Problems of ionization of nova envelopes in the nebular stage. The models for Nova Delphini 1967

A sequence of stationary photoionization models of the envelope of Nova Del 1967 in the nebular stage have been constructed. The results of calculations have been compared with spectroscopic observations of the nova made in June, 1969 - August, 1975. The observed intensities of emission lines cannot be entirely matched by the models of the envelope in which a single blackbody spectrum of the ionizing radiation is assumed; the computed intensities of [Ne V], [O II] and [N II] lines are then 4-100 times smaller than the observed ones. After detailed discussion the author comes to the conclusion that the ionizing radiation is probably produced by two sources with different temperatures. The observational data obtained in June-July, 1969 have been well matched by the models assuming that the ionizing radiation is a sum of two blackbody distributions with temperatures: approximately 2.5 x 105 K and approximately 4 x 104 K. The hotter spectrum is attributed to the hot atmosphere of the primary whereas the cooler one is probably produced by the heated secondary and/or the outburst remnant transformed into a differentially rotating disk. (Auth.)