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

5. Conclusions

We have presented the results of an HNCO survey of high mass star-forming cores at frequencies from 22 to 461 GHz. The main conclusions are the following:

1. HNCO is widespread in dense cores forming high mass stars. The detection rate was %. There is no significant galactic gradient in its abundance as indicated by the fact that abundances derived for the sources which belong to the inner and to the outer Galaxy, respectively, are about the same.

2. Transitions in higher ladders, up to , are detected. The excitation energy reaches  K above the ground level.

3. HN¹³CO is tentatively detected towards G 301.12-0.20. This implies an optical depth in the HNCO line in this source. The optical depth in the transition is for the sources detected in this line as inferred from the hyperfine ratios.

4. The sources are compact with sizes .

5. HNCO rotational temperatures vary from  K to  K. Typical relative abundances are . These increase with increasing velocity dispersion.

6. The emission in the ladders is best explained by FIR radiative excitation. In order to provide a sufficiently large dust optical depth at FIR wavelengths taking into account the limitations on the source size, the gas density should be  cm^-3; a temperature  K is needed to excite the emission in Orion KL. The transitions can be collisionally excited. The required densities are  cm^-3.

7. HNCO correlates well with SiO and does not correlate with CS which is a typical high density probe. HNCO abundances are enhanced in high velocity gas. Probably HNCO production is related to shocks as for SiO. A plausible pathway is gas-phase neutral-neutral reactions at high ( K) temperatures to overcome an activation barrier that is likely inhibiting the reaction in a cool interstellar medium.

© European Southern Observatory (ESO) 2000

Online publication: October 10, 2000
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