Abstract
We surveyed 81 dense molecular cores associated with regions of massive star formation and Sgr A in the J(K-1K1) = 5(05) - 4(04) and 10(010) - 9(09) lines of HNCO. Line emission was detected towards 57 objects. Selected subsamples were also observed in the 1(01) - 0(00), 4(04) - 3(03), 7(07) - 6(06), 15(015) - 14(014), 16(016) - 15(015) and 21(021) - 20(020) lines, covering a frequency range from 22 to 461 GHz. HNCO lines from the K-1 = 2,3 ladders were detected in several sources. Towards Orion-KL, K-1 = 5 transitions with upper state energies E-u/k similar to 1100 and 1300 K could be observed.
Five HNCO cores were mapped. The sources remain spatially unresolved at 220 and 461 GHz (10(010) - 9(09) and 21(010) - 20(020) transitions) with beam sizes of 24 " and 18 ", respectively.
The detection of hyperfine structure in the 1(01) - 0(00) transition is consistent with optically thin emission under conditions of Local Thermodynamic Equilibrium (LTE). This is corroborated by a rotational diagram analysis of Orion-KL that indicates optically thin line emission also for transitions between higher excited states. At the same time a tentative detection of interstellar (HNCO)-C-13 (the 10(0,10) - 9(0,9) line at 220 GHz toward G 310.12-0.20) suggests optically thick emission from some rotational transitions.
Typical HNCO abundances relative to H-2 as derived from a population diagram analysis are similar to 10(-9). The rotational temperatures reach similar to 500 K The gas densities in regions of HNCO K-1 = 0 emission should be n greater than or similar to 10(6) cm(-3) and in regions of K-1 > 0 emission about an order of magnitude higher even for radiative excitation.
HNCO abundances are found to be enhanced in high-velocity gas. HNCO integrated line intensities correlate well with those of thermal SiO emission. This indicates a spatial coexistence of the two species and may hint at a common production mechanism, presumably based on shock chemistry.