Date of Award


Degree Name

Doctor of Philosophy



First Advisor

Dr. Manuel Bautista

Second Advisor

Dr. Kirk Korista

Third Advisor

Dr. Thomas Gorczyca

Fourth Advisor

Dr. Mark Voit


We study time-dependent photoionization of gaseous nebulae, i.e. the physical conditions and spectra of astronomical plasmas photoionized by a time-dependent source of ionizing radiation. Our study proceeds in two chief steps: First, we start with a simplified model of plasmas of pure H. Second, we develop a more realistic model of plasmas composed of a mixture of chemical elements. For the first step, we wrote a time-dependent photoionization code (TDP) that solves the coupled system of equations for ionization, energy balance, and radiation transfer in their full time-dependent forms For the second step, we developed a more realistic code (TDXSTAR) to solve for the excitation, ionization, thermal, and radiative transfer equations in their full time-dependent forms using full atomic model. The TDXSTAR code is based on the well known steady-state code XSTAR and capable of including all chemical elements from hydrogen (Z=1) to nickel (Z=28).

TDP and TDXSTAR simulations of a pure hydrogen nebula with constant density show that ionization and thermal fronts are created due to flux variations and propagate (often supersonic) through the cloud over time scales that vary widely and non-linearly across the nebula. Further, simulations for slabs initially in pressure equilibrium show that thermal and pressure fronts propagate through the plasma, which become particularly pronounced across the ionization front (IF). In addition, periodic variations in the ionizing flux lead to the following conclusions, (1) the instantaneous physical conditions of the plasma are different from any steady-state solution, (2) the time-averaged conditions are different from any steady-state solution and characterized by over-ionization and a broader IF with respect to the steady-state solution for a mean value of the radiation flux. (3) the dispersions in the physical conditions from their time-averaged values are increase period of variations in the flux, (4) variations in physical conditions are asynchronous along the slab due to the combination of non-linear propagation times for thermal and ionization fronts and equilibration times.

Further, we used TDXSTAR to study the physical conditions in planetary nebulae (PNe) that are experiencing a steady decline in stellar temperature. Simulations show that spectra of different ions respond differently to the stellar evolution; while some lines decay others are enhanced. Differences in initial stellar temperatures don’t affect the spectral lines trend. However, higher initial stellar temperature yield more intense forbidden lines due to the higher electron temperature. Moreover, models with different gas densities show that gas densities determine the time scale of the gas response to the changes in the ionizing radiation. Furthermore, models of different fixed luminosities produce similar conditions throughout the nebulae where luminosities and distances scale in a way such that the radiation field stays nearly the same across the nebulae, with the exception He II lines.

In addition, we studied the time-dependent effects in H II regions and planetary nebulae ionized by binary systems. Simulations of different periods from a few days to decades show that short-period binaries (with periods much shorter than equilibration times in the gas) have no noticeable effects on the gas conditions. In contrast, binary systems with long periods (comparable to equilibration times in the nebula) have noticeable effects on gas conditions. The time-averaged temperature profiles are different from the steady-states corresponding to the mean flux. Further, time-averaged temperature profiles show two peaks and flatter profiles compared to that of the steady-state corresponding to the mean of the ionizing flux. The two-peak average temperature profiles suggest binaries as probable sources of temperature fluctuations. Furthermore, the average temperature weighted by collisionally excited and recombination lines integrated over nebular volume exhibit clear discrepancies in the temperature. Thus, adopting an average temperature for spectroscopic abundances determinations are expected to lead to discordances in the estimated abundances.

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