Abstract

Hubble Space Telescope, IUE, and ground-based observations of the central star of the LMC planetary nebula N66 (CS N66), obtained in different epochs, are presented. Since 1990, CS N66 has displayed remarkable short- and long-term spectroscopic and photometric changes amounting to more than 3 mag in the optical. Expanding model atmospheres have been constructed to fit observations from different epochs. Fits provide the chemical composition, the fundamental stellar parameters L*, T*, R*, the mass-loss rate, and the wind velocity. From our best models we find that CS N66 is a very luminous He star (X/Y <= 0.1), with a small amount of N, undergoing a violent and unstable mass-loss event. The photospheric chemical abundances correspond to the equilibrium CNO nuclear burning values, while the nebula has a normal chemical composition. Models fitting data from different epochs indicate that the fundamental stellar parameters remain constant with time, with values log (L*/L⊙) = 4.53 +/- 0.10, T* = 93,300 K, and R* = 0.71 R⊙. The short- and long-term stellar variations are produced by large changes in the mass-loss rate, which varies by large factors, from Ṁ<=8×10-7 M⊙ yr-1 in 1983 (preoutburst epoch) to Ṁ=2.5×10-5 M⊙ yr-1 in early 1995 (maximum stellar brightness). No evidence was found to support the suggestion that the outburst was due to a late thermal pulse. We propose that the event taking place in CS N66 was produced by an atmospheric instability similar to that triggering the giant eruptions of Population I luminous blue variable stars. The possible mechanism causing the atmospheric instability is briefly discussed. Based on observations made with the NASA/ESA Hubble Space Telescope obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.