STRUCTURE OF THE MOLECULAR ENVELOPE OF CRL-2688 BASED ON (CO)-C-13 OBSERVATIONS

We have studied the proto-planetary nebula CRL 2688 by mapping observa tions of the (CO)-C-13 J = 1-0 line. Data obtained by single-dish (Nob eyama 45 m) and interferometric (Nobeyama Millimeter Array) observatio ns are combined and reduced to maps of the whole flux of (CO)-C-13 emi ssion in the envelope. The resultant maps reveal that weak (CO)-C-13 e mission is extended by about 20 '' (corresponding to 3 x 10(17) cm) fr om 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 M.. Numerical mo del analyses of the velocity-channel maps are performed to obtain a th ree-dimensional density distribution in the envelope. They indicate th at the mass loss of the central star was drastically increased about 3 000 yr ago, reached a maximum rate of about 3 x 10(-4) M. yr(-1), and then abruptly stopped approximately 200 yr ago. The present (CO)-C-13 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-infrare d wavelengths, may be very small, less than a few arcseconds in size. It is suggested that the high-velocity flow detected in the (CO)-C-13 interferometric observations originates from the gas in the thick enve lope heated by the shock with collimated outflows.