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Astron. Astrophys. 337, 338-348 (1998)

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2. Data sample

The data we refer to for the comparison with log(N/O) versus 12+ log(O/H) are taken from Kobulnicky and Skillman (1996).

Thuan at al. (1995) presented high quality spectrophotometric observations of 15 supergiant HII regions in 14 BCG. We compared our model results with their data relative to O and Fe. Their data showed that none of the ratios N/O, Fe/O, Ne/O, Ar/O, S/O, depends on the oxygen abundance indicating that abundance ratios in these galaxies show the pollution only from massive stars and that all the elements, including nitrogen, should have a primary origin (see paragraph 3.1). However, there is a warning concerning this kind of interpretation which implicitely assumes that the star formation history in all the objects considered has been the same. In fact, it is very likely that the star formation history has been different in different galaxies and therefore one can draw this kind of conclusions only after comparing with detailed chemical evolution models.

We compared our predicted log(C/O) versus 12+log(O/H) with data from Garnett et al. (1995) who presented UV observations of 7 HII regions in low-luminosity dwarf irregular galaxies and the Magellanic Clouds (HST data). This allowed them to measure the carbon abundances and discuss the evolution of the C/O ratio in these galaxies. They found that the C/O ratio increases continuously with increasing O/H and suggested that this can be due to the fact that in the most metal poor galaxies massive star nucleosynthesis predominates (oxygen production), while in more metallic objects the delayed contribution of intermediate mass stars to carbon is more evident. However, their comparison with chemical evolution models is not really appropriate since they have adopted, for such comparison, models relative to the solar neighbourhood, where the star formation mechanism and evolution are definitively different from those of dwarf irregular galaxies. We considered also recent data on C/O measured in the NW and SE components of IZw18 by Garnett et al. (1997), again obtained by means of ultraviolet spectroscopy (HST data).

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© European Southern Observatory (ESO) 1998

Online publication: August 17, 1998