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 toward the proto-planetary nebula CRL 618 during a study of vibrationally excited carbon chains. The J = 8, 10, 11, 12, 13, and 14 ΔJ = 0 transitions of HCN in its first excited bending mode v2 = 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. In addition, 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 × 1018 cm-2.