Optical spectroscopy of the candidate luminous white dwarf in the young Large Magellanic Cloud cluster NGC 1818

Abstract

An optical spectrum of the Elson et al. (1998) candidate luminous white dwarf in the young LMC cluster NGC1818 shows conclusively that it is not a degenerate star. A model atmosphere fit gives Teff 31,500K and log g 4.4, typical of a garden-variety main sequence B star. However, if it is a true LMC member then the star is underluminous by almost three magnitudes. Its position in the cluster colour-magnitude diagram also rules out the possibility that this is an ordinary B star. The luminosity is, however, consistent with a ~0.5M⊙ post-AGB or post-EHB object, although if it has evolved via single star evolution from a high mass (7.6−9.0M⊙) progenitor then we might expect it to have a much higher mass, ~0.9M⊙. Alternatively, it has evolved in a close binary. In this case the object offers no implications for the maximum mass for white dwarf progenitors, or the initial-final mass relation. Finally, we suggest that it could in fact be an evolved member of the LMC disk, and merely projected by chance onto NGC1818. Spectroscopically, though, we cannot distinguish between these evolutionary states without higher resolution (echelle) data.