Classical novae in outburst: evolution of the ultraviolet emission lines in CO novae

Abstract

A comparative study is made of the temporal changes of the ionization level in seven CO novae well monitored with the IUE satellite at low resolution. The investigation is based on the evolution of the fluxes in eight strong ultraviolet emission lines covering a wide range of ionization potentials, from Mg II (7.6 eV) to N V (77.5 eV). We find that the rate of increase of the ionization conditions in the emitting regions is uniform in the sample of novae considered, provided the time scale is normalized to the optical decay time t_3. The implications of this result for the nova ejected masses and for the nuclear turnoff time are discussed.