2003A&A...409..969H

The central star of the planetary nebula N66 (alias WS35, SMP83 and HV5967) in the Large Magellanic Cloud enhanced its brightness dramatically in 1993 and 1994. Within the subsequent four years it returned to the previous level. Its spectrum resembles that of a Wolf-Rayet star of the nitrogen sequence (WN4.5). We monitored the object intensively from ground and with the Hubble Space Telescope. Now we present the complete set of spectroscopic observations from the different epochs before, during and after the brightness outburst of N66. The stellar spectra from the different epochs are analyzed in detail by means of most advanced non-LTE models for expanding stellar atmospheres. The main results are: the luminosity, logL/L_☉=4.6, before and after the outburst is exceptionally high for a central star of a planetary nebula. During the outburst in 1994, it even climbed up to logL/L_☉=5.4 for about one year. The effective temperature of about 112kK remained roughly constant, i.e. the luminosity mainly increased because of a larger effective stellar radius. The mass loss rate increased from 10^–5.7M_☉/yr in the quiet state to 10^–5.0M_☉/yr during the outburst. The chemical composition of the stellar atmosphere is that of incompletely CNO-processed matter: it is dominated by helium with a rest of hydrogen, nitrogen being slightly enhanced and carbon strongly depleted. We extensively discuss possible scenarios for the nature and evolutionary origin of N66, which should explain the exceptional stellar parameters, the atmospheric composition, the outburst mechanism, and the existence of the bipolar nebula which was ejected only a few thousand years ago and contains about 0.6 solar masses of hydrogen-rich matter. If being a single star, N66 might be (I) a low-mass star after the Asymptotic Giant Branch, as usually adopted for central stars of planetary nebulae, (II) a massive, i.e. non-degenerate star, or (III) a merger produced from two white dwarfs. Although there are no direct indications for binarity, we alternatively discuss whether N66 might be (IV) a massive star which lost its hydrogen envelope in a recent common-envelope phase with a less massive companion, or (V) a white dwarf accreting mass from a companion with a high rate. None of the scenarios is free of any contradiction to at least one of the observational facts. However, the binary scenarios pose less severe problems. If N66 is a white dwarf accreting matter in a close-binary system, its present accretion rate would bring it to the Chandrasekhar limit within a few hundred thousand years. Thus N66 might be a candidate for a future type Ia supernova explosion in our cosmic neighborhood.