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

TL;DR

This study reports the first detection of methyl isocyanide in a solar-type protostar using ALMA, compares its abundance with methyl cyanide, and models its chemistry to understand formation mechanisms and evolutionary stages.

Contribution

It provides the first chemical modeling of methyl isocyanide and compares its abundance with methyl cyanide in a protostellar environment.

Findings

Methyl isocyanide detected in IRAS 16293-2422 B for the first time.

Methyl isocyanide is at least 20 times more abundant in source B than in source A.

The CH$_3$CN/CH$_3$NC ratio is 200 in source B and >5517 in source A.

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.