SIMBAD references

A high-dispersion optical (3700-10100 Å) spectrum was secured with the Hamilton echelle spectrograph at the coudé focus of the 120 inch (3 m) Shane telescope at Lick Observatory, with the goal of deriving chemical abundances for the compact planetary nebula IC 4846. We also remeasured the UV spectra from the IUE archive. Diagnostics indicate that the entire planetary nebula may be represented by an electron density N_ε≃9000 cm^–3 and electron temperature T_ε≃10,500 K. However, diagnostics seem to imply that the [O II] and [S II] zone electron temperatures are higher than the [O III] radiating strata. The electron density of the [O II] and [S II] zone might be higher as well (for which N_ε≃20,000 cm^–3). The photoionization model, which represents most of the observed line intensities, seems to confirm this physical condition, although the actual situation may be much more complicated. On the basis of the semiempirical ionization correction method and the photoionization model calculation, we derived the chemical abundances in this planetary nebula. Our optical data were compared with the image-tube scanner data from Aller & Czyzak. Apparently, there is a long-term spectral line variation in IC 4846. Chemical abundances derived in the present study agree, within 30%, with those previously estimated in Aller & Czyzak, except for carbon; the C, N, O, and Ne ratios are lower than both the average planetary nebula and the solar abundance. IC 4846 is a metal-deficient planetary nebula that may have evolved from a Population II progenitor.