Simulating chemistry in star forming environments

Abstract

Chemistry plays an important role in the interstellar medium (ISM), regulating the heating and cooling of the gas and determining abundances of molecular species that trace gas properties in observations. One of the most abundant and important molecules in the ISM is \CO. \CO is a main coolant for the molecular ISM, and the \CO(J=1−0) line emission is a widely used observational tracer for molecular clouds. In Chapter 2, we propose a new simplified chemical network for hydrogen and carbon chemistry in the atomic and molecular ISM. We compare results from our chemical network in detail with results from a full photodissociation region (PDR) code, and also with the \citet{NL1999} (NL99) network previously adopted in the simulation literature. We show that our chemical network gives similar results to the PDR code in the equilibrium abundances of all species over a wide range of densities, temperature, and metallicities, whereas the NL99 network shows significant disagreement. We also compare with observations of diffuse and translucent clouds. We find that the \CO, \CHx and \OHx abundances are consistent with equilibrium predictions for densities n=100−1000 \mrcm−3, but the predicted equilibrium \CI abundance is higher than observations, signaling the potential importance of non-equilibrium/dynamical effects. In Chapter 3, we apply our new chemistry network to a study of the X\CO conversion factor, which is used to convert the \CO luminosity to the total \Ht mass. We use numerical simulations to investigate how X\CO depends on numerical resolution, non-equilibrium chemistry, physical environment, and observational beam size. Our study employs 3D magnetohydrodynamics (MHD) simulations of galactic disks with solar neighborhood conditions, where star formation and the three-phase interstellar medium (ISM) is self-consistently generated by the interaction between gravity and stellar feedback. Synthetic \CO maps are obtained by post-processing the MHD simulations with chemistry and radiation transfer. We find that \CO is only an approximate tracer of \Ht. Nevertheless, $\langle X_\CO \rangle=0.7-1.0\times 10^{20}~ \mr{cm^{-2}K^{-1}km^{-1}s}$ consistent with observations, insensitive to the evolutionary ISM state or the far-ultraviolet (FUV) radiation field strength. Our numerical simulations successfully reproduce the observed variations of X\CO on parsec scales, as well as the dependence of X\CO on extinction and the \CO excitation temperature.