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 $n=100-1000~\mr{cm^{-3}}$, 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.