High-resolution CO Observation of the Carbon Star CIT 6 Revealing the Spiral Structure and a Nascent Bipolar Outflow

CIT 6 is a carbon star in the transitional phase from the asymptotic giant branch (AGB) to the protoplanetary nebulae (pPNs). Observational evidences of two point sources in the optical, circumstellar arc segments in an HC₃N line emission, and a bipolar nebula in near-infrared provide strong support for the presence of a binary companion. Hence, CIT 6 is very attractive for studying the role of companions in the AGB-pPN transition. We have carried out high-resolution ¹²CO J = 2-1 and ¹³CO J = 2-1 observations of CIT 6 with the Submillimeter Array combined with the Submillimeter Telescope (single-dish) data. The ¹²CO channel maps reveal a spiral-shell pattern connecting the HC₃N segments in a continuous form and an asymmetric outflow corresponding to the near-infrared bipolar nebula. Rotation of the ¹²CO channel peak position may be related to the inner spiral winding and/or the bipolar outflow. An eccentric orbit binary is suggested for the presence of an anisotropic mass loss to the west and a double spiral pattern. The lack of interarm emission to the west may indicate a feature corresponding to the periastron passage of a highly eccentric orbit of the binary. Spatially averaged radial and spectral profiles of ¹²CO J = 2-1 and ¹³CO J = 2-1 are compared with simple spherical radiative transfer models, suggesting a change of ¹²CO/¹³CO abundance ratio from ∼30 to ∼50 inward in the CSE of CIT 6. The millimeter continuum emission is decomposed into extended dust thermal emission (spectral index ∼ -2.4) and compact emission from radio photosphere (spectral index ∼ -2.0).