Interferometric observations of warm deuterated methanol in the inner regions of low-mass protostars

Abstract

Methanol is a key species in astrochemistry since it is the most abundant organic molecule in the interstellar medium and is thought to be the mother molecule of many complex organic species. Estimating the deuteration of methanol around young protostars is of crucial importance because it highly depends on its formation mechanisms and the physical conditions during its moment of formation. We analyse several dozens of transitions from deuterated methanol isotopologues coming from various existing observational datasets obtained with the IRAM-PdBI and ALMA submm interferometers to estimate the methanol deuteration surrounding three low-mass protostars on Solar System scales. A population diagram analysis allows us to derive a [CH2DOH]/[CH3OH] abundance ratio of 3 − 6 % and a [CH3OD]/[CH3OH] ratio of 0.4 − 1.6 % in the warm inner (≤ 100 − 200 AU) protostellar regions. These values are typically ten times lower than those derived with previous single-dish observations towards these sources but they are one to two orders of magnitude higher than the methanol deuteration measured in massive hot cores. Dust temperature maps obtained from Herschel and Planck observations show that massive hot cores are located in warmer molecular clouds than low-mass sources, with temperature differences of ∼10 K. The comparison of our measured values with the predictions of the gas-grain astrochemical model GRAINOBLE shows that such a temperature difference is sufficient to explain the different deuteration observed in low-to high-mass sources. This suggests that the physical conditions of the molecular cloud at the origin of the protostars mostly govern the present observed deuteration of methanol and, therefore, of more complex organic molecules. Finally, the methanol deuteration measured towards young solar-type protostars on Solar System scales seems to be higher by a factor of ∼ 5 than the upper limit in methanol deuteration estimated in comet Hale-Bopp. If this result is confirmed by subsequent observations of other comets, this would imply that an important reprocessing of the organic material likely occurred in the solar nebula during the formation of the Solar System.

A survey of CH2DOH towards starless and pre-stellar cores in the Taurus molecular cloud

“…Single-dish observations out to ∼ 1000 AU by Parise et al (2002Parise et al ( , 2004Parise et al ( , 2006 indeed show higher deuterium fractions (∼ 0.3-0.6) for methanol in Class 0 protostars. Ultimately, the deuterium fraction of methanol is predicted to decrease towards the warm central regions (< 100 AU) of the core envelope as the protostar evolves (Taquet et al 2014), which has been confirmed by recent interferometric observations of Class 0/I protostars (Bianchi et al 2017a,b;Jørgensen et al 2018;Persson et al 2018;Taquet et al 2019;Manigand et al 2020;van Gelder et al 2020).…”

“…The deuteration of the complex organic molecules -the replacement of one or more hydrogen atoms with deuterium -is one aspect of complex organic molecule chemistry that has not been well studied to date during the starless core phase. Deuterated COMs are readily observed during low-mass protostellar phases (Loinard et al 2002;Parise et al 2002Parise et al , 2006Bianchi et al 2017b;Jørgensen et al 2018;Manigand et al 2019;Taquet et al 2019;Manigand et al 2020;van Gelder et al 2020) but there are few observations towards prestellar cores (cf. Bizzocchi et al 2014;Chacón-Tanarro et al 2019).…”

Prestellar grain-surface origins of deuterated methanol in comet 67P/Churyumov–Gerasimenko

“…The smallest uncertainties stem from spectroscopy (<10 per cent for T < 150 K and <20 per cent for T < 300 K for all methanol variants as long as the rigid rotor approximation is not adopted; Müller private communication) and from the assumption of LTE (less than a factor of 2). However, Taquet et al (2019) showed that if the rigid rotor approximation, that is to be avoided for methanol, is made for the partition function of CH 3 OD, differences as large as a factor of 5 in its column density can be incurred at T = 150 K. The largest uncertainties in derived column densities originate from optical depth effects of the observed methanol gas and dust along the line of sight. If not treated with care, column densities can be easily underestimated by as much as an order of magnitude (most problematic near protostars for the case of normal methanol).…”

“…With interferometric Atacama Large Millimeter/submillimeter Array (ALMA) observations, IRAS 16293-2422 A could be studied separately from B to derive similar (at most a factor of 1.2 apart) CH 2 DOH/CH 3 OH ratios towards both protostars (Jørgensen et al 2018;Manigand et al 2020). For NGC 1333-IRAS 4A, interferometric IRAM Plateau de Bure Interferometer (IRAM-PdBI; now called the NOrthern Extended Millimeter Array, NOEMA) observations determined a CH 2 DOH/CH 3 OH ratio of 0.037 (Taquet et al 2019). Subsequent interferometric ALMA observations separated the source into its binary components A1 and A2 with ratios of 0.0058 and 0.048, respectively (Sahu et al 2019), indicating that the two binary components are starkly different from one another (and that earlier IRAM-PdBI observations were dominated by the emission from A2).…”

“…The same situation holds for CH 3 OD. It has been speculated that non-LTE effects may be non-negligible for this isotopologue due to high critical densities for some of its transitions (based on observations of low-mass protostars NGC 1333-IRAS 2A and -IRAS 4A; Taquet et al 2019). Low-level non-LTE effects were also thought to explain partially the scatter in the methanol deuterated isotopologue rotational diagrams from observations of IRAS 16293-2422 (Parise et al 2002(Parise et al , 2006; however, such scatter is more likely explained by dust optical thickness and both components of the binary being probed jointly in the earlier IRAM 30m observations (Jørgensen et al 2016).…”