Detection of interstellar HN¹³C

Abstract

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 groups^3–5 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.