2024A&A...686A.289H

Context. Complex organic molecules (COMs) have been found toward low-mass protostars, but the origins of the COM emission are still unclear. It can be associated with, for example, hot corinos, outflows, and/or accretion shock and disk atmospheres.
Aims. We aim to disentangle the origin of the COM emission toward the chemically rich protobinary system SVS13A using six O-bearing COMs.
Methods. We conducted NOrthern Extended Millimeter Array observations toward SVS13A as part of the PROtostars & DIsks: Global Evolution (PRODIGE) program. Our previous DCN observations reveal a possible infalling streamer, which may affect the chemistry of the central protobinary by inducing accretion outbursts and/or shocked gas. We further analyzed six O-bearing COMs: CH₃OH, aGg'- (CH₂OH)₂, C₂H₅OH, CH₂(OH)CHO, CH₃CHO, and CH₃OCHO. Although the COM emission is not spatially resolved, we constrained the source sizes to ≲0.3-0.4 arcsec (90-120 au) by conducting uv-domain Gaussian fitting. Interestingly, the high-spectral-resolution data reveal complex line profiles with multiple peaks; although the line emission is likely dominated by the secondary, VLA4A, at V_LSR = 7.36 km s^–1, the numbers of peaks (∼2-5), the velocities, and the linewidths of these six O-bearing COMs are different. The local thermodynamic equilibrium (LTE) fitting unveils differences in excitation temperatures and emitting areas among these COMs. We further conducted multiple-velocity-component LTE fitting to decompose the line emission into different kinematic components. As a result, the emission of these COMs is decomposed into up to six velocity components from the LTE modeling. The physical conditions (temperature, column density, and source size) of these components from each COM are obtained, and Markov chain Monte Carlo sampling was performed to test the fitting results.
Results. We find a variety in excitation temperatures (100-500 K) and source sizes (D ∼ 10-70 au) from these kinematic components from different COMs. The emission of each COM can trace several components, and different COMs most likely trace different regions.
Conclusions. Given this complex structure, we suggest that the central region is inhomogeneous and unlikely to be heated by only protostellar radiation. We conclude that accretion shocks induced by the large-scale infalling streamer likely exist and contribute to the complexity of the COM emission. This underlines the importance of high-spectral-resolution data when analyzing COM emission in protostars and deriving relative COM abundances.