LINE ANALYSIS AND NEBULAR ABUNDANCES IN H-II GALAXIES

Abstract

We present spectrophotometry of 42 southern H II galaxies (H II Gs) recently discovered in the Calan-Tololo Spectroscopic Survey. Physical conditions and He, N, Ne and O abundances of the ionized regions of these galaxies were derived. We found that most of the objects have 7.9 less-than-or-equal-to log O/H less-than-or-equal-to 8.3, that is, a factor between 10 and 4 lower than the solar value. The sample shows a tight correlation between Ne and O abundances that is well fitted by the linear regression log Ne/O = (1.18 +/- 0.07) log O/H - (2.21 +/- 0.54), with a mean value <log Ne/log O> = -0.70 +/- 0.08. For 10 H II Gs of the sample we found no correlation between N/O ratio and O abundance. The mean value of log N/O for these objects is - 1.41 +/- 0.10. Two other objects show a much larger N/O ratio. For a sample that includes the objects in this work and those reported by Campbell et al. (1986), we discuss the relation between some oxygen emission line intensity ratios and the physical conditions of the ionized region by comparing the observational data with the predictions of a grid of ionization structure models. An attempt is made to study the behavior of the effective temperature of the ionizing cluster and the ionization parameter for different metallicities. It is found that the mean T(eff) of the high O abundance H II Gs is lower than that in low abundance objects. We have estimated the ionization parameter for the regions in the sample and we derive a mean value of 0.005. From the H II Gs showing the highest ionization degree we deduced that, for a given O abundance, there is an upper limit, <U>max, for the ionization parameter. This upper limit increases smoothly with decreasing metallicity. In the range 7.8 less-than-or-equal-to log O/H less-than-or-equal-to 9.0, the behavior of <U>max is well represented by the expression: log <U>max = (-1.19 +/- 0.09)log O/H + (7.52 +/- 0.78).