2007ApJ...654..978P

In previous works we have modeled the different gas regions of the proto-planetary nebula CRL 618 by studying the large number of lines from the cyanopolyynes detected in a millimeter-wave line survey of this object. In this work we retrieve the rotational temperatures (T_r) and abundance ratios R with respect to HC₃N (used as reference) for all molecular species detected in the survey by running grids of models in the (R,T_r) space to find the minimum of a weighted χ² defined for this analysis. This provides the best knowledge to date of the (polar) molecular composition of CRL 618 thanks to the uniform calibration of the whole survey and the large number of lines available from each species, allowing comparisons with predictions made by chemical models of C-rich post-AGB objects. A significantly lower value of the ¹²C/¹³C ratio has been revealed in the gas closest to central star with respect to the colder and outer envelope. It can be due to ¹³C-rich material, produced in a late CNO cycling occurred in the central star, being currently injected into this inner gas envelope.