolution in a relatively low density environment .