HST Spectrophotometric Data of the Central Star of the Planetary Nebula LMC-N66

Abstract

N66 (WS 35, SMP 83) is a Type I (He-N rich) PN in the LMC with a high ionization degree. It shows a bipolar morphology with a filamentary structure (Dopita et al. 1993). Its central star has shown very impressive changes, in short time scale, that have been investigated. Here we describe the history of these changes:

• 1. -In 1987, as derived from nebular parameters (Peña & Ruiz 1987), the central star had a weak continuum with T_* ~ 120 000 K and L_* ~ 25 000 L_⨀. From evolutionary tracks, a stellar mass of ~ 1M _⨀ was obtained, which is one of the highest of those that have been determined with reasonable accuracy.

• 2. -In 1990 the star began a spectacular mass-loss event. Its UV and optical continua increased by factors ranging from 2 to 4 magnitudes and it developed WR features resembling a Population I WN4.5 (or earlier) star (Peña et al. 1995).

• 3. -The strong wind has persisted for more than 5 years, showing some variations in the emission line ionization degree and a sudden disappearance of the WR features which lasted some weeks in August-September, 1995. From ground-based telescopes and the IUE satellite we have followed the central star evolution; the changes in the stellar fluxes are presented in Table 1.

• 4. -N66 central star is the only nucleus of a PN showing a WN type spectrum. All the other CSPNe with WR features (in the Galaxy and the Magellanic Clouds) are of WC type. 5.- These spectacular changes have no theoretical explanation at present. The possibilities that the star is undergoing a late thermal pulse or suffering a violent instability due to its high luminosity are being explored.