IUE observations of the shells of RR Pic and GK Per

Abstract

We present ultraviolet, observations of the resolved shells of the old novae RR Pic (1925) and GK Per (1901). On the assumption that the RR Pic nebula is photoionized by the ultraviolet continuum emitted by the stellar remnant, we conclude that nitrogen is overabundant relative to solar in both the ‘equatorial ring’ and ‘polar blob’ regions; there are steep gradients in the C/O ratio, however. We also find possible evidence for emission by molecular hydrogen in the ‘blob’. There is, in addition, some evidence for abundance gradients in the GK Per nebula, in which excitation is likely to be by shocks rather than by the weak ultraviolet emission of the stellar remnant. The existence of abundance gradients in old nova shells is consistent with the infrared behaviour of recent novae, and must arise during the thermonuclear runaway itself.