Abstract
The chemical composition of forming planets depends on how material is processed as it moves from molecular clouds into protoplanetary disks. However, how physical conditions regulate this process, and how these effects translate into observable chemical signatures, remains unclear. This thesis investigates how physical environments shape chemistry using a combination of modeling, infrared observations, and laboratory experiments, focusing on the impact of external radiation fields on disk chemistry, quantifying ice absorption features across protostellar environments, and how ice composition influences the desorption of complex organic molecules.
First, I use gas-grain chemical models to examine how low to moderate external far-ultraviolet (FUV) radiation fields (1–100x the interstellar background) impact gas-phase chemistry in protoplanetary disks across disk structures. For this project, I analyze the response of key ionization tracers, including HCO+, N2H+, and C+, and use the radiative transfer code LIME to simulate corresponding ALMA observations. The results from this first project show that external irradiation significantly alters molecular abundances and observable line emission in the outer disk, while inner regions remain comparatively shielded.
Second, I extend this modeling project to the solid phase by examining how external radiation impacts the distribution and observability of the main ice carriers H2O, CO2, and CO in disk midplanes. Using radiative transfer calculations with RADMC-3D, I simulate near-infrared ice absorption features for a representative disk model in a highly inclined geometry, where ice is observable with instruments such as JWST/NIRSpec. I vary the external irradiation across this range and show how this impacts the spatial distribution of the ice and the resulting spatially extracted spectra, linking disk structure to observable ice features.
Sulfur is a cosmically abundant element, yet only a small fraction is observed in known gas-phase and ice species, indicating that most sulfur resides in an unidentified reservoir. To assess how ice composition varies across protostellar environments, I characterize the ice inventory of a sample of Class 0 protostars using mid-infrared spectra from the JWST CORINOS program. I find that both SO2 and CH4 ice abundances show limited variation across sources spanning different environments, indicating that their abundances are not strongly regulated by local protostellar conditions and are likely established earlier in the evolutionary sequence. By performing a systematic analysis that explicitly accounts for continuum baseline uncertainty, this work provides one of the first robust constraints on SO2 ice abundances in embedded protostars, corresponding to ~1% of the total volatile sulfur budget.
Finally, observations of complex organic molecules in cold gas-phase regions further indicate that molecules must be released from ices under conditions where temperatures are too low for thermal desorption from water-rich ice, requiring alternative mechanisms or ice environments that modify the trapping and release of more volatile species like methyl formate. In the final project, I investigate methyl formate (MF) ices through laboratory measurements to determine the binding energies (BE; the energy needed for a molecule to leave the surface and desorb into gas) on water and methanol surfaces using a laboratory technique called temperature-programmed desorption (TPD). These experiments show that a methanol-rich ice environment exhibits reduced trapping of MF, enhanced mobility, and lower effective binding energies compared to water-dominated ices, enabling desorption at temperatures of ~85 K compared to ~95–100 K for water-rich ice, where trapping can further delay release to higher temperatures.
Overall, these projects demonstrate how physical environments regulate distinct processes that regulate the distribution and balance of ice and gas. By linking disk irradiation to observable chemical signatures, characterizing ice compositions across protostellar environments, and demonstrating that molecular desorption of complex species depends on the composition of the ice matrix, this work provides new constraints on how chemical inventories are processed and observed in planet-forming systems.
