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.

arXiv comment29 pages, 24 figures, accepted for publication in Astronomy & Astrophysics

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• The First Mid-Infrared Detection of HNC in the Interstellar Medium: Probing the Extreme Environment Towards the Orion Hot Core
• The Role of Terahertz and Far-IR Spectroscopy in Understanding the Formation and Evolution of Interstellar Prebiotic Molecules
• Chemical modeling of the complex organic molecules in the extended region around Sagittarius B2
• Astrochemical model to study the abundances of branched carbon-chain molecules in a hot molecular core with realistic binding energies
• Reexploring Molecular Complexity with ALMA: Insights into chemical differentiation from the molecular composition of hot cores in Sgr B2(N2)
• Thermal desorption of astrophysical relevant ice mixtures of acetaldehyde and acetonitrile from olivine dust
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• Atomic Carbon in the Central Molecular Zone of the Milky Way : Possible Cosmic-ray Induced Chemistry or Time-Dependent Chemistry Associated with SNR Sagittarius A East
• Ice Chemistry Modeling of Active Phase Comets: Hale-Bopp
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