The two chemical phases of dark interstellar clouds

We show that there exist two phases in the chemical equilibrium state of dark interstellar clouds, according to the local value of the density, nH, and cosmic ray ionization rate, ζ. For a given value of ζ, the ion-neutral chemistry at high density is driven by proton transfer with |H⁺₃|⁠, while at low density charge transfer with H+ dominates. The transition between these two regimes occurs abruptly. Much higher values of the ratio n(C)/n(CO) are attained just below the critical density than just above it. The value of the critical density increases with ζ. When |ζ=2.5×10⁻¹⁷ s⁻¹| the transition occurs at |n_ H 10⁴⁻³|⁠. Thus the chemical equilibrium state of the gas may differ qualitatively from point to point in dark clouds or from one region to another in the Galaxy.