First Detection of Vibrationally Excited HNC in Space

We report the first detection of vibrationally excited HNC in space, found toward the proto-planetary nebula CRL 618. Three lines of HNC in the v2 = 1 vibrationally excited state have been found, making the detection unambiguous. The lines are emitted in a hot region close to the inner radius of the molecular envelope of CRL 618, but quantitative estimates of the column density are not possible, since the lines are most likely optically thick. In other astrophysical environments, HNC is found only in cold gas, and the presence of HNC in the hot layer toward CRL 618 is most likely due to time-dependent photochemistry in the boundary between the inner H II region and the molecular envelope.