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Astron. Astrophys. 363, L37-L39 (2000)

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1. Introduction

In 1971, the observation of a new interstellar line at 90665[FORMULA]1 MHz (U90.7) was reported by Snyder and Buhl (1971, 1972). By structural considerations (estimation of the rotational constant B), this line was attributed to HNC, a molecule that had been observed previously only by means of matrix-isolation spectroscopy in the laboratory (Milligan and Jacox 1963, 1967). Shortly thereafter, Zuckerman et al. (1972) derived an improved value of 90663.9[FORMULA]0.5 MHz for its rest frequency from observations of the young cluster NGC 2264. The first millimeter-wave investigations to confirm this tentative assignment were performed by Blackman et al., Creswell et al., and Saykally et al. in 1976. Since these early days, only one additional work on the pure rotational spectrum of HNC up to [FORMULA] has been reported by Okabayashi and Tanimoto (1993). The authors detected rotational transitions of the molecule in its [FORMULA], [FORMULA], [FORMULA], and [FORMULA] vibrational states.

The isomeric pair HCN/HNC is of enormous astrophysical interest (e.g. Schilke et al. 1992, Turner et al. 1997, Hirota et al. 1998, Talbi and Herbst 1998, Herbst et al. 2000), but the laboratory data available on HNC is by far not as extensive as the data on HCN. Very recently, Maiwald et al. (2000) reported on the submm-wave spectrum of HCN up to 1.95 THz whereas for HNC 365 GHz marks the upper limit (Okabayashi and Tanimoto 1993). The pure rotational spectrum of HNC in its ground vibrational state has been observed in the protoplanetary nebula CRL 618 by Herpin and Cernicharo (2000) up to [FORMULA], which corresponds to a frequency of approximately 2 THz, using the Infrared Space Observatory, ISO. This very transition has been measured in the laboratory in the present study (see Table 1 and Fig. 1). The rotational energy of the [FORMULA] ground state transition of HNC is about 698 [FORMULA] (1005 K) and therefore significantly higher than the energy of 464 [FORMULA] for the bending fundamental. In comparison, the fundamental bending mode of HCN lies at 712 [FORMULA]. Nevertheless, features due to this and even higher vibrational states have been detected in astronomical sources (e.g. Ziurys and Turner 1986, Schilke et al. 2000). This indicates, that it could be possible to detect vibrational satellites of HNC in warmer regions of the interstellar and circumstellar medium provided it can survive radiative penetration.

[FIGURE] Fig. 1. Examples of HNC spectra recorded close to two terahertz. The upper trace shows the [FORMULA] ground state transition at 1990360 MHz and the lower its [FORMULA] vibrational satellite at 1989846 MHz.


Table 1. Ground state and first excited bending state assignments, frequencies (MHz) (a), and residuals o-c (kHz) of HNC

Further extension of the frequency range up to the far infrared (THz range) by laboratory spectroscopy is required by future missions such as SOFIA (Stratospheric Observatory For Infrared Astronomy) and FIRST (Far InfraRed and Submillimetre Telescope) to unambiguously identify the carriers of spectral features and to allow for proper kinematical analyses.

In this Letter, we report the investigation of pure rotational transitions of HNC in its [FORMULA] and [FORMULA] states up to 2 THz.

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© European Southern Observatory (ESO) 2000

Online publication: December 5, 2000