Variable H-alpha Line Emission from the Central Star of the Helix Nebula

Abstract

The central star of the Helix Nebula is a hot white dwarf with unexpected hard X-ray emission. With an effective temperature of ~100,000 K, the star is a powerful source of H-ionizing radiation; the atmosphere of a stellar or planetary companion, if present, will be ionized and emit recombination lines. To probe the origin of hard X-ray emission from the Helix Nebula's central star, we have obtained multi-epoch high-dispersion spectra of the star, and found temporal variation in the H-alpha line profile over a time span of one week. The observed width and strength of the variable H-alpha emission component are consistent with the hypothesized dMe companion proposed to explain the hard X-ray emission. A dMe companion, however, cannot explain the possible detection of variable HeII and [NII] emission. Follow-up spectroscopic monitoring of the Helix Nebula central star is needed to better establish the identification of the spectral lines and their temporal behavior in order to determine the origin of the optical variability and hard X-ray emission.