This thesis has investigated the chemical evolution of the inner regions (r smaller or equal to 10 AU) of a modelled protoplanetary disk surrounding a low-mass T Tauri star; a phase our own solar system underwent some 4.5-4.6 billion years ago. A 1+1- dimensional physical model of a radially-accreting protoplanetary disk was combined with a chemical model consisting of gas-phase reactions extracted from the RATE95 UMIST database for Astrochemistry, and to this, a network of gas-grain interaction and deuterated reactions were added.Influenced by the knowledge that radionuclides may have been abnormally abundant in our early solar system compared with the interstellar medium, and that the energy expelled from their decay is sufficient to ionize molecules, a 1-dimensional simulation along the disk midplane was performed, comparing the chemistry with and without radionuclides, as a function of the radionuclide ionization rate. The molecules C4H2, HC3N, C3H, HCN, CH4, C2H2 and N2 were found to be particularly sensitive. Of these, HCN and C2H2 have already been detected in protoplanetary disks.Motivated by observations which suggest that T Tauri systems vary from faster to slower accretion rates, the chemical distributions of two disks with stellar accretion rates of 10-7 M⊙ yr-1 and 10-8 M⊙ yr-1 were compared. Allowing the mass accretion rate (and thus physical conditions) to vary in time, starting from 10-7 M⊙ ￼yr-1, and evolving to a mass accretion rate of 10-8 M⊙ yr-1, the molecules CN, HCN, H2CO and NH3 were found to be particularly sensitive when compared to a standalone 10-8 M⊙ yr-1 simulation.With the use of a 1-D CASA LTE algorithm for ALMA, the sensitivities of HCN and H2CO were transcribed into integrated intensity differences, as a function of emissivity and optical depth. The largest differences were associated with the largest feasible transitions of HCN and H2CO; J = 8-7 and JK = 10010-909 respectively, but could not be converted into potentially observable integrated fluxes due to the restrictions of this model. Both molecules were found to trace different regions of the disk.Using the stellar accretion rate-to-age linear relation evaluated from observations, the calculation times for the 10-7 M⊙ yr-1 and 10-8 M⊙ yr-1 were re-evaluated, of which CN and NH3 emerged as the most sensitive molecules. Thus, CN and NH3 may be a possible tracer of calculation time, and disk age.
|Date of Award||1 Aug 2014|
- The University of Manchester
|Supervisor||Andrew Markwick (Supervisor)|