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