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Appendix A:

The rotational diagram in Fig. A.1 is complemented with ground-based data for the frequency range 300-400 GHz observed at the CSO with comparable beam size by Phillips et al. (1992). The non-detection at 307 MHz is surprising, but possibly an effect of optical depth. The data in Fig. A.1, except 307 GHz, are well fitted by a single rotational temperature of 239 K, suggesting that the observed levels are populated according to an exponential distribution. The derived temperature and column density infer an optical depth of for all lines except at 307 GHz. The fitted line (dashed) corresponds to a column density of  cm^-2, consistent with the value derived by Phillips et al. (1992). This leads to a beam averaged H₃O⁺ abundance of relative to H, to be compared with the theoretical value of reported by Bruderer et al. (2010a).

	Figure A.1: Rotational diagram of H3O+. Numbers indicate the frequencies in GHz of the observed lines.
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