The infrared spectrum of Nova Cygni 1975

Reference is made to Grasdalen and Joyce (Nature 259:187 (1976)) who in observations of Nova Cygni noted a number of infrared lines, in addition to recombination lines of H as the dominant spectral feature. They tentatively identified these lines with forbidden transitions in highly ionised forms of Mg, Ca, Al, and Si. The prerequisite for such coronal lines is an emitting region of low density and extremely high temperature ( approximately 108 K) which must also span a considerable temperature range in order for species such as Al V, VI, VII, and IX to exist simultaneously. It is here proposed that these lines are all lines of He I, with one possible exception, and that their sudden appearance and increase in relative intensity accord with somewhat more temperate conditions ranging from approximately 104 K to 105 K. Such an identification is also consistent with the behaviour of very fast novas. The exception may relate to Ca IV. (U.K.)