Spectroscopic Flickering in the WC8 Nucleus of the Planetary Nebula NGC 40

Abstract

The flat-topped profile of the nuclear C III λ5696 Å line in the WC8 nucleus of NGC 40 shows profile shape variations on time scales of hours. Relatively narrow intensity peaks and/or valleys (i.e., "features") appear first near the center of the 1400 km s^-1 wide C III line. Over the next ≈3 hr the features slide toward either the red or blue line edge with a Doppler shift that increases linearly with time. The apparent acceleration is nearly the same for all features. The size of the acceleration zone is at least 5 R_sun. At least one feature is visible at any time and the timing of the formation of features appears to be random. The fluxes of C III and many other nuclear lines are also variable on time scales of days and months. The optical attributes of the C III λ5696 Å line from the nucleus of NGC 40 are very similar to line profile variations reported in some normal" late-type WC stars by Robert, Moffat, and their collaborators. This suggests that the same wind acceleration mechanism (which they believe to be radiative acceleration of "blobs" forming in an unstable outfiowing wind) is an inherent property of local conditions in WR atmospheres. We were unable to detect any variability in the UV spectrum of NGC 40 using the lUE at low dispersion on two consecutive days.