The ALMA-PILS survey: first detection of methyl isocyanide (CH${}_3$NC) in a solar-type protostar

Abstract

Methyl isocyanide (CH${}_3$NC) is the isocyanide with the largest number of atoms confirmed in the interstellar medium (ISM), but it is not an abundant molecule, having only been detected towards a handful of objects. Conversely, its isomer, methyl cyanide (CH${}_3$CN), is one of the most abundant complex organic molecules detected in the ISM, with detections in a variety of low- and high-mass sources. We use ALMA observations from the Protostellar Interferometric Line Survey (PILS) to search for methyl isocyanide and compare its abundance with that of its isomer methyl cyanide. We use a new line catalogue from the Cologne Database for Molecular Spectroscopy (CDMS) to identify methyl isocyanide lines. We also model the chemistry with an updated version of the three-phase chemical kinetics model {\em MAGICKAL}, presenting the first chemical modelling of methyl isocyanide to date. We detect methyl isocyanide for the first time in a solar-type protostar, IRAS 16293$-$2422 B, and present upper limits for its companion protostar, IRAS 16293$-$2422 A. Methyl isocyanide is found to be at least 20 times more abundant in source B compared to source A, with a CH${}_3$CN/CH${}_3$NC abundance ratio of 200 in IRAS 16293--2422 B and >5517 in IRAS 16293$-$2422 A. We also present the results of a chemical model of methyl isocyanide chemistry in both sources, and discuss the implications on methyl isocyanide formation mechanisms and the relative evolutionary stages of both sources.

arXiv commentAccepted to A&A on 5th July 2018

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• The ALMA-PILS survey: First detection of the unsaturated 3-carbon molecules Propenal (C${}_2$H${}_3$CHO) and Propylene (C${}_3$H${}_6$) towards IRAS 16293$-$2422 B
• The ALMA-PILS survey: The first detection of doubly-deuterated methyl formate (CHD2OCHO) in the ISM
• Detection of Interstellar HC${}_4$NC and an Investigation of Isocyanopolyyne Chemistry under TMC-1 Conditions
• The role of C/O in nitrile astrochemistry in PDRs and planet-forming disks
• The ALMA-PILS survey: First tentative detection of 3-hydroxypropenal (HOCHCHCHO) in the interstellar medium and chemical modeling of the C${}_3$H${}_4$O${}_2$ isomers
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• Exploring molecular complexity with ALMA (EMoCA): Complex isocyanides in Sgr B2(N)
• Methyl cyanide (CH3CN) and propyne (CH3CCH) in the low mass protostar IRAS 16293-2422
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• Photodesorption of acetonitrile CH3CN in UV-irradiated regions of the Interstellar Medium: an experimental evidence
• Exploring the Origins of Earth's Nitrogen: Astronomical Observations of Nitrogen-bearing Organics in Protostellar Environments
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• Reexploring Molecular Complexity with ALMA: Insights into chemical differentiation from the molecular composition of hot cores in Sgr B2(N2)
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• Cyanopolyyne line survey towards high-mass star-forming regions with TMRT