Spectrophotometric studies of gaseous nebulae. XXIV. The amorphous low-excitation planetary IC 4593.
Photoelectrically calibrated line intensity measurements are presented for IC 4593, whose spectrum has also been observed photographically (Mount Wilson) and with the Wampler-Robinson super-scanner device (Lick). This relatively homogeneous nebula shows little evidence of dumpiness, as illustrated by the characteristic radiations of [0 1], [S II], and [N II]. An estimate of the chemical composition from ionic concentrations, based on extrapolation procedures of the type suggested by Seaton, is compared with results of a model calculation by Buerger. For most elements the agreement is reasonably good, but there is an order of magnitude discordance for [N II]. It is suggested that great caution must be used with extrapolation of [N II] lines to estimate nitrogen abundance in gaseous nebulae. It is concluded that nitrogen abundances estimated from extrapolation of [N II] line data can sometimes be in error. Subject headings: abundances, nebular - planetary nebulae