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Context. Propyne (CH$_3$CCH) has been detected in a variety of environments,from Galactic star-forming regions to extragalactic sources. Such molecules areexcellent tracers of the physical conditions in star-forming regions.  Aims. This study explores the emission of CH$_3$CCH in the low-massprotostellar binary, IRAS 16293$-$2422, examining the spatial scales traced bythis molecule, as well as its formation and destruction pathways.  Methods. ALMA observations from the Protostellar Interferometric Line Survey(PILS) are used to determine the abundances and excitation temperatures ofCH$_3$CCH towards both protostars, exploring spatial scales from 70 to 2400 au.The three-phase chemical kinetics model MAGICKAL is also used, to explore thechemical reactions of this molecule.  Results. CH$_3$CCH is detected towards both IRAS 16293A and IRAS 16293B andis found to trace the hot corino component around each source in the PILSdataset. Eighteen transitions above 3$\sigma$ are detected, enabling robustexcitation temperatures and column densities to be determined in each source.In IRAS 16293A, an excitation temperature of 90 K and a column density of7.8$\times$10$^{15}$ cm$^{-2}$ best fits the spectra. In IRAS 16293B, anexcitation temperature of 100 K and 6.8$\times$10$^{15}$ cm$^{-2}$ best fitsthe spectra. The chemical modelling finds that in order to reproduce theobserved abundances, both gas-phase and grain-surface reactions are needed.  Conclusions. CH$_3$CCH is a molecule whose brightness and abundance in manydifferent regions can be utilised to provide a benchmark of molecular variationwith the physical properties of star-forming regions. It is essential whenmaking such comparisons, that the abundances are determined with a goodunderstanding of the spatial scale of the emitting region, to ensure thataccurate abundances are derived.

The ALMA-PILS survey: propyne (CH$_3$CCH) in IRAS 16293$-$2422