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

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Abstract
Context . Methyl cyanide (CH 3 CN) is one of the most abundant nitrogen-bearing complex organic molecules observed in the gas phase around both low- and high-mass protostars. However, very few observations exist with a broad enough frequency coverage and sufficiently high sensitivity to systematically characterize its minor isotopologues toward such regions. Aims . We add to this number by analyzing data toward BHR71-IRS1 taken with the Atacama Large Millimeter/submillimeter Array (ALMA) as part of the Complex Organic Molecules in Protostars with ALMA Spectral Surveys (COMPASS) Large Program over a ∼33 GHz frequency range. Methods . We studied as many isotopologues of methyl cyanide as possible toward BHR71-IRS1. We found the column densities and excitation temperatures via spectral fitting and assuming local thermodynamic equilibrium. We then compared the derived column density ratios with those toward IRAS 16293-2422 A and B, V883 Ori, G31.41+0.31, and Sgr B2(N2b). Results . We detect CH 3 CN, CH 3 CN v 8 =1, 13 CH 3 CN, CH 3 13 CN, CH 3 C 15 N, CH 2 DCN, and CHD 2 CN. The column density ratio of 13 CH 3 CN/CH 3 13 CN is around unity, in agreement with previous observations. The CH 2 DCN/CH 3 CN ratios are similar for all the low-mass objects but are ∼1 order of magnitude higher than those toward Sgr B2(N2b). This could be due to a warmer pre-stellar phase or a different formation timescale of Sgr B2(N2b). Conclusions . The D/H ratios found from methyl cyanide isotopologues agree well with those of methanol toward BHR71-IRS1, likely pointing to the formation and deuteration of methyl cyanide in a similar star-formation phase as methanol (i.e., the pre-stellar phase). We find an increase in the methyl cyanide D/H ratios with the number of deuterium atoms, which suggests that the deuterated isotopologues may form later in the pre-stellar phase. Future COMPASS studies will increase the sample size of low-mass systems with methyl cyanide analysis and potentially examine some of the trends observed here statistically.
Funding Information
  • NKFIH Excellence Grant (TKP2021-NKTA-64)
  • Center for Informatics and Computation in Astronomy
  • Riken Special Postdoctoral Researcher Program
  • Ministry of Education of Taiwan (110J0353I9)
  • NKFIH NKKP (ADVANCED 149943)
  • National Science and Technology Council, Taiwan (NSTC111-2112-M-007-014-MY3, NSTC113-2639-M-A49-002-ASP, and NSTC113-2112-M-007-027)
  • European Union (101004719 [ORP])
  • European Southern Observatory (ESO Fellowship)
  • Gruber Foundation (IAU Gruber Foundation Fellowship)
  • European Research Council (949278)
  • CNRS Physique
  • CNRS Chimie
  • Centre National d’Etudes Spatiales
  • CEA
  • National Research Foundation of Korea (RS-2026-25490557 and RS-2024-00416859)
  • National Research Foundation of Korea (RS-2025-25402622)
  • European Cooperation in Science and Technology (PLANETS CA22133)
  • Max Planck Society
  • Ministry of Education, Culture, Sports, Science, and Technology of Japan (20H05845, 20H05844)

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