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Exploring the Chemical Evolution of Molecular Clouds

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  • Figure 1: post-processed column density projection of HCO+.
  • Figure 2: evolution of 400 selected tracer particles with regard to their local total hydrogen density and HCO+ abundance, plotted upon a heatmap of this relation for all tracer particles at a simulation time of 4 megayears.

Marco Panessa

Modern astrophysical simulations have made enormous strides in accurately modeling the dynamical evolution of the interstellar medium. However, these simulations rarely model the local chemistry, apart from hydrogen- and carbon-bearing species which influence the gas temperature, since the computational cost is prohibitive.

We have developed a post-processing method which takes for its initial conditions the cloud's dynamical parameters and the limited simulation chemistry, and calculates the time-dependent abundances of numerous species in a much more extensive chemical network. By applying a robust network of 37 species and about 300 reactions (Seifried & Walch 2016) to the KROME chemical rate equation solver (Grassi et al. 2014), we expand the chemistry of a molecular cloud far beyond what is achieved during the simulation itself.

Our post-processing methodology is currently in active development, and has been tested upon the SILCC-Zoom simulations of Seifried et al. 2017. These simulations track the evolution of individual molecular clouds over several megayears, at a maximal resolution of 0.06 pc.

In Fig. 1, we show a post-processed column density projection of HCO+, a species present in our extended network but not in the original simulation. By comparing this to projections of H2 and CO, we find that HCO+ accurately traces the densest molecular gas.

The SILCC-Zoom simulations include about one million passive tracer particles which flow with the local density field. Because these particles report the time-dependent local dynamical and chemical properties of the molecular cloud, our post-processing method lets us analyze the evolution of individual elements of the simulated gas.

Fig. 2 displays the evolution of 400 selected tracer particles with regard to their local total hydrogen density and HCO+ abundance, plotted upon a heatmap of this relation for all tracer particles at a simulation time of 4 megayears. The selected particles are those with the highest HCO+ abundance at this time. By plotting their evolution, we see that these tracer particles reported much lower HCO+ abundance at earlier times, and generally undergo an increase of five orders of magnitude in HCO+ abundance while remaining around a total hydrogen density of 1000-10000 cm-3. In further work, we are studying the distribution of these evolutionary tracks to measure the formation timescale of HCO+.

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