Spectrophotometric data of the central star of the large Magellanic Cloud planetary nebula N66: Quantitative analysis of its WN-type spectrum

Abstract

Hubble Space Telescope, IUE, and ground-based observations of the central star of the LMC planetary nebula N66 (CS N66), obtained in di†erent 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 Ðt observations from di†erent epochs. Fits provide the chemical composition, the fundamental stellar parameters L the mass- * , T* , R* , loss rate, and the wind velocity. From our best models we Ðnd that CS N66 is a very luminous He star (X/Y ¹0.1), 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 Ðtting data from di†erent 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 The short- and long-term stellar variations are produced by large changes *\0.71 R_. in the mass-loss rate, which varies by large factors, from M0 ¹8]10~7 M yr~1 in 1983 (preoutburst _ epoch) to M0 \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 brieÑy discussed.