The infrared spectrum of Nova Cygni 1975

Abstract

GRASDALEN and Joyce¹ have observed emission lines from Nova Cygni 1975 in the 2 and 3 µm spectral regions. In mid-September 1975, the dominant spectral features were recombination lines of hydrogen. Grasdalen and Joyce noted the presence of a number of additional lines in a later spectrum and identified these with forbidden transitions in highly ionised forms of Mg, Ca, Al, and Si. The prerequisite for formation of such coronal lines is an emitting region of low density and extremely high temperature, T∼10⁶ K, which, moreover, must span a considerable range of temperature in order for species such as A1 V, VI, VIII and IX to exist simultaneously. This would seem to preclude an isothermal shock as the mechanism responsible for a high temperature region in Nova Cygni. We propose a more conservative identification of these new lines, namely that they are all lines of He I with one possible exception. Furthermore, their sudden appearance and increase in relative intensity are in accord with somewhat more temperate conditions ranging from T∼10⁴ K in September 1975 to T∼10⁵ K in October. This identification is also more consistent with the known behaviour of very fast novae, which rarely exhibit optical coronal emission lines².