tokens: which could make the analyses more complicated. We analyzed the PILS data for lines of CH3SD similarly to what Drozdovskaya et al. (2018) did for CH3SH. We adopted an excitation temperature T ex = 125 K, a source size of 0.5′′, and a full line width at half maximum (FWHM) of 1 km s −1 . We did not detect CH3SD, but we derived an upper limit on its column density of 8.8 × 10 14 cm −2 (±0.88). The 12 most constraining lines are shown in Fig. 3 . Drozdovskaya et al. (2018) determined a column density of 4.8 × 10 15 cm −2 (±0.48) for CH3SH 2 . Our upper limit corresponds thus to a D/H ratio of less than 0.18 ± 0.025; the uncertainty was estimated as previously ( Drozdovskaya et al. 2018 ). This value is probably not particularly constraining compared with the deuteration of H2CS per H atom of ∼0.05 ± 0.007 ( Drozdovskaya et al. 2018 ). Deeper interferometric observations are required in order to conclusively verify whether the level of deuteration of H2CS is lower, higher, or preserved at the next level of chemical complexity into CH3SH. Fig. 3. Sections of the Protostellar Interferometric Line Survey (PILS) toward a position slightly off-set from source B displaying the 12 most constraining lines of CH3SD. 5. Conclusion and outlook In the present work, the rotational spectrum of CH3SD has been investigated in the frequency range 150−510 GHz in order to provide accurate predictions for astronomical searches. Extensive assignments have been made for the ground, first, and second excited torsional states up to high J and K a quantum numbers (see Table 1 ). The detailed modeling of the absorption spectra of the CH3SD has been performed in a global fit of a dataset of 4905 rotational transitions to the RAM Hamiltonian containing 78 parameters. The overall weighted standard deviation of the global fit is 0.9 in the range 150–510 GHz, indicating that our set of parameters reproduces the assigned lines within experimental uncertainties. Transition frequencies calculated from these parameters should be reliable for astronomical observations. A first attempt to search for CH3SD in the PILS data turned out to be negative. The upper limit, though not unreasonable, is not very constraining either. Future, even more sensitive observations may provide insight into the deuteration of CH3SH. A search for the potentially more abundant CH2DSH was hampered by insufficient laboratory data reported for this isotopolog. Calculations of the rotational spectrum of CH3SD will be available in the catalog section of the Cologne Database for Molecular Spectroscopy, CDMS, ( Endres et al. 2016 ). The input and output files of the fit as well as auxiliary files are available in the data section of the CDMS 3 . Appendix A: Additional tables Table A.1. Spectroscopic parameters of CH3SD (cm −1 ). Notes. (a) n = t + r , where n is the total order of the operator, t is the order of the torsional part and r is the order of the rotational part, respectively. The ordering scheme of
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