We have studied the proto-planetary nebula CRL 2688 by mapping observations of the 13CO J = 1-0 line. Data obtained by single-dish (Nobeyama 45 m) and interferometric (Nobeyama Millimeter Array) observations are combined and reduced to maps of the whole flux of 13CO emission in the envelope. The resultant maps reveal that weak 13C0 emission is extended by about 20" (corresponding to 3 x 1017 cm) from the central star. Part of the extended emission spreads toward the south of the source, although no counterpart is seen in the north. The mass of this component is estimated to be about 0.08 Msun. Numerical model analyses of the velocity-channel maps are performed to obtain a three- dimensional density distribution in the envelope. They indicate that the mass loss of the central star was drastically increased about 3000 yr ago, reached a maximum rate of about 3 x 10-4 Msun yr-1, and then abruptly stopped approximately 200 yr ago. The present 13CO observations show no evidence for the presence of a large axisymmetric disk in the bright core region. The disk, which is inferred from the shape of the bipolar reflection nebula at the optical and near-infrared wavelengths, may be very small, less than a few arcseconds in size. It is suggested that the high-velocity flow detected in the 13CO interferometric observations originates from the gas in the thick envelope heated by the shock with collimated outflows.
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Yamamura et al. (1996) studied this question.