The search for interstellar molecules and the effect of physical and chemical properties of the interstellar medium (ISM) on the abundance of molecules lie at the heart of Astrochemistry. One essential motivation is to understand how the molecules can be found and the physical properties of the ISM can be affected and traced by the chemistry of the high-mass star-forming region. While the number of species detected in the interstellar medium is increasing, we require an understanding of the chemistry inside the interstellar medium.
In high-mass star-forming regions, the density and temperature of molecules are high, leading to more complex molecules such as methanol, ethanol, Nitrogen-bearing molecules, and Sulfur-bearing molecules. In particular, Sagittarius B2 (Sgr B2) is the most active high mass star-forming region in our Galaxy. Moreover, Sgr B2 is embedded in the Center Molecular Zone (CMZ) region, which leads to Sgr B2 being an excellent target for studying the production of complex molecules under extreme physical conditions (high densities, strong radiation field, high cosmic-ray flux) has long been a favorite region to search for new molecules.
In this research, we take advantage of the high angular resolution and high sensitivity of the Atacama Large Millimeter/submillimeter Array (ALMA). In astonishing detail, the observed ALMA data allow us to examine the spatial distribution and structure of molecules in the massive star-forming regions Sgr B2 (Main). I concentrate on sulfur-bearing molecules and derive the physical and chemical properties of the molecules in Sgr B2(Main).
The second part of the thesis analyzes their physical evolution from the cold pre-stellar phase to the warm-up phase. Using the astrochemical code Saptarsy, we simulate the chemical evolution of selected sulfur-bearing molecules in 106 years and analyze the impacts of physical conditions of the molecular cloud. We can constrain the physical parameter of the cloud, such as the cosmic rays, the density distribution of the molecular cloud, the thermal evolution of the cloud, and the minor impacts of the molecules’ abundance: the binding energy of species to water ice surfaces, which is the intrinsic parameter of the molecules and impact the gas-grain chemical models.
In addition, among the physical parameters of the cloud that we have constrained, we found that cosmic rays ionization is essential, which significantly influences the cloud chemistry producing the ions H+, H3+, H2+, and He+. These ions take part in the formation and destruction processes of COMs.
We ran the astrochemical models and investigated the dependence of the abundances on the cosmic-ray ionization rate inside the molecules cloud. We compared the astrochemical model results to the abundance of molecules derived from the observations of the hot cores. Our results showed that cosmic-ray ionization rates of 1.3 10^{−16} s^{-1} match the observed abundance of Sulfur-bearing molecules in Sgr B2(Main)
