2020A&A...640A..75D

Context. The interstellar complex organic molecules (iCOMs) are C-bearing molecules containing at least six atoms; two main proposals for their formation are suggested: a direct formation in the icy mantle of the dust grains and formation through the reaction in gas phase of released grain mantle species. The shocked gas along outflows driven by low-mass protostars is a unique environment to study how the iCOMs can be formed as the composition of the dust mantles is sputtered into the gas phase.
Aims. The chemical richness in shocked material associated with low-mass protostellar outflows has been so far studied in the prototypical L1157 blue-shifted outflow to investigate the iCOM formation routes. To understand whether the case of L1157-B1 is unique, we imaged and studied the IRAS 4A outflows in the NGC 1333 star forming region.
Methods. We used the NOrthern Extended Millimeter Array interferometer as part of the IRAM Seeds Of Life in Space (SOLIS) Large Program to image the large-scale bipolar outflows driven by the IRAS 4A system in the 3 mm band, and we compared the observation with the GRAINOBLE+ astrochemical model.
Results. We report the first detection, in the IRAS 4A outflows, of several iCOMs: six lines of methanol (CH₃OH), eight of acetaldehyde (CH₃CHO), one of formamide (NH₂CHO), and four of dimethyl ether (CH₃OCH₃), all sampling upper excitation energy up to ∼30 K. We found a significant chemical differentiation between the southeast outflow driven by the IRAS 4A1 protostar, showing a richer molecular content, and the north-southwest one driven by the IRAS 4A2 hot corino. The CH₃OH/CH₃CHO abundance ratio is lower by a factor of ∼4 in the former; furthermore, the ratio in the IRAS 4A outflows is lower by a factor of ∼10 with respect to the values found in different hot corinos.
Conclusions. After L1157-B1, the IRAS 4A outflow is now the second outflow to show an evident chemical complexity. Given that CH₃OH is a grain surface species, the astrochemical gas-phase model run with GRAINOBLE+ reproduced our observation assuming that acetaldehyde is formed mainly through the gas-phase reaction of the ethyl radical (CH₃CH₂) and atomic oxygen. Furthermore, the chemical differentiation between the two outflows suggests that the IRAS 4A1 outflow is likely younger than that of the IRAS 4A2. Further investigation is needed to constrain the age of the outflow. In addition, observation of even younger shocks are necessary. In order to provide strong constraints on the CH₃CHO formation mechanisms it would be interesting to observe CH₃CH₂, but given that its frequencies are not known, future spectroscopic studies on this species are needed.