HNCO in massive galactic dense cores

We surveyed 81 dense molecular cores associated with regions of massive star formation and Sgr A in the J_K-1K_-1 = 5₀₅-4₀₄ and 10₀₁₀-9₀₉ lines of HNCO. Line emission was detected towards 57 objects. Selected subsamples were also observed in the 1₀₁-0₀₀, 4₀₄-3₀₃, 7₀₇-6₀₆, 15₀₁₅-14₀₁₄, 16₀₁₆-15₀₁₅ and 21₀₂₁-20₀₂₀ 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 ~ 1100 and 1300 K could be observed. Five HNCO cores were mapped. The sources remain spatially unresolved at 220 and 461 GHz (10₀₁₀-9₀₉ and 21₀₁₀-20₀₂₀ transitions) with beam sizes of 24'' and 18\arcsec, respectively. The detection of hyperfine structure in the 1₀₁-0₀₀ 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 HN¹³CO (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 H₂ as derived from a population diagram analysis are ~ 10^-9. The rotational temperatures reach ~ 500 K. The gas densities in regions of HNCO K_-1=0 emission should be n>~ 10⁶ 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. Based on the observations collected at the European Southern Observatory, La Silla, Chile and on observations with the Heinrich-Hertz-Telescope (HHT). The HHT is operated by the Submillimeter Telescope Observatory on behalf of Steward Observatory and the MPI für Radioastronomie. Tables 1, 2, 5, 6 are also available in electronic form and Tables 7-14 are only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/Abstract.html