Near-infrared spectroscopy of the proto-planetary nebula CRL 618 and the origin of the hydrocarbon dust component in the interstellar medium

A new 2.8-3.8 mu m spectrum of the carbon-rich protoplanetary nebula CRL 61 8 confirms the previous detection of a circumstellar 3.4 mu m absorption fe ature in this object (Lequeux & Jourdain de Muizon). The high resolution an d high signal-to-noise ratio of our spectrum allow us to derive the detaile d profile of this absorption feature, which is very similar to that observe d in the spectrum of the Galactic center and also resembles the strong 3.4 mu m emission feature in some post-asymptotic giant branch stars. A weak 3. 3 mu m unidentified infrared band, marginally detected in the CRL 618 spect rum of Lequeux & Jourdain de Muizon, is present in our spectrum. The existe nce of the 3.4 pm feature implies the presence of relatively short-chained, aliphatic hydrocarbon materials (-CH2-/-CH3 similar or equal to 2-2.5) in the circumstellar environment around CRL 618. It also implies that the carr iers of the interstellar 3.4 pm feature are produced at least in part in ci rcumstellar material, and it calls into question whether any are produced b y the processing of interstellar ices in dense interstellar clouds, as has been previously proposed. Other features in the spectrum are recombination lines of hydrogen, rotational and vibration-rotation lines of molecular hyd rogen, and a broad absorption probably due to a blend of HCN and C2H2 bands .