The ALMA-PILS survey: propyne (CH$_3$CCH) in IRAS 16293$-$2422
  Context. Propyne (CH$_3$CCH) has been detected in a variety of environments,
from Galactic star-forming regions to extragalactic sources. Such molecules are
excellent tracers of the physical conditions in star-forming regions.
  Aims. This study explores the emission of CH$_3$CCH in the low-mass
protostellar binary, IRAS 16293$-$2422, examining the spatial scales traced by
this molecule, as well as its formation and destruction pathways.
  Methods. ALMA observations from the Protostellar Interferometric Line Survey
(PILS) are used to determine the abundances and excitation temperatures of
CH$_3$CCH towards both protostars, exploring spatial scales from 70 to 2400 au.
The three-phase chemical kinetics model MAGICKAL is also used, to explore the
chemical reactions of this molecule.
  Results. CH$_3$CCH is detected towards both IRAS 16293A and IRAS 16293B and
is found to trace the hot corino component around each source in the PILS
dataset. Eighteen transitions above 3$\sigma$ are detected, enabling robust
excitation temperatures and column densities to be determined in each source.
In IRAS 16293A, an excitation temperature of 90 K and a column density of
7.8$\times$10$^{15}$ cm$^{-2}$ best fits the spectra. In IRAS 16293B, an
excitation temperature of 100 K and 6.8$\times$10$^{15}$ cm$^{-2}$ best fits
the spectra. The chemical modelling finds that in order to reproduce the
observed abundances, both gas-phase and grain-surface reactions are needed.
  Conclusions. CH$_3$CCH is a molecule whose brightness and abundance in many
different regions can be utilised to provide a benchmark of molecular variation
with the physical properties of star-forming regions. It is essential when
making such comparisons, that the abundances are determined with a good
understanding of the spatial scale of the emitting region, to ensure that
accurate abundances are derived.
