Astron. Astrophys. 320, 575-579 (1997)

1. Introduction

The infrared source, CRL 2688, is a reflection nebula (the "Egg Nebula") with remarkably symmetric optical lobes which are separated by dark lane (Ney et al.1975). The Hubble Space Telescope found search-light beams and multiple arclets in this source (Sahai et al. 1995). The central object is an evolved star with a spectral type of F5 (Crampton et al. 1975). Radio line observations revealed that the circumstellar envelope consists of rich carbon-bearing molecules (Zuckerman et al. 1976; Nguyen-Q-Rieu et al. 1988). High-resolution radio observations found a weak bipolarity in the high-velocity molecular outflow (Heiligman et al 1986), or an longated feature along the optical dark lane. (Kawabe et al. 1987, Bieging & Nguyen-Q-Rieu 1988). The overall distribution of CO emission is, however, found to be rather spherically symmetric (Truong-Bach et al. 1990). The high velocity components of about 20-30 km s^-1 of ¹³ CO are not oriented along the the bipolar axis (Yamamura et al. 1995, 1996). It has been believed that the CS molecule is a good probe for the dense disk in young stellar objects (e. g., Kaifu et al. 1983). However, in the evolved objects, a potential of the CS molecule as a disk probe is still unknown. The mapping observation of the CS 2-1 line for the alternative protoplanetary object, CRL 618, has shown that emission is not necessarily associated with the disk (Hajian et al. 1995). To study the CS distribution for the representative protoplanetary nebula CRL 2688, we have made high spatial-resolution observations.

© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998
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