2003ApJ...586..338T

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 2x10¹⁸ cm^–2.