1996A&A...314..265W

We have observed the (3,3) and (4,4) lines of ¹⁵NH₃ toward a selection of ultracompact HII regions using the Effelsberg 100m telescope. The lines were detected in 6 sources. Toward the strong source G10.47+0.03, all metastable inversion transitions from (1,1) to (6,6) were detected. In addition, ¹⁴NH₃(3,3) was observed in 6 sources and the (4,2) and (5,4) non-metastable transitions of ¹⁴NH₃ were detected in G10.47+0.03 and G31.41+0.31. We find beam averaged ¹⁵NH₃ column densities of order 10¹³cm^–2 and temperatures between 100 and 300K. Towards G10.47+0.03, the rotational temperature between metastable levels is found to be 185±20K. Comparison with an LVG model allows us to deduce a kinetic temperature of 240±40K. No evidence is found for maser characteristics in the ¹⁵NH₃(3,3) transition like those found in NGC 7538 by Mauersberger et al. (1986). The non-detection of ¹⁵NH₃(4,3) in G10.47+0.03 and G31.41+0.31 allows us to set an upper limit of 5x10⁷cm^–3 to the molecular hydrogen density in the hot cores. In comparison to column densities based upon optical depths derived from the relative intensities of hyperfine satellites of ¹⁴NH₃ we find column densities lower by a factor of 3-4 using ¹⁵NH₃. Possible explanations of this discrepancy are discussed.