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Sakurai's Object—A Possible Final Helium Flash in a Planetary Nebula Nucleus

UBVRIJK photometry and optical spectroscopy of Sakurai's object and direct imaging of the surrounding nebulosity are presented. The progenitor star is identified as a faint blue object of mj = 21. The circular planetary nebula has a diameter of 32'', its distance, derived from the Hβ flux, is 5.5 kpc. At present, the object is a luminous supergiant of approximate spectral type F2, having MV = -4.1, and has clear indications of hydrogen underabundance, with carbon and oxygen overabundance. Its photospheric radial velocity is the same as that of the surrounding planetary nebula, indicating that they are associated, and that there is currently no mass loss by a thick wind. A marginal IR excess suggests the presence of some hot dust.