First Detection of CH3OD in Prestellar Cores

TL;DR

This study reports the first secure detection of CH$_3$OD in a prestellar core (L1448) with a tentative detection in B213-C6, enabling the first constraints on the CH$_2$DOH/CH$_3$OD ratio and the D/H fractions in singly deuterated methanol at the prestellar stage. By combining IRAM 30 m observations from two programs and LTE-based column-density analyses, the authors derive CH$_2$DOH and CH$_3$OD abundances, revealing D/H ratios of a few percent and CH$_2$DOH/CH$_3$OD ratios generally in the range of 3–9 for low-mass cores. The results support that methanol deuteration is largely inherited from the prestellar phase in low-mass star formation, with abstraction chemistry providing a plausible formation route for the observed patterns, while high-mass sources show different trends likely due to higher dust temperatures. The work provides a baseline for CH$_3$OD in prestellar environments and offers crucial constraints for astrochemical models of ice processing and deuteration across the star formation sequence.

Abstract

The isotopic ratios of deuterated methanol derived around protostars are commonly used to infer the physical conditions under which they formed in the earlier prestellar stage. However, there is a discrepancy in the ratio of the singly deuterated methanol isotopologues, CH2DOH/CH3OD, between low- and high-mass protostars, which puts into question whether prestellar isotopic ratios are generally preserved during the star- and planet-forming process. Resolving this puzzle is only made harder by the complete lack of data on this ratio in the prestellar stage. This work presents observations with the IRAM 30m telescope that securely detect CH3OD in the prestellar core L1448 in Perseus and tentatively in B213-C6 in Taurus. This work constrains the ratio of CH2DOH/CH3OD and the D/H ratios for both singly deuterated methanol isotopologues for the first time at the prestellar stage. Column densities calculated under the assumption of local thermal equilibrium lead to a CH2DOH/CH3OD ratio of 2.8-8.5 in L1448 and $\leq$ 5.7 in B213-C6. The values are marginally consistent with the statistically expected ratio of 3, but most assumptions put the values in an elevated range in line with values found around low-mass protostars. The D/H ratio in CH2DOH is between 3.6% and 6.8% in L1448 and in the range of 2.4-5.8% in B213-C6. The D/H ratio derived for CH3OD is lower, namely 1.4-4.4% in L1448 and $\leq$ 3.8% in B213-C6.

Figures (6)

• Figure 1: Detected transitions of CH$_2$DOH (top two rows) and CH$_3$OD (bottom row) towards L1448. The line at 110 GHz is a tentative detection. The observed spectra plotted in black are overlaid by the Gaussian fits (see Table \ref{['tab:detectedlines']}) in red. The silver dashed lines represent the value of $\pm$1 $\times$ rms, and the dotted, black line represents the value of 3 $\times$ rms. The purple dash-dotted vertical line represents the line rest velocity of CH$_3$OH (Kulterer et al. submitted).
• Figure 2: Detected transitions of CH$_2$DOH (top row) and CH$_3$OD (bottom left panel) towards B213-C6. The observed spectra plotted in black are overlaid by the Gaussian fits (see Table \ref{['tab:detectedlines']}) in red. The silver dashed lines represent the value of $\pm$1 $\times$ rms, and the dotted, black line represents the value of 3 $\times$ rms. The purple dash-dotted vertical line represents the line rest velocity of CH$_3$OH (Spezzano22).
• Figure 3: D/H ratios for CH$_2$DOH (left panel) and CH$_3$OD (right panel) across the star-forming sequence. The spread per source type was calculated using all column densities listed in Table \ref{['tab:DHreferences']}. The black circle denotes the average of the values. The values of CH$_2$DOH have been statistically corrected. Violin plots demonstrate the density distribution of the data: the wider the area of the violin, the more data points are around this value; narrow regions show that data are sparse at this value.
• Figure 4: CH$_2$DOH/CH$_3$OD ratio across the star-forming sequence. Dark blue data points represent prestellar cores, sky blue data points depict low-mass protostars, and high-mass protostars are shown in light blue. Squared markers indicate observations with single-dish facilities, circles highlight observations taken with an interferometer. The filled data points show the newly presented values from this work. The column densities used to calculate the remaining ratios and their corresponding references can be found in Table \ref{['tab:DHreferences']} in the Appendix. The dashed, black line corresponds to the statistically expected value of 3. If multiple data points were available for a source, the range is represented by a line; and the marker shows the average; the lines show the range of values including errors. If a value stems from CH$_3$OD column densities that have been calculated based on the spectroscopy published in Ilyushin24, it is framed in orange.
• Figure 1.1: Left: HNC transition at 90.6636 GHz and its corresponding absorption feature produced during the folding of the frequency-switched data in L1448. The red line denotes the position of the CH$_3$OD transition at 90.6699 GHz. Right: Zoomed-in picture of the image on the left. Again, the red line denotes the CH$_3$OD line that may have been affected; the dashed gray lines represent the flux at 0 K and at 3 $\times$ rms.