Contributed Talk - Splinter ISM

Thursday, 24 September 2020, 09:34   (virtual room F)

Tracing phases: Synthetic observations of CO, C, and C+ in molecular clouds

Ebagezio, S., Seifried, D., Walch, S.
1. Physikalisches Institut, Universität zu Köln

Molecular clouds (MCs) present a complex chemical variety and the abundance of different species is related to the different phases of the clouds. Observations of CO, C, and C+ can be used to trace the CO-bright and CO-dark H2 gas in these clouds. In this work, we produce synthetic observations of CO, C, and C+ for 3D-MHD simulations of MCs within the SILCC – Zoom project. These simulations of turbulent clouds include (MHD) and exclude (HD) magnetic fields, as well as stellar feedback, and include a detailed chemical network of H2, C+, C, and CO. We show in detail which density and temperature regimes are traced best by either C+, C, or CO. In particular, we show that there is a density regime around n = 10^3 cm^-3 in which C dominates over CO and C+. The X_CO factor shows a large scatter of a factor ~2, due to optical thickness, around a typical mean of about 3*10^20 cm^2 K^-1 km^-1 s for HD simulations, and lower values for MHD runs. The X_C factor, which has achieved only little attention in literature so far, strongly varies over time and the X_C+ factor is affected both by the large scatter between the line-of-sights and the time dependence. Hence, neither CO nor C and C+ allow for a accurate determination as the H2 mass. We will discuss how a combination of the simultaneous observations of C+, C, and CO allows for a detailed determination of the chemical state of MCs: in particular, the CO/C and the CO/C+ line emission ratio on cloud scales may give an estimation of the H2 fractional abundance in a cloud with an uncertainty of ~10 %. They allow to make conclusions about the relative abundance of those species, further constraining the evolutionary stage of MCs.