Abstract
The transformation of the energy dependence of the cosmic ray proton flux in the keV to GeV region is investigated theoretically when penetrating inside molecular clouds ( mag). The computations suggest that energy losses of the cosmic ray particles by interaction with the matter of the molecular cloud are principally caused by the inelastic (electronic) interaction potential; the transformed energy distribution of energetic protons is determined mainly by the column density of the absorbing medium. A cutoff of the cosmic ray spectrum inside clouds by their magnetic fields is also phenomenologically taken into account. This procedure allows a determination of environment-dependent ionization rates of molecular clouds. The theoretically predicted ionization rates are in good agreement with those derived from astronomical observations of absorption lines in the spectrum of the cloud connected with the Herbig Be star LkH 101.
Highlights
The physical characteristics of interstellar clouds such as temperature, pressure, and radiation balance as well as energy deposition from cosmic ray (CR) particles and ultraviolet photons (UV) depend strongly on the state of hydrogen [4]
UV does not penetrate into the deeper layers, energetic CR particles can induce an ionization and destruction of molecular hydrogen in the interior of molecular clouds (1); here, p(E) presents a proton of the CR with a kinetic energy before (E) and after (E ) the ionization process
Since the range is inversely proportional to the density of a medium, we can utilize (11) to scale the range R1 of a proton of an energy E in a medium of a density ρ1to compute the range R2 of the same proton with an identical energy E in a medium of a density ρ2 via (12). Applying this procedure for molecular clouds with number density of 103 cm−3, we find that R2 ≈ 2.7 · 1016 · R1
Summary
The physical characteristics of interstellar clouds such as temperature, pressure, and radiation balance as well as energy deposition from cosmic ray (CR) particles and ultraviolet photons (UV) depend strongly on the state of hydrogen (atomic versus molecular, neutral versus ionized) [4]. Interstellar regions in which the gas exists predominantly in molecular form, that is primarily molecular hydrogen (H2), are referred to as molecular clouds These areas hold average number densities of 103–104 cm−3 and temperatures of about 10 K [8]. As intermediate objects between the low-extinction diffuse clouds (Av < 1) and the optically thick molecular clouds (Av > 5), van Dishoeck and Black [9] introduced the class of translucent clouds with visual extinction spanning the range Av∼1–5 mag The interior of these clouds are entirely blocked from the external UV field at 13.6 eV (the ionization energy of atomic hydrogen) and beyond; recall that the column density along the line of sight, N (in cm−2), is related to AV (in mag) by N = 2 · 1021Av [10]. The ionization rates in appropriate astrochemical models should be chosen taking into account both CR particles energy loss processes and the cutting off their spectral fluxes by magnetic fields of the clouds
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
