Master thesis defense by Simon Hilding-Nørkjær
Title: Galactic heritage of chemistry around young stars
Abstract
The chemical composition of planetary systems is set by the material in proto-planetary disks, which itself inherits a chemical history shaped by millions of years of evolution through the interstellar medium (ISM). In particular, the deuterium fractionation of water, measured as HDO/H2O and D2O/HDO, serves as a sensitive tracer of the thermal and radiative history experienced by interstellar material before it is incorporated into forming stellar systems. Observations from flagship facilities including the James Webb Space Telescope reveal substantial diversity in water deuteration across both Solar System objects and proto-stellar environments, and a systematic discrepancy has been identified between isolated and clustered proto-stars. Understanding what drives this diversity requires connecting large-scale galactic environments to the physicochemical evolution of material on its path from diffuse gas to proto-planetary disks — a link that simple static chemical models cannot provide.
A (250 pc)3 magnetohydrodynamical RAMSES simulation of the turbulent ISM, containing approximately 109 AMR cells, 250 million Lagrangian tracer particles, and more than 10,000 young stars, is post-processed using continuum dust radiative transfer across the full domain. This includes all stellar sources and background radiation to estimate the dust temperature and the far-ultraviolet (FUV) radiation field throughout the simulation volume. Tracer particles accreted by selected protostars are back-propagated through 125 simulation snapshots spanning 3.75 Myr of evolution, and their physical histories — including density, dust temperature, and FUV intensity — are reconstructed using cloud-in-cell interpolation. Along these trajectories, a large gas-grain chemical network is evolved using KEMIMO, following the formation and deuteration of water across 1866 species and more than 52,000 reaction pathways. The analysis targets proto-stellar systems embedded in molecular cloud environments of differing virial parameter to investigate how cloud-scale dynamics influence chemical outcomes.
The reconstructed tracer trajectories exhibit substantial environmental diversity in shielding history, thermal evolution, and radiation exposure, with the degree of variation reflecting the dynamical state of the host molecular cloud. Evolving the chemical network along these trajectories reveals significant star-to-star variation in both gas-phase and ice deuteration ratios, indicating that different ISM environments lead to chemically distinct evolutionary pathways. Ice deuteration ratios are found to remain comparatively stable after their initial cold-cloud build-up, while gas-phase ratios show considerable late-time variability driven by local thermal and radiative processing. The spread in final deuteration level across proto-stellar systems in the high-virial cloud spans nearly two orders of magnitude, substantially exceeding that seen in the lower-virial environment, consistent with a more turbulently heterogeneous accretion history.
The results indicate that much of the observable water deuteration signature is established prior to proto-stellar collapse and reflects long-term environmental evolution through the ISM and molecular cloud phases rather than local conditions near the forming star. This work demonstrates the feasibility of combining galaxy-scale MHD simulations, full-domain radiative transfer, Lagrangian trajectory reconstruction, and large gas-grain chemical networks into a unified framework for studying the environmental origin of proto-planetary chemistry. Identifying which environmental quantities dominate the chemical evolution ultimately requires substantially larger tracer statistics, motivating a transition from individual trajectory analysis toward population-level statistical inference connecting environmental histories directly to observable chemical signatures.
Censor
Hans Kjeldsen, AU