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Astron. Astrophys. 361, 1079-1094 (2000)

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HNCO in massive galactic dense cores *  **

I. Zinchenko 1,2, C. Henkel 3 and R.Q. Mao 3,4

1 Institute of Applied Physics of the Russian Academy of Sciences, 46 Ulyanov str., 603600 Nizhny Novgorod, Russia
2 Helsinki University Observatory, Tähtitorninmäki, P.O.Box 14, 00014 University of Helsinki, Finland
3 Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, Bonn, Germany
4 Purple Mountain Observatory, 210008 Nanjing, P.R. China

Received 29 February 2000 / Accepted 29 June 2000

Abstract

We surveyed 81 dense molecular cores associated with regions of massive star formation and Sgr A in the [FORMULA] and [FORMULA] lines of HNCO. Line emission was detected towards 57 objects. Selected subsamples were also observed in the [FORMULA], [FORMULA], [FORMULA], [FORMULA], [FORMULA] and [FORMULA] lines, covering a frequency range from 22 to 461 GHz. HNCO lines from the [FORMULA] ladders were detected in several sources. Towards Orion-KL, [FORMULA] transitions with upper state energies [FORMULA] and 1300 K could be observed.

Five HNCO cores were mapped. The sources remain spatially unresolved at 220 and 461 GHz ([FORMULA] and [FORMULA] transitions) with beam sizes of 24" and 18", respectively.

The detection of hyperfine structure in the [FORMULA] 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 [FORMULA] 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 [FORMULA]. The rotational temperatures reach [FORMULA] K. The gas densities in regions of HNCO [FORMULA] emission should be [FORMULA] cm-3 and in regions of [FORMULA] 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.
** Table 1, Table 2, Table 5, and Table 6 are also available in electronic form and Table 7-Table 14 are only available in electronic form at the CDS via anonymous ftp to ftp://cdsarc.u-strasbg.fr/pub/cats/J/A+A/361/1079 (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/361/1079

Send offprint requests to: I. Zinchenko

Correspondence to: zin@appl.sci-nnov.ru

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© European Southern Observatory (ESO) 2000

Online publication: October 10, 2000
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