  A detailed interpretation of the detected emission lines of environments in
which propyne (or methyl acetylene, CH$_3$CCH) is observed requires a knowledge
of its collisional rate coefficients with the most abundant species in the
interstellar medium, He or H$_2$. We present the first three-dimensional
potential energy surface (3D-PES) for the CH$_3$CCH-He molecular complex, study
the dynamics of the collision, and report the first set of rate coefficients
for temperatures up to 100 K for the collisional excitation of the lowest 60
ortho rotational levels and 60 para rotational levels of CH$_3$CCH by He atoms.
We computed the 3D-PES with the explicitly correlated coupled-cluster with
single-, double-, and perturbative triple-excitation method, in conjunction
with the augmented correlation-consistent triple zeta basis set
(CCSD(T)-F12a/aug-cc-pVTZ). The 3D-PES was fitted to an analytical function.
Scattering computations of pure rotational (de-)excitation of CH$_3$CCH by
collision with He atoms were performed and the state-to-state cross sections
were computed using the close coupling method for total energies up to 100
cm$^{-1}$ and with the coupled states approximation at higher energy for both
ortho and para symmetries of CH$_3$CCH. The PES obtained is caracterized by a
large anisotropy and a potential well depth of 51.04 cm$^{-1}$. By thermally
averaging the collisional cross sections, we determined quenching rate
coefficients for kinetic temperatures up to 100 K. A strong even $\Delta j$
propensity rule at almost all collision energies exists for CH$_3$CCH-He
complex. To evaluate the impact of rate coefficients in the analysis of
observations, we carried out non-LTE radiative transfer computations of the
excitation temperatures and we demonstrate that LTE conditions are typically
not fulfilled for the propyne molecule.
