A thermal pulse in progress in the nucleus of the LMC planetary nebula N66

Abstract

Optical and spectrophotometric ata, gathered in the 1987-1994 interval, are presented for the planetary nebula N66 (WS 35 or SMP 83) in the Large Magellanic Cloud. These data show that the central star has suffered dramatic changes. Before 1987 it showed a featureless continuum while its present spectrum shows intense W-R features and P-Cygni profiles which resemble a WN4.5 although the N lines are fainter than the He II lines as compared to normal Galactic WN stars. From the C IV lambda1550 P-Cygni profile it is found that upsilon(infinity) = 4200 +/- 500 km s-1. The intensity of the UV and optical continua have been increasing systematically and the absolute visual magnitude, M(V), has changed from + 1.15 +/- 0.30 mag in 1987 August to -2.01 +/- 0.20 mag in 1994 January. The temperature of the central object has diminished from 120,000 +/- 20,000 K to 50,000 +/- 10,000. The bolometric luminosity has decreased from log L/L. = 4.40 +/- 0.17 to log L/L. = 4.19 +/- 0.12 while the radius has increased from 0.37 +/- 0.07 to 1.7 +/- 0.5 R.. The object is probably undergoing a final helium flash. Most of the UV line intensities are 3 times higher in 1993 than in 1983; on the other hand, no significant variations have been detected in the nebular line intensity ratios in this period. The interesting and unusual behavior of N66 should be closely followed.