Optical Recombination Lines of Heavy Elements in Giant Extragalactic H II Regions

We present high-resolution observations of the giant extragalactic H II regions NGC 604, NGC 2363, NGC 5461, and NGC 5471, based on observations taken with the ISIS spectrograph on the William Herschel Telescope. We have detected, for the first time, C II and O II recombination lines in these objects. We find that recombination lines give larger C++ and O++ abundances than collisionally excited lines, suggesting that temperature variations may be present in the objects. We detect [Fe IV] lines in NGC 2363 and NGC 5471, the most confident detection of optical lines of this kind in H II regions. Considering the temperature structure, we derive their H, He, C, N, O, Ne, S, Ar, and Fe abundances. From the recombination lines of NGC 5461 and NGC 5471, we determine the presence of C/H and O/H gradients in M101. We calculate the ΔY/ΔO and ΔY/ΔZ values considering the presence of temperature variations and under the assumption of constant temperature. We obtain a better agreement with models of galactic chemical evolution by considering the presence of temperature variations than by assuming that the temperature is constant in these nebulae.