Subluminous Wolf-Rayet Stars: Observations

Abstract

Fully ten percent of the Wolf-Rayet stars known in our galaxy are central stars of planetary nebulae. These stars are highly-evolved stars of approximately one solar mass. The existence of WR characteristics in the spectra of central stars as well as in massive, younger stars proves conclusively that WR spectra are a phenomenon that occurs in hot stars of widely different masses and evolutionary histories. Just what this phenomenon is — the nature of the instability that drives the wind — is not known, but the phenomenon is associated with a prior event in the star's evolutionary history. The common event that links Wolf-Rayet stars is loss of most, if not all of the outer, hydrogen-rich envelope just prior to the onset of WR characteristics. As Tutakov (this volume) and others have emphasized, there are many ways for a star to lose its H-rich envelope. These ways include mixing, ejection via a wind, or, in the case of a close binary, ejection via Roche-lobe overflow. The existence of WR-type central stars implies simply that we should include one more path leading to the Wolf-Rayet phenomenon, and that is, ejection to form a planetary nebulae.