CRL 2688 and CRL 618 - Proto-planetary nebulae

Abstract

The letter reports observations of 2.6-mm CO emission from the peculiar IR objects CRL 2688 (the Egg Nebula and CRL 618. A parabolic line shape is fitted to the observed CO emission spectrum of CRL 2688, molecular-envelope expansion velocities of approximately 20 km/s are deduced for both objects, and physical parameters of the CRL 2688 molecular envelope are estimated, including the mass-loss rate. The large line width, parabolic line shape, and other factors are shown to indicate optically thick emission from uniformly expanding envelopes which presumably originated from the central stars. It is suggested that the F5 Ia central star in CRL 2688 is in, or has recently passed through, the carbon-star stage, that the B0 central star in CRL 618 has evolved in less than 10,000 years from a cool (perhaps carbon-rich) star, and that CRL 618 is an extremely young planetary nebula. It is concluded that both objects may represent some intermediate stage between red giants and planetary nebulae and that the central stars can be identified with points along the theoretical evolutionary track of transitions from a red giant to a blue nucleus after ejection of a planetary nebula.