A survey of CN in circumstellar envelopes

We have conducted a survey of CN N=2-1 and N=1-0 line emission in the envelopes of evolved stars. The sample consists of 42 objects, including C-rich and O-rich envelopes, S-stars, detached envelopes, and proto-planetary nebulae. Confident detections have been achieved in 30 objects. Both CN lines are bright in C-rich envelopes, and the 2-1 line has been detected in 5 O-rich objects (previously, CN had been detected in only one O-rich envelope). The excitation temperature T-rot, evaluated from the 2-1/1-0 intensity ratio, is similar to 3-6 K in most carbon stars, and greater than or equal to 10-20 K in O-rich envelopes. We find that the CN spectra display anomalies in the rotational, fine, and hyperfine line ratios. Anomalies in the rotational excitation appear in W Ori and UU Aur, two stars which are known to present HCN upsilon=0 J=1-0 masers. The excitation of the CN 2-1 line is unusually high in both objects, and UU Aur may present a weak maser effect in this line. Anomalies are also observed in the intensity ratios of the fine and hyperfine components. If such anomalies were due to the envelope thickness, the required line opacities would be excessively high, in particular for low mass-loss rate objects. We thus suggest that the observed anomalies are the result of an anomalous excitation. Pumping through the optical and near-IR bands seems to play a dominant role in the CN excitation. A comparison with previously published HCN data shows that the CN/HCN ratio of the total numbers of molecules in C-rich stars tends to be larger in the objects with lower mass-loss rate, supporting the idea that CN is mainly formed from the photodissociation of HCN. The average peak abundance of CN is similar to 1.9 10(-5) in C-rich objects, and is about 300 times smaller (similar to 6.6 10(-8)) in O-rich envelopes. The CN/HCN peak abundance ratio is similar to 0.45 in C-rich stars, in agreement with photodissociation chemical models, and similar to 0.04 in O-rich objects. This last value is about two orders of magnitude smaller than the predictions of standard chemical models, and suggest that CN is destroyed by additional mechanisms than photodissociation in O-rich envelopes.