Energy Deposition by Cosmic Rays in the Molecular Cloud Using GEANT4 Code and Voyager I Data

Molecular clouds (MCs) are exposed to Galactic and extragalactic cosmic rays (CR) that trigger several physical and physicochemical changes, including gas and grain heating and molecular destruction and formation. Here we present a theoretical model describing the energy delivered by CRs, composed of protons, alphas, and electrons taken from Voyager I measurements, into a typical MC with 5400 M _☉ (composed mainly of H with a density law of r ^−1.2) and size around 1 × 10⁶ au. The calculation was performed employing the Monte Carlo toolkit GEANT4 to obtain the energy deposition per mass from several types of secondary particles (considering nuclear and hadron physics). The results indicate that incoming protons contribute to most of the energy delivered in the MC in all regions (maximum ∼230 MeV g⁻¹ s⁻¹ at outer regions of the cloud). Secondary electrons are the second most important component for energy deposition in almost all layers of the MC and can deliver an energy rate of ∼130 MeV g⁻¹ s⁻¹ in the outer region of the MC. Other cascade particles have their major energy delivery in the central and denser core of the MC. From a temperature model (considering CR data from Voyager I), we observed (i) a small bump in temperature at the distance of 3 × 10³–2 × 10⁴ au from the center, (ii) a rapid temperature decrease (roughly 7 K) between the outer layer and the second most outer layer, and (iii) that, at a distance of 5 × 10⁴ au (Av > 10), the gas temperature of the MC is below 15 K.