2024A&A...686A..59N

Context. Complex organic molecules (COMs) have been detected ubiquitously in protostellar systems. However, at shorter wavelengths (∼0.8 mm), it is generally more difficult to detect larger molecules than at longer wavelengths (∼3 mm) because of the increase in millimeter dust opacity, line confusion, and unfavorable partition function.
Aims. We aim to search for large molecules (more than eight atoms) in the Atacama Large Millimeter/submillimeter Array (ALMA) Band 3 spectrum of IRAS 16293-2422 B. In particular, the goal is to quantify the usability of ALMA Band 3 for molecular line surveys in comparison to similar studies at shorter wavelengths.
Methods. We used deep ALMA Band 3 observations of IRAS 16293-2422 B to search for more than 70 molecules and identified as many lines as possible in the spectrum. The spectral settings were set to specifically target three-carbon species such as i- and n-propanol and glycerol, the next step after glycolaldehyde and ethylene glycol in the hydrogenation of CO. We then derived the column densities and excitation temperatures of the detected species and compared the ratios with respect to methanol between Band 3 (∼3 mm) and Band 7 (∼1 mm, Protostellar Interferometric Line Survey) observations of this source to examine the effect of the dust optical depth.
Results. We identified lines of 31 molecules including many oxygen-bearing COMs such as CH₃OH, CH₂OHCHO, CH₃CH₂OH, and c-C₂H₄O and a few nitrogen- and sulfur-bearing ones such as HOCH₂CN and CH₃SH. The largest detected molecules are gGg-(CH₂OH)₂ and CH₃COCH₃. We did not detect glycerol or i- and n-propanol, but we do provide upper limits for them which are in line with previous laboratory and observational studies. The line density in Band 3 is only ∼2.5 times lower in frequency space than in Band 7. From the detected lines in Band 3 at a ≳ 6σ level, ∼25-30% of them could not be identified indicating the need for more laboratory data of rotational spectra. We find similar column densities and column density ratios of COMs (within a factor ∼2) between Band 3 and Band 7.
Conclusions. The effect of the dust optical depth for IRAS 16293-2422 B at an off-source location on column densities and column density ratios is minimal. Moreover, for warm protostars, long wavelength spectra (∼3 mm) are not only crowded and complex, but they also take significantly longer integration times than shorter wavelength observations (∼0.8 mm) to reach the same sensitivity limit. The 3 mm search has not yet resulted in the detection of larger and more complex molecules in warm sources. A full deep ALMA Band 2-3 (i.e., ∼3-4 mm wavelengths) survey is needed to assess whether low frequency data have the potential to reveal more complex molecules in warm sources.