Computational rate coefficients of isocyanodiacetylene HC4NC by collisions with He and non-LTE modelling

Abstract We present quantum close-coupling calculations for the rotational excitation of the interstellar molecule HC4NC by He collisions using a high-accuracy potential energy surface. State-to-state cross sections (for total energies E ≤ 200cm−1, J ≤ 20) and rate coefficients (for kinetic temperatures T = 2–40K) were calculated. De-excitation rate coefficients exhibit a clear propensity for even-ΔJ transitions. In particular, ΔJ = −2 transitions dominate over ΔJ = −1, maintaining large values (~10−11–10−10cm3 s−1) with weak J-dependence owing to the small rotational constant (B0 = 0.0467384cm−1). Scaling these rates for H2 collisions, we performed non-LTE radiative transfer modelling of HC4NC emission in TMC-1 (Tkin = 10 K). A χ2 minimisation yields n(H2) = 2.56 × 105 cm−3 and a column density N(HC4NC) = 4.78 × 1010 cm−2. This column density is a factor of ~6.3 lower than previous LTE estimates, demonstrating that sub-thermal excitation significantly impacts abundance derivations in cold dark clouds.