Successive deuteration in low-mass star-forming regions: The case of D₂-methanol (CHD₂OH) in IRAS 16293-2422

Abstract

Context. Di-deuterated molecules are observed in the earliest stages of star formation at abundances of a few percent relative to their nondeuterated isotopologs, which is unexpected considering the scarcity of deuterium in the interstellar medium. With sensitive observations leading to the detection of a steadily increasing number of di-deuterated species, it is becoming possible to explore successive deuteration chains. Aims. The accurate quantification of the column density of di-deuterated methanol is a key piece of the puzzle that is missing in the otherwise thoroughly constrained family of D-bearing methanol in the deeply embedded low-mass protostellar system and astrochemical template source IRAS 16293-2422. A spectroscopic dataset for astrophysical purposes was built for CHD2OH and made publicly available to facilitate the accurate characterization of this species in astrochemical surveys. Methods. The newly computed line list and partition function were used to search for CHD2OH toward IRAS 16293-2422 A and B in data from the Atacama Large Millimeter/submillimeter Array (ALMA) Protostellar Interferometric Line Survey (PILS). Only nonblended, optically thin lines of CHD2OH were used for the synthetic spectral fitting. Results. The constructed spectroscopic database contains line frequencies and strengths for 7417 transitions in the 0–500 GHz frequency range. ALMA-PILS observations in the 329–363 GHz range were used to identify 105 unique, nonblended, optically thin line frequencies of CHD2OH for synthetic spectral fitting. The derived excitation temperatures and column densities yield high D/H ratios of CHD2OH in IRAS 16293-2422 A and B of 7.5 ± 1.1% and 7.7 ± 1.2%, respectively. Conclusions. Deuteration in IRAS 16293-2422 is not higher than in other low-mass star-forming regions (L483, SVS13-A, NGC 1333-IRAS2A, -IRAS4A, and -IRAS4B). Di-deuterated molecules consistently have higher D/H ratios than their mono-deuterated counterparts in all low-mass protostars, which may be a natural consequence of H–D substitution reactions as seen in laboratory experiments. The Solar System’s natal cloud, as traced by comet 67P/Churyumov–Gerasimenko, may have had a lower initial abundance of D, been warmer than the cloud of IRAS 16293-2422, or been partially reprocessed. In combination with accurate spectroscopy, a careful spectral analysis, and the consideration of the underlying assumptions, successive deuteration is a robust window on the physicochemical provenance of star-forming systems.

Methanol deuteration in high-mass protostars

“…Using the database entry of CHD 2 OH provided by Drozdovskaya et al (2022), transitions from CHD 2 OH can also be searched for in a few COM-rich low-mass protostars. Here, this is done for B1-c, Serpens S68N (hereafter S68N), and B1-bS from the 2017.1.01174.S ALMA program.…”

“…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).…”

Complex organic molecules detected in twelve high-mass star-forming regions with Atacama Large Millimeter/submillimeter Array (ALMA)

“…Since we observed interior region, close to the massive protostar, these unusually high isotopic ratios may come from significantly different physical and chemical conditions deviated from the Galactic scale environment. In addition, the isotope fractionations differ by species, but the values adopted for the Galactic and local ISM ratios are derived from atoms, and thus, they could be very different from the ratios in COMs (e.g., Nomura et al 2022;Drozdovskaya et al 2022).…”

“…The D/H ratios of CH 3 OH and NH 2 CHO measured in IRAS 16293B are ∼0.02, lower than those of most our sources. The ratios of different COMs range over 0.005-0.2 in IRAS 16293B (Drozdovskaya et al 2022). Within a source, the varied deuteration of individual species imply the different formation timescales of those molecules (Jørgensen et al 2020).…”