Planetary Nebula with a Peculiar Spectrum in the Ultraviolet - CN:3-1

Abstract

We have obtained IUE ultraviolet spectra of the low-excitation planetary nebula CN 3-1. The recordings show the well-known doublet 2800 MgII and resonance line 2852 MgI as strong absorption lines. We show that these lines cannot be of interstellar origin and that they may be formed in two envelopes surrounding the main nebula: in the transition zone (doublet 2800 MgII) and in the neutral envelope (line 2852 MgI). These envelopes possess an important property: they contain dust particles, and even a moderate amount of such particles may influence the strength of the absorption lines of MgII and MgI. The emission line 4686 HeII has no relation to the nebula CN 3-1 and belongs to its nucleus, a star of type WR. It is very probable that the nucleus of this nebula is a binary system with a WR component (T_*=50 000 K), exciting the helium lines, and a star of B0 or B2 type (T_*=26 000 K) exciting the nebular linesN₁+N₂ [OIII], 3727 [OII], hydrogen lines, etc.