(NH3)-N-15 towards ultracompact HII regions

We have observed the (3,3) and (4,4) lines of (NH3)-N-15 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, (NH3)-N-14(3,3) was observed in 6 sources and the (4,2) and (5,4) non-metastable transitions of (NH3)-N-14 were detected in G10.47 + 0.03 and G31.41 + 0.31. We find beam averaged (NH3)-N-15 column densities of order 10(13) cm(-2) and temperatures between 100 and 300 K. Towards G10.47+0.03, the rotational temperature between metastable levels is found to be 185 +/- 20 K. Comparison with an LVG model allows us to deduce a kinetic temperature of 240 +/- 40 K. No evidence is found for maser characteristics in the (NH3)-N-15(3,3) transition like those found in NGC 7538 by Mauersberger et al. (1986). The non-detection of (NH3)-N-15(4,3) in G10.47 + 0.03 and G31.41 + 0.31 allows us to set an upper limit of 5 . 10(7) cm(-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 (NH3)-N-14, We find column densities lower by a factor of 3-4 using (NH3)-N-15. Possible explanations of this discrepancy are discussed.