INTERSTELLAR hydrogen isocyanide (HNC) was first tentatively identified1 on the basis of a calculated frequency for the J =1→0 transition of the most abundant isotopic species H14N12C. The observed interstellar line was quite strong and widespread in the galaxy. A more definitive identification of the observed U90.7 emission line (90,663.6 MHz)2 by searching for other isotopes was not possible because of the large uncertainties in the calculated molecular structure of HNC, and the corresponding large uncertainties in the calculated transition frequencies for H14N13C and H15N12C. Hydrogen isocyanide is very short lived in terrestrial laboratory conditions, and therefore it has proven difficult to measure the rotational spectrum directly. The J = 0→1 transition has, however, been recently measured independently by three groups3–5 and the frequency of the major isotope is in excellent agreement with that of U90.7. The laboratory spectroscopy has also yielded the 13C isotope transition frequency4. Immediately following this measurement, we searched for and detected the HN13C J =1→0 rotational spectral line at 87,090.6 MHz in the galactic molecular source W51 (Fig. 1) on the NRAO 11-m telescope in March, 1976. Because of calibration problems with our receiver, we simultaneously observed the 12C isotope so that relative abundance determinations might ultimately be made from our data. Appropriately, W51 was the source in which Snyder and Buhl first detected the J = 1→0 transition of HN12C. In addition, the HN13C line was detected in DR21, Sag B2, and possibly also NGC2264 and IRC + 10°216 (Figs 2–5). All of these sources also showed the HN12C emission line. In addition, we detected HN12C in W3 (Fig. 6), but were unable to detect the 13C isotope there due to equipment failure. These results are summarised in Table 1.
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Snyder, L. E., and Buhl, D., Ann. N. Y. Acad. Sci., 194, 17 (1972).
Zuckerman, B., Morris, M., Palmer, P., and Turner, B. E., Astrophys. J. Lett., 173, L125 (1972).
Saykally, R. J., Szanto, T. G., Anderson, T. G., and Woods, R. C., Astrophys. J. Lett., 204, L143 (1976).
Blackman, G. L., Brown, R. D., Godfrey, P. D., and Gunn, H. I., Nature (in the press).
Creswell, R. A., Pearson, E. F., Winnewisser, M., and Winnewisser, G., Z. Naturforsch., 31a, 221 (1976).
Snyder, L. E., and Hollis, J. M., Astrophys. J. Lett., 204, L139 (1976).
Turner, B. E., Astrophys. J. Lett., 193, L83 (1974).
Turner, B. E., Zuckerman, B., Palmer, P., and Morris, M., Astrophys. J., 186, 123 (1973).
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