Helium triplet lines of the Nova Delphini 1967

This paper studies the neutral helium triplet emission lines identified in the spectrum of the envelope of Nova Delphini. By comparing the observed flux of the neutral helium lines with that calculated theoretically by Robbins, it is found that the optical thickness in the center of the line lambda 3889 is of the order of 21.50 for summer 1969. The optical thickness obtained by this method is here denoted tausub(tran)(lambda3889). On the other hand, the number of neutral helium atoms in the 23S state [N(23S)] is obtained by considering the equilibrium between the mechanisms that populate and depopulate this state. It is then found that the depopulation by photoionization due to the radiation of Lyα (H I), transitions to the 21S,21p and 23p states by electron collision, photoionization due to the continuum radiation of the central star, are 82.70%, 13.20%,2.40%,0.90% and 0.80% respectively. It is found that the mechanism of the photoionization by Lyα is the dominant mechanism of depopulation of 23S state. The author calculated tau(lambda 3889) of the order of 82.37 from N(23S), obtained in the preceeding paragraph. The optical thickness obtained by this method is here denoted tausub(bal)(lambda). The difference between tausub(tran)(lambda 3889) and tausub(bal)(lambda 3889) is very large and it cannot be attributed to calculation errors. It is concluded that the difference between tausub(tran)(lambda 3889) and tausub(bal)(lambda 3889) is due to the heterogeneity of the envelope of the Nova, already found in a previous study of the profiles of the permitted and forbidden lines. (Auth.)