Abstract
Understanding how stars and planetary systems form requires linking the structure of young stellar environments to the processes that shape their observable spectra. This is especially challenging in embedded protostellar systems, where the excitation of atomic and molecular species and the transfer of radiation through dense, dusty material complicate the interpretation of their spectra. More generally, the wide diversity of structures and physical conditions across star-forming environments leads to corresponding diversity in their observed spectra and spatial emission. With the James Webb Space Telescope (JWST), it is now possible to probe these environments in incredible detail at infrared wavelengths, where both dust and gas leave strong observational signatures. This dissertation uses JWST integral field spectroscopy to investigate how infrared atomic, molecular, and continuum emission can disentangle the structure of protostellar systems, the attenuation of light by circumstellar and foreground material, and the processes that excite their gas.
I begin with the Class I protostar TMC1A, where JWST observations provided the first evidence for a deeply embedded atomic jet. Analysis of the [Fe II] emission revealed that the bipolar outflow is intrinsically asymmetric, even after accounting for geometry and differential foreground attenuation. More generally, extinction corrections are needed to recover the intrinsic line intensities used to infer gas properties, and thus accurate characterization of the extinction curve, which describes how dust attenuation varies with wavelength, is essential for interpreting embedded protostars. However, the extinction laws commonly applied are largely derived from non-star-forming environments and may not be representative of protostellar envelopes, the dense infalling material that surrounds young stars and feeds the planet-forming disk. In a follow-up study, we used the embedded TMC1A jet itself as a background beacon to place the first direct constraints on the extinction curve through a protostellar envelope, tracing dust attenuation from ~1.2 to 26 microns and finding significant deviations from standard interstellar extinction laws. These results suggest that dust in the envelope likely already evolved beyond the properties of diffuse interstellar and more dense molecular cloud material.
I then turn to the more evolved Class II stage, where the surrounding envelope has largely dissipated and the disk and outflow can be more directly separated across near- and mid-infrared wavelengths using different atomic and molecular tracers. In recent work, we found that individual H2 ro-vibrational transitions trace both extended disk gas and outflow structure. Comparisons between observed H2 line ratios, spatial distributions, and excitation models indicate that the disk emission cannot be explained by a single mechanism, but instead requires contributions from both ultraviolet irradiation and cosmic rays. This provides the first inferred evidence for cosmic-ray excited H2 emission in a protoplanetary disk and suggests that cosmic rays may penetrate deeper into such disks than is often assumed, with important implications for disk chemistry and evolution. Building on this, my ongoing work extends the analysis to five edge-on disk systems and incorporates a broader range of atomic and molecular emission, enabling comparisons across sources with diverse outflow structures and between different emission tracers.
Together, these results demonstrate the power of JWST integral field spectroscopy to separate overlapping components of protostellar systems and connect infrared line emission to the physical processes that shape star- and planet-forming environments. More broadly, this work shows that the diversity of infrared spectra in young stellar objects reflects corresponding diversity in structure, dust properties, and excitation conditions, and provides a strong foundation for interpreting the increasingly detailed observations of star and planet forming regions.
