Numerical Modeling of Chemistry-coupled Radiative Transfer

In this thesis a fundamental physical process, radiative transfer, is modeled numerically. The implementation as a code module for the hydrodynamical simulation code Flash 4 is presented. The coupling to an efficient chemical network that explicitly tracks the three hydrogen species H, H_2, H+ and the two carbon species C+ and CO is described as well as the modeling of all relevant thermal stellar feedback mechanisms, i.e. photoelectric heating, pumping of molecular hydrogen by UV photons, photoionization and H_2 dissociation heating. These modeled processes coupled to the chemical network, make it possible to capture the non-equilibrium time-dependent thermal and chemical state of the present-day interstellar medium and dense molecular clouds affected by radiative feedback of massive stars. All included radiative feedback processes are extensively tested. The results obtained with this code module are compared to ones calculated from dedicated photo-dissociation region (PDR) codes. Good agreement is found in all modeled hydrogen species once the radiative transfer solution reaches equilibrium. In addition, it is shown that the implemented radiative feedback physics is insensitive to the spatial resolution of the simulation mesh and under which conditions a well-converged evolution in time can be obtained. The last test cases explore the robustness of the developed numerical scheme in treating the combined ionizing and non-ionizing radiation. In a follow-up study, different simplified numerical radiative transfer models are compared in the context of ionization front instabilities. The growth of unstable modes is found to be strongly dependent on the coupling of the thermal state to the ionization state. Depending on the implemented model, radically different conclusions can be drawn. For an equilibrium ionization model with a bimodal temperature structure for ionized and ambient gas, the swept up surrounding shell is found to be unstable. However, if the temperature of the ionized gas is calculated from the equilibrium ionization heating rate no instability is found. Finally, a damped ionization front instability is obtained from the newly implemented code module, which is unable to impact and perturb the shell sufficiently for it to break up

Feeding versus Falling: The Growth and Collapse of Molecular Clouds in a Turbulent Interstellar Medium

In order to understand the origin of observed molecular cloud (MC) properties, it is critical to understand how clouds interact with their environments during their formation, growth, and collapse. It has been suggested that accretion-driven turbulence can maintain clouds in a highly turbulent state, preventing runaway collapse and explaining the observed non-thermal velocity dispersions. We present 3D, adaptive-mesh-refinement, magnetohydrodynamical simulations of a kiloparsec-scale, stratified, supernova-driven, self-gravitating, interstellar medium (ISM), including diffuse heating and radiative cooling. These simulations model the formation and evolution of a MC population in the turbulent ISM. We use zoom-in techniques to focus on the dynamics of the mass accretion and its history for individual MCs. We find that mass accretion onto MCs proceeds as a combination of turbulent flow and near free-fall accretion of a gravitationally bound envelope. Nearby supernova explosions have a dual role, compressing the envelope and increasing mass accretion rates, but also disrupting parts of the envelope and eroding mass from the cloud's surface. It appears that the inflow rate of kinetic energy onto clouds from supernova explosions is insufficient to explain the net rate of change of the cloud kinetic energy. In the absence of self-consistent star formation, the conversion of gravitational potential into kinetic energy during contraction seems to be the main driver of non-thermal motions within clouds. We conclude that although clouds interact strongly with their environments, bound clouds are always in a state of gravitational contraction, close to runaway, and their properties are a natural result of this collapse

The SILCC project - IV. Impact of dissociating and ionizing radiation on the interstellar medium and Ha emission as a tracer of the star formation rate

We present three-dimensional radiation-hydrodynamical simulations of the impact of stellar winds, photoelectric heating, photodissociating and photoionizing radiation, and supernovae on the chemical composition and star formation in a stratified disc model. This is followed by a sink-based model for star clusters with populations of individual massive stars. Stellar winds and ionizing radiation regulate the star formation rate at a factor of similar to 10 below the simulation with only supernova feedback due to their immediate impact on the ambient interstellar medium after star formation. Ionizing radiation (with winds and supernovae) significantly reduces the ambient densities for most supernova explosions to. 10-25 g cm(-3), compared to 10-23g cm(-3) for the model with only winds and supernovae. Radiation from massive stars reduces the amount of molecular hydrogen and increases the neutral hydrogen mass and volume filling fraction. Only this model results in a molecular gas depletion time-scale of 2 Gyr and shows the best agreement with observations. In the radiative models, the Ha emission is dominated by radiative recombination as opposed to collisional excitation (the dominant emission in non-radiative models), which only contributes similar to 1-10 per cent to the total Ha emission. Individual massive stars (M >= 30M(circle dot)) with short lifetimes are responsible for significant fluctuations in the Ha luminosities. The corresponding inferred star formation rates can underestimate the true instantaneous star formation rate by a factor of similar to 10

LAUNCHING COSMIC-RAY-DRIVEN OUTFLOWS FROM THE MAGNETIZED INTERSTELLAR MEDIUM

We present a hydrodynamical simulation of the turbulent, magnetized, supernova (SN)-driven interstellar medium (ISM) in a stratified box that dynamically couples the injection and evolution of cosmic rays (CRs) and a self-consistent evolution of the chemical composition. CRs are treated as a relativistic fluid in the advection-diffusion approximation. The thermodynamic evolution of the gas is computed using a chemical network that follows the abundances of H+, H, H-2, CO, C+, and free electrons and includes (self-) shielding of the gas and dust. We find that CRs perceptibly thicken the disk with the heights of 90% (70%) enclosed mass reaching greater than or similar to 1.5 kpc (greater than or similar to 0.2 kpc). The simulations indicate that CRs alone can launch and sustain strong outflows of atomic and ionized gas with mass loading factors of order unity, even in solar neighborhood conditions and with a CR energy injection per SN of 10(50) erg, 10% of the fiducial thermal energy of an SN. The CR-driven outflows have moderate launching velocities close to the midplane (less than or similar to 100 km s(-1)) and are denser (rho similar to 10(-24)-10(-26) g cm(-3)), smoother, and colder than the (thermal) SN-driven winds. The simulations support the importance of CRs for setting the vertical structure of the disk as well as the driving of winds