Rotational and rovibrational spectroscopy of CD₃OH with an account of CD₃OH toward IRAS 16293−2422

Abstract

Solar-type protostars have been shown to harbor highly deuterated complex organics, as evidenced, for instance, by the high relative abundances of doubly and triply deuterated isotopologs. While this degree of deuteration may provide important clues in studying the formation of these species, spectroscopic information on multiply deuterated isotopologs is often insufficient. In particular, searches for triply deuterated methanol, CD3OH, are hampered to a large extent by the lack of intensity information from a spectroscopic model. The aim of the present study is to develop a spectroscopic model of CD3OH in low-lying torsional states that is sufficiently accurate to facilitate further searches for CD3OH in space. We performed a new measurement campaign for CD3OH involving three spectroscopic laboratories that covers the 34 GHz−1.1 THz and the 20−900 cm−1 ranges. The analysis was performed using the torsion-rotation Hamiltonian model based on the rho-axis method. We determined a model that describes the ground and first excited torsional states of CD3OH, up to quantum numbers J ≤ 55 and Ka ≤ 23, and we derived a line list for radio-astronomical observations. The resulting line list is accurate up to at least 1.1 THz and should be sufficient for all types of radio-astronomical searches for this methanol isotopolog. This line list was used to search for CD3OH in data from the Protostellar Interferometric Line Survey of IRAS 16293−2422 using the Atacama Large Millimeter/submillimeter Array. Specifically, CD3OH is securely detected in the data, with a large number of clearly separated and well-reproduced lines. We not only detected lines belonging to the ground torsional state, but also several belonging to the first excited torsional state. The derived column density of CD3OH and abundance relative to the non-deuterated isotopolog confirm the significant enhancement of this multiply deuterated variant. This finding is in line with other observations of multiply deuterated complex organic molecules and may serve as an important constraint on their formation models.

Rotational spectroscopy of mono-deuterated oxirane ($c$-C$_2$H$_3$DO) and its detection towards IRAS 16293$-$2422 B

“…A similar result was obtained for the deuteration of NH 2 to ND 2 compared with that to NHD (Melosso et al 2020). Additional support comes from the PILS data of CH 3 OH and CH 2 DOH (Jørgensen et al 2016), CHD 2 OH (Drozdovskaya et al 2022) and CD 3 OH (Ilyushin et al 2022). It has been argued that the higher degree of deuteration per H atom for multiply deuterated isotopologues compared with the respective singly deuterated ones has been inherited from the prestellar phase.…”

“…The deuterated species detected in the PILS data are the mono-deuterated isotopomers of the oxygen-bearing organics glycolaldehyde (Jørgensen et al 2016), ethanol, ketene, formic acid and of mono-deuterated acetaldehyde species CH 3 CDO (Jørgensen et al 2018) and CH 2 DCHO (Coudert et al 2019;Manigand et al 2020), of the nitrogenbearing organics isocyanic acid DNCO and the monodeuterated isotopomers of formamide (Coutens et al 2016) and the cyanamide isotopologue HDNCN (Coutens et al 2018) and sulfur-containing species such as the hydrogen sulfide isotopologue HD 34 S (Drozdovskaya et al 2018). Also, the PILS data reveal the presence of doubly-deuterated organics including the methyl cyanide species CHD 2 CN (Calcutt et al 2018), the methyl formate species CHD 2 OCHO (Manigand et al 2019) and the dimethyl ether species CHD 2 OCH 3 (Richard et al 2021) and enable new and more accurate constraints on the doubly-and triply-deuterated variants of methanol in the warm gas close to the protostars (Drozdovskaya et al 2022;Ilyushin et al 2022). These systematic studies also enabled a more detailed comparison across the different species.…”

Methanol deuteration in high-mass protostars

“…The angular resolution of the data ranges from 0.5−1.25 , corresponding to about ∼ 2500 − 10000 au at the range of distances covered (2−12 kpc; Mège et al 2021), and the data have a sensitivity of ∼ 0.2 K. The ALMAGAL data cover several transitions of CH 3 OH, four transitions of 13 CH 3 OH, nine transitions of CH 18 3 OH, 21 transitions of CH 2 DOH, and 22 transitions of CHD 2 OH (see Appendix A). Also nine transitions of CD 3 OH are covered (Ilyushin et al 2022), but these are not detected toward any of the sources. No transitions of CH 3 OD are covered in the observed frequency range.…”

“…Mono deuterated methanol, CH 2 DOH and CH 3 OD, have been observed in the warm inner regions of both low-mass and high-mass protostellar systems (e.g., Fuente et al 2014;Belloche et al 2016;Bøgelund et al 2018;van Gelder et al 2020;. Similarly, both doubly and triply deuterated methanol have been detected in hot cores (e.g., Parise et al 2002Parise et al , 2004Bianchi et al 2017a;Drozdovskaya et al 2022;Ilyushin et al 2022). Moreover, CH 2 DOH has also been detected in both low-mass prestellar cores (e.g., Bizzocchi et al 2014;Lattanzi et al 2020;Ambrose et al 2021) and high-mass starless cores (e.g., Fontani et al 2015).…”

“…The D/H ratio is on the order of 10% for low-mass prestellar cores, low-mass protostars, and comets (e.g., Bianchi et al 2017a,b;Jørgensen et al 2018;Taquet et al 2019;Manigand et al 2020;van Gelder et al 2020;Lattanzi et al 2020;Ambrose et al 2021;Drozdovskaya et al 2021). Interestingly, successive deuteration toward CHD 2 OH and CD 3 OH seems to be quite effective in low-mass protostars (about 15-25%; Drozdovskaya et al 2022;Ilyushin et al 2022). On the other hand, the D/H ratio derived from CH 2 DOH is as low as 0.1 − 0.01% for high-mass starless cores and high-mass protostars (Fontani et al 2015;Neill et al 2013;Belloche et al 2016;Bøgelund et al 2018).…”