Detection of HCN Directl‐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 Δ 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. 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 × 10 18 cm -2 .