SIMBAD references

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 multiepoch, high-dispersion spectra of the star and have found temporal variation in the Hα line profile over a time span of 1 week. The observed width and strength of the variable Hα 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 He II and [N II] 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.