PhD Defense by Sébastien Manigand

Title: The origin of complex organic molecules in star forming regions

Complex organic molecules (COMs, i.e. molecules with six or more atoms with at least one carbon) are present in the gas around young stars in the earliest stages of their evolution. Similar molecules are also found in meteorites and comets in our Solar system. Their evolution throughout low-mass star formation is closely linked to what could have occurred during the formation of the Solar system. Thus, it is important to study the chemistry of COMs of the earliest evolutionary stage to understand how these molecules are formed, how they can form prebiotic molecules, such as amino acids, and investigate whether they potentially could be important for the origin of life elsewhere. In this thesis, the inventory of oxygen-bearing COMs is studied towards the low-mass Class 0 protostellar binary IRAS 16293–2422 using the interferometric observations from the Protostellar Interferometric Line Survey (PILS), carried out by the Atacama Large Millimeter/submillimeter Array (ALMA). 

The high spectral resolution, and the high sensitivity of the observations, revealed ∼10 000 lines belonging to more than 100 molecular species, including a number of rarer isotopologues of different COMs. In particular, the first detection of doubly-deuterated methyl formate (CHD₂OCHO) is reported. The increased deuteration of the multiply deuterated variants were also seen for other species and is discussed in the context of theoretical models. The formation of COMs is studied by using a chemical model in the context of the detection of propenal (C₂H₃CHO) and propylene (C₃H₆). The comparison between the COMs abundances found towards two binary components and the spatial extent of the different molecular emissions suggests that the hot corino region shows a layered structure in terms of its molecular compositions. This layer-structure seems to be correlated with the desorption temperatures of the different species. Finally, the kinematics of the infalling gas is studied through the analysis of the spectroscopic line observations, in particular the fit of inverse P Cygni profiles. This study demonstrated that the differentiation exists in the temperatures where different groups of organic molecules (oxygen-bearing and nitrogen-bearing) are present.

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