PhD defense by Manar el Akel

Title: Unlocking the sulphur chemistry in a protostellar environment: physical and chemical conditions

Abstract:
The origin of our solar system hosting the only habitable planet, the Earth, has been, for centuries, the source of human interest for space. Several studies have been conducted to better understand the universe, from the study of our solar system to the Milky Way and other galaxies. A main focus over the years has been how the stars evolve from their birth. In this study, we focus on the early stage of the star, from a chemical and physical point of view. The protostellar environment is governed by the
interaction of the gas and dust grains, leading to rich gas and surface chemistries. It is through the evolution of such processes that it will be possible to understand how the star evolves, to ultimately form a protostellar system. The chemistry occurring in protostars is vast, therefore, our attention is focused on one specific family of species: the sulphur-bearing molecules. These have been detected in several regions of the protostar, displaying different physical conditions. However, it is unclear the extent of the role this family plays in the protostellar evolution and how.

To unveil the importance of this molecular family in the protostellar environment, I conducted several studies on the sulphur chemistries based on millimeter and sub-millimiter observations, combined with in-situ laboratory experiments and modeling. Through observation, different regions of different protostars could be analysed, from the cold part (T∼10 K) to the warm region (T≥100 K) of the protostar, including the strong outflow region. In such a way, the sulphur chemistry could be analysed. Although the observational approach has been the common thread of all the previous studies, these were complemented with laboratory experiments simulating the ISM conditions and studying the reactivity of H2S, the initial product of atomic sulphur with one of the most abundant elements in the ISM, atomic hydrogen. Moreover, the modeling aspect was used in parallel with the observations for the study of an outflow, as together with the observations, they provided physical constraints on the region and understanding on the physical processes occurring there. By combining such three methods, it was possible to gain a more extensive understanding of the studied region. All the selected sources for this research are famous class 0 protostars, located in the star forming region at proximity of our solar system.

In this research, we studied the protostellar environment in two stages. On one side, we investigated the sulphur chemistry in the cold and hot environments of theprotostar. We pointed out the high reactivity of H2S in a cold surface to produce the organo-sulphur, OCS. We could determine which sulphur bearing molecules trace the cold envelope and the hot core of a protostar, by comparing several sources of the Cygnus-X complex. In this way, these molecules were quantified towards the different environments and possible chemical paths could be identified. The combination of laboratory and observational methods have shown the importance of H2S in the sulphur chemistry, as a key element for the production of organo-sulphurs
on the icy dust grains. We highlighted the early production of H2S in a protostellar environment. The study of the sulphur chemistry in the cold envelope and hot core of the protostar is an essential springboard for future work on the sulphur chemistry, and for understanding how it has evolved during the protostar evolution.

On the other side, we discussed the chemistry and physical characteristics of a well-known strong outflow, with a focus on S-bearing molecules. The combination of high-resolution observations with shock models allowed us to pinpoint the physical conditions of the outflow and provided us insights on the effects of the shock history on the chemical evolution of the region. This work demonstrated the efficient combination of both methods to interpret such regions. It therefore constitutes a reliable path for future work on outflows.

The full dissertation can be accessed at https://sid.erda.dk/share_redirect/fjorfFksxm

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