Date of Award
Doctor of Philosophy (PhD)
Physics and Astronomy
Star and planet formation is intimately tied to the accretion of material from the environments in which they form. During the formation process, disks of gas and dust develop in young stellar objects through which material is facilitated to the star and forming planets. Theoretical models of these accretion processes invoke viscous spreading via hydrodynamics, as well as more complex interactions with magnetic fields be it from the stellar component or the formation environment in order to catalyze these mass flows. These accretion models predict various scenarios including magnetospheric accretion as well as supersonic accretion flows in the disk atmosphere which can be observed using high resolution optical and infrared spectroscopy.
In this dissertation, I report the results of my investigation of the accretion signatures of T Tauri (< 2 Msun), intermediate mass T Tauri (IMTT), and Herbig (> 2 Msun) systems. The Class I source, GV Tau, is located in the Taurus molecular cloud, and is a binary system of T Tauri stars that are embedded in a molecular envelope. Previous observations of GV Tau North have uncovered a rich spectrum containing many molecules which incorporate carbon, hydrogen, oxygen, and nitrogen. In the mid-infrared, these molecules contain low- and high-velocity red-shifted spectral components whose properties are consistent with MHD simulations of supersonic surface accretion flows in the disk atmosphere. Here I present the high resolution near-infrared iSHELL spectrum of GV Tau North and investigate the inferred properties of its protoplanetary disk using a curve of growth analysis alongside a synthetic spectral model of the system. The M-band spectrum reveals hundreds of absorption lines of fundamental 12CO, 13CO, and C18O transitions, as well as a nominal detection of the 12CO v=2-1 transitions. All of the CO isotopologues exhibit a red-shifted absorption feature that is convolved with at least three Doppler shifted components which have supersonic in-flow velocities. Results from our synthetic spectral model infer accretion rates for these red-shifted CO components on the order of 10-8 Msun yr-1 - in agreement with the rates inferred from the mid-infrared species. Coincidentally, a high-velocity blue-shifted component consistent with the properties of a molecular outflow is observed simultaneously in the fundamental 12CO. This is the first empirical evidence that the supersonic surface accretion flow observed in GV Tau N may be related to an MHD disk wind in the inner few au of the disk.
In addition to the detailed analysis of the near-infrared spectrum of GV Tau N, a study of the accretion rates of IMTTSs and Herbig stars is presented. The results find that the median accretion rate of IMTTSs (1.1 x 10-8 Msun yr-1) is lower than that of Herbig stars (2.2 x 10-7 Msun yr-1) by roughly an order of magnitude. It is shown that the pre-main sequence evolution of intermediate mass stars, specifically the hardening of the far ultra-violet field as their pre-main sequence temperature increases, coincides with the trends observed in their accretion rates as they evolve across the HR-diagram. A consequence of this model of accretion in intermediate mass stars is the prediction of a large population of low or non-accreting post Herbig stars. A comparison of the population statistics of intermediate mass stars in the local neighborhood (d < 300 pc) suggests that hybrid and/or debris disks may represent the low or non-accreting phase of intermediate mass stars just as they evolve onto the main sequence.
Kern, Joshua, "An Investigation of the Accretion Processes in T Tauri and Herbig Ae/Be Systems Using High Resolution Optical and Near-Infrared Spectroscopy" (2023). All Dissertations. 3525.
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