Exploring molecular complexity with ALMA (EMoCA): Complex isocyanides in Sgr B2(N)

Abstract

We used the EMoCA survey data to search for isocyanides in Sgr B2(N2) and their corresponding cyanide analogs. We then used the coupled three-phase chemical kinetics code MAGICKAL to simulate their chemistry. Several new species, and over 100 new reactions have been added to the network. In addition, a new single-stage simultaneous collapse/warm-up model has been implemented, thus eliminating the need for the previous two-stage models. A variable, visual extinction-dependent $\zeta$ was also incorporated into the model and tested. We report the tentative detection of CH$_3$NC and HCCNC in Sgr B2(N2), which represents the first detection of both species in a hot core of Sgr B2. Our updated chemical models can reproduce most observed NC:CN ratios reasonably well depending on the physical parameters chosen. The model that performs best has an extinction-dependent cosmic-ray ionization rate that varies from ~2 $\times$ 10$^{-15}$ s$^{-1}$ at the edge of the cloud to ~1 $\times$ 10$^{-16}$ s$^{-1}$ in the center. Models with higher extinction-dependent $\zeta$ than this model generally do not agree as well, nor do models with a constant $\zeta$ greater than the canonical value of 1.3 $\times$ 10$^{-17}$ s$^{-1}$ throughout the source. Radiative transfer models are run using results of the best-fit chemical model. Column densities produced by the radiative transfer models are significantly lower than those determined observationally. Inaccuracy in the observationally determined density and temperature profiles is a possible explanation. Excitation temperatures are well reproduced for the true ``hot core'' molecules, but are more variable for other molecules such as HC$_3$N, for which fewer lines exist in ALMA Band 3.