Detection of HCN Direct l - Type Transitions Probing Hot Molecular Gas in the Proto - Planetary Nebula CRL 618

We report the detection of direct ℓ-type transitions towards the proto-planetary nebula CRL 618 during a study of vibrationally excited carbon chains. ∆J = 0 transitions of HCN in its first excited bending mode v 2 = 1 were detected in absorption against the continuum of the central H ii region making use of the Effelsberg 100 m telescope and the Very Large Array. Additionally, the J = 9 direct ℓ-type transition was detected in emission presumably indicating a weak maser. All lines are blueshifted with respect to the systemic velocity of CRL 618 indicating that the lines originate from a hot, expanding circumstellar envelope. The HCN column density along the line of sight in front of the continuum is 2 × 10 18 cm −2 .