Contributed Talk - Splinter ISM

Thursday, 24 September 2020, 09:51   (virtual room F)

Investigating HCO+ and OH in 3D-MHD simulations of molecular cloud evolution

Marco Panessa, Daniel Seifried, Stefanie Walch
Universitaet zu Koeln

The interstellar medium broadly consists of atomic hydrogen traced by 21-cm hyperfine emission, and molecular clouds traced by CO line emission. However, because H2 can survive through self-shielding at a lower extinction than CO, in many clouds we expect the presence of H2 between roughly AV=0.1 to AV=2 which is not traced by CO. This extinction regime is referred to as the Dark Molecular Gas, or DMG. Many authors (Li 2018, Wolfire 2010, and others) note that the presence of H2 also connotes the presence of HCO+ and OH. Using the KROME chemistry analysis package (Grassi et al. 2014) in conjunction with a chemical network of about 300 reactions, we postprocess 3D-MHD simulations of molecular clouds from the SILCC-Zoom project (Seifried et al. 2015). We investigate the evolution of chemical species like HCO+ and OH which so far have never been studied in the context of actual 3D-MHD simulations. We examine the utility of HCO+ and OH as tracers of H2 in the boundaries of molecular clouds. Both show fractional abundances - with respect to both H2 and the total hydrogen density - which allows their use as tracers of H2 in the ranges of n = 1-1000 cm^-3 and Av = 0.1-1. We will also show how the abundance of HCO+ increases over time in the lower-density regions due to in-situ formation rather than turbulent mixing, a question recently debated in literature. Finally, we will present preliminary results on synthetic emission maps of the investigated species.