2008ApJ...684.1221H

We present a new method for the simultaneous calculation of the cosmic ray ionization rate, ζ_H2, and the ionization fraction, χ_e_, in dense molecular clouds. A simple network of chemical reactions dominant in the creation and destruction of HCNH⁺ and HCO⁺ is used in conjunction with observed pairs of rotational transitions of several molecular species in order to determine the electron abundance and the H⁺₃abundance. The cosmic ray ionization rate is then calculated by taking advantage of the fact that, in dark clouds, it governs the rate of creation of H⁺₃. We apply this technique to the case of the star-forming region DR 21(OH), where we successfully detected the (J=3⟶2) and (J=4⟶3) rotational transitions of HCNH⁺. We also determine the C and O isotopic ratios in this source to be ¹²C/¹³C=63±4 and ¹⁶O/¹⁸O=318±64, which are in good agreement with previous measurements in other clouds. The significance of our method lies in the ability to determine N(H⁺₃) and χ_e directly from observations, and estimate ζ_H2accordingly. Our results, ζ_H2=3.1x10^–18/s and χ_e=3.2x10^–8, are consistent with recent determinations in other objects.