Time-dependent behavior and physical conditions of the LMC planetary nebular N66

Abstract

Optical and UV spectrophotometric data for the planetary nebula N66 in the Large Magellanic Cloud gathered in the 1987-1994 interval are analyzed. The central star, which had a featureless weak continuum, has developed intense W-R features and P Cygni profiles that were not present before 1990. He- and N-rich material, processed during the CNO cycle, is being ejected; the ejecta are very H-deficient, with X/Y < 0.1. A spectral type of WN4.5 is confirmed for the central object. From the optical broad emission lines a wind velocity of about 3000 km s-1 is found in fair agreement with the value upsilon(infinity) = 4200 km s-1 found from the C IV lambda 1550 P Cygni profile. The intensity of the UV and optical continua have been increasing systematically, and the absolute stellar visual magnitude, M(V), has changed from + 1.24 mag in 1987 August to -22.23 mag in 1994 March. The stellar temperature has diminished from greater-than-or-equal-to 120,000 K to congruent-to 50,000 K, while the radius has increased from 0.37 to 1.7 R.. The object is probably undergoing a final helium shell flash. The present evolutionary stage of N66 is discussed. On the other hand, the intensities of the nebular emission lines have shown no significant variation in the last 20 years. The nebular chemical abundances, calculated by considering no temperature fluctuations, are He/H = 0.116 +/- 0.004, log C/H = 7.45 +/- 0.10, log N/H = 7.95 +/- 0.12, log O/H = 8.24 +/- 0.06, log Ne/H = 7.70 +/- 0.06, log Ar/H = 6.12 +/- 0.04, and log S/H = 6.70 +/- 0.10. N66 shows He and N enrichment, while C appears very depleted relative to LMC H II regions and planetary nebulae; the other heavy element abundances (O, Ne, Ar, and S) are slightly lower than in H II regions, confirming that N66 is a type I planetary nebula. No evidence of C enrichment of the stellar atmosphere due to third dredge-up episode is detected.