Complex Organic Molecules in Protostars with ALMA Spectral Surveys (COMPASS)

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Abstract
Context . There have been no robust identifications of fully deuterated methanol, CD 3 OD, in the interstellar medium (ISM) to date. Aims . Our goal is to continue the search for CD 3 OD in star-forming regions to determine the deuteration pattern of methanol and set constraints on the deuteration process. Methods . We used a sensitive imaging spectral survey of the protostellar system IRAS4A in the NGC1333 molecular cloud performed as part of the large program carried out with the Atacama Large Millimeter/submillimeter Array (ALMA) called Complex Organic Molecules in Protostars with ALMA Spectral Surveys (COMPASS). The column densities of methanol and several of its less abundant isotopologs, including deuterated ones, were derived under the assumption of local thermodynamic equilibrium (LTE). Results . Along with the detection of several methanol isotopologs toward the binary component IRAS4A2, we report the first interstellar detection of CD 3 OD. The deuteration pattern of methanol in IRAS4A2 is similar to the pattern measured in other low-mass protostars, with an increase in the apparent D/H ratio from the singly to multiply deuterated isotopologs. Chemical models of prestellar and protostellar cores focused on deuteration do not reproduce this pattern quantitatively. This discrepancy is even more exacerbated by the detection of CD 3 OD. Our LTE modeling of the spectra yields low column density ratios of methanol to its 13 C and 18 O isotopologs while the 13 C to 18 O and 18 O to 17 O ratios are consistent with the isotopic ratios expected in the local ISM. We suspect that the column density of methanol is underestimated. One possible explanation is that the emission is clumpy rather than monolithic. Conclusions . The first detection of a fully deuterated COM highlights the power of deuterium fractionation as a diagnostic tool for investigating the formation and inheritance of organic molecules during the process of star formation.
Funding Information
  • Horizon 2020 (101004719)
  • European Research Council (949278)
  • National Research Foundation of Korea (2021R1A2C1011718)
  • National Research Foundation of Korea (RS-2024-00416859)
  • National Research Foundation of Korea (RS-2025-25402622)
  • PCMI
  • Ministry of Education of Taiwan (110J0353I9)
  • National Science and Technology Council, Taiwan (NSTC111-2112-M-007-014-MY3)
  • National Science and Technology Council, Taiwan (NSTC113-2639-M-A49-002-ASP)
  • National Science and Technology Council, Taiwan (NSTC113-2112-M-007- 027)
  • IAU Gruber Foundation
  • Max-Planck-Gesellschaft
  • Ministry of Education, Culture, Sports, Science, and Technology of Japan (20H05845)
  • Ministry of Education, Culture, Sports, Science, and Technology of Japan (20H05844)
  • RIKEN

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