Previous abstract Next abstract
Iron and Nickel abundances in H~II regions and supernova remnants (SNRs) are known to be abnormal compared to cosmic abundances. Estimates of the Iron abundance are up to ten times lower than solar (thought to be due to grain depletion), and the abundance of Nickel ranges from about solar to several times solar for many different objects, such as Seyfert galaxies and SNR's. The deduced Ni/Fe ratio could therefore be up to 50 times the solar value. Following our earlier work, Bautista et al. (1994) and Bautista \& Pradhan (1995), using new atomic data for Fe~II and Ni~II, we estimate the abundance of Iron with respect to Oxygen in samples of H~II regions and SNRs by using Optical and IR [Fe~II] lines emanating from Partially Ionized Zones (PIZs). The same methods are also used to derive the Nickel abundance from [Ni~II] lines. It is found that the Fe/O ratio is about solar for all the objects studied, in contrast to the estimates by other authors. The Ni/Fe abundance ratio, however, is still greater than solar, ranging from about five times solar in the Orion nebula and P Cyg, to 10 times solar in most SNRs, and to 20 times solar in the Crab. We also investigate the recently proposed model of continuum fluorescent excitation of [Ni~II] (Lucy, 1995), and the same mechanism for [Fe~II], and find that such excitation mechanism is likely to be unimportant in the objects considered, and that the enhanced Fe and Ni emission is dominated by the high density regions. We conclude that the Fe/O gas abundance ratio in H~II region and SNRs may be much closer to the value observed in stars, close to solar, than previously estimated. The Nickel abundance, on the other hand, remains uncertain and further investigation concerning other excitation mechanisms for the [Ni~II] lines and the ionization conditions in the PIZs is necessary.\\ $\star$ Work supported by NSF grant PHY-9421898.
Wednesday program listing