Interstellar Isocyanic Acid

Abstract

Interstellar isocyanic acid (HNCO) has been detected in emission through the 4 0 4-3o3 ground-state rotational transition at 3.4 mm. Out of nine galactic sources surveyed, HNCO has been observed only in the direction of Sgr B2 and possibly W51. The Sgr B2 emission pattern is fairly extensive, and our observations indicate that the peak HNCO emission is ~2' north of the OH position. A search for the 7-6 transition of interstellar OCS yielded negative results. Earlier we reported the detection of the 4 04-3 0 3 transition of interstellar isocyanic acid (HNCO) in emission at 87,925 MHz during an observing period in 1971 May (Snyder and Buhl 19716). In 1971 December, we completed a preliminary survey for HNCO in a number of galactic molecular sources, and our results are reported here. Both sets of observations were made with the NR AO 36-foot (11-m) radio telescope; the nominal half-power beamwidth at 3.4 mm is 78" with pointing uncertain by ± 10". Frequency switching of the first local oscillator was used in the May observations ; a ferrite beam switch with 15' feed separation was used in December. In both sets of observations, the radiometer feed was alternately moved on and off source to subtract a cold-sky comparison spectrum. The single-sideband noise temperature of the radiometer was ~7000° K for both observing periods. The filter bank used for the observations has 40 filters spaced 1 MHz apart and covers a velocity range of 136 km s" 1 at 3.4 mm. The 4 0 4-3o3 ground-state rotational transition of HNCO definitely has been detected in Sgr B2 (see fig. 1) and possibly in W51. Our initial identification of interstellar HNCO was based on the excellent agreement between our astronomical rest frequency and the laboratory frequency of 87,925.45 ± 0.5 MHz determined by Kewley, Sastry, and Winnewisser (1963); the subsequent detection of the loi-0 O o transition at 21,982 MHz (see fig. 2) has confirmed our identification (Buhl, Snyder, and Edrich 1972) and is discussed in the following paper. A nine-point map of the region around Sgr B2(OH) (the OH and H 2 0 emission position in the direction of Sgr B2) shows that the HNCO emission is fairly extensive over a region > 4' in diameter. The observations are summarized in table 1, where the mapping coordinates are listed as displacements from Sgr B2(OH). Column (4) gives the observed antenna temperature (uncorrected for atmospheric extinction), and the last two columns list radial velocity and line width at half-maximum intensity for those spectral features with the best signal-to-noise ratio. The radial velocities are uncertain by ±1.7 km s-1 and temperature calibration by ~ 20 percent. We included the W51 results in table 1, but they must be confirmed by future observations.