Rotational excitation of interstellar benzonitrile by helium atoms

Interstellar aromatic molecules such as polycyclic aromatic hydrocarbons and polycyclic nitrogen and oxygen bearing molecules are thought to be abundant in the interstellar medium. In this class of molecules, benzonitrile ($c$-C$_6$H$_5$CN) plays an important role as a proxy for benzene. It has been detected through rotational emission in several astrophysical sources and is one of the simplest N-bearing polar aromatic molecules. Even in the cold ISM, the population of the rotational levels of benzonitrile might not be at equilibrium. Consequently, modeling its detected emission lines requires a prior computation of its quenching rate coefficients by the most abundant species in the ISM (He or H$_2$). In this paper, we focus on the excitation of c-C$_6$H$_5$CN by collision with He. We compute the first potential energy surface (PES) using the explicitly correlated coupled cluster method in conjunction with large basis sets. The PES obtained is characterized by a potential well depth of -97.2 cm$^{-1}$ and an important anisotropy. Scattering computations of the rotational (de-)excitation of c-C$_6$H$_5$CN by He atoms are performed by means of the coupled states approximation that allow to obtain collisional rates for rotational states up to $j$ = 9 and temperatures up to 40 K. These rate coefficients are then used to examine the effect of C$_6$H$_5$CN excitation induced by collisions with para-H$_2$ in molecular clouds by carrying out simple radiative transfer calculations of the excitation temperatures and show that non-equilibrium effects can be expected for H$_2$ densities up to 10$^5$-10$^6$ cm$^{-3}$.