SPECTROPHOTOMETRIC DATA OF THE CENTRAL STAR OF THE LARGE-MAGELLANIC-CLOUD PLANETARY-NEBULA N66 - QUANTITATIVE-ANALYSIS OF ITS WN-TYPE SPECTRUM

Hubble Space Telescope, IUE, and ground-based observations of the cent ral star of the LMC planetary nebula N66 (CS N66), obtained in differe nt 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 t he chemical composition, the fundamental stellar parameters L-, T-*, R-, the massloss rate, and the wind velocity. From our best models we find that CS N66 is a very luminous He star (X/Y less than or equal t o 0.1), with a small amount of N, undergoing a violent and unstable ma ss-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 indica te that the fundamental stellar parameters remain constant with time, with values log (L-/L-circle dot) = 4.53 +/- 0.10, T-* = 93,300 K, an d R- = 0.71 R-circle dot. The short-and long-term stellar variations are produced by large changes in the mass-loss rate, which varies by l arge factors, from (M)over dot less than or equal to 8 x 10(-7) M. yr( -1) in 1983 (preoutburst epoch) to (M)over dot = 2.5 x 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 pu lse. We propose that the event taking place in CS N66 was produced by an atmospheric instability similar to that triggering the giant erupti ons of Population I luminous blue variable stars. The possible mechani sm causing the atmospheric instability is briefly discussed.