Aperture synthesis observations of molecules in CRL 2688

We have used the BIMA-Hat Creek interferometer to map the line emission of H¹³CN, HC₃N, HNC, and SiS at 3 mm toward the bipolar object CRL 2688 (the " Egg nebula "). The molecular emission distributions show a slight departure from spherical geometry and have a major axis (∼10″) almost aligned with the direction of the dust lane of the Egg nebula. The SiS emission is less extended than that of the organic molecules and is confined to the central region of the envelope. A pedestal emission at both redshifted (∼ -15 to ∼ -5 km s^-1) and blueshifted (∼ -65 to ∼ -45 km s^-1) wings detected in the H¹³CN, and HC₃N spectra may arise from a bipolar outflow or a time-variable wind corresponding to a high-speed wind phase. The abundances of H¹³CN, HC₃N, and HNC in CRL 2688 are comparable to those observed in IRC + 10216. SiS is depleted in CRL 2688 by an order of magnitude with respect to the average SiS abundance observed in IRC +10216. It is shown that the determination of the abundances of HCN and H¹³CN is strongly affected by the self-absorption of the optically thick blended hyperfine components in the expanding envelope. If self-absorption is ignored, the HCN and H¹³CN abundances could be underestimated by a factor of ∼3.