I. Zinchenko 1,2,
C. Henkel 3 and
R.Q. Mao 3,4
Received 29 February 2000 / Accepted 29 June 2000
We surveyed 81 dense molecular cores associated with regions of massive star formation and Sgr A in the and lines of HNCO. Line emission was detected towards 57 objects. Selected subsamples were also observed in the , , , , and lines, covering a frequency range from 22 to 461 GHz. HNCO lines from the ladders were detected in several sources. Towards Orion-KL, transitions with upper state energies and 1300 K could be observed.
Five HNCO cores were mapped. The sources remain spatially unresolved at 220 and 461 GHz ( and transitions) with beam sizes of 24" and 18", respectively.
The detection of hyperfine structure in the 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 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 . The rotational temperatures reach K. The gas densities in regions of HNCO emission should be cm-3 and in regions of 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.
Key words: stars: formation ISM: clouds ISM: molecules radio lines: ISM
* 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.
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© European Southern Observatory (ESO) 2000
Online publication: October 10, 2000