Simultaneous Determination of the Cosmic Ray Ionization Rate and Fractional Ionization in DR 21(OH)

We present a new method for the simultaneous calculation of the cosmic ray ionization rate, ζ H 2 , 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 3 + abundance. 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 12 C/ 13 C = 63 ± 4 and 16 O/ 18 O = 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 H 2 accordingly. Our results, ζ H 2 = 3.1 × 10 −18 s −1 and χ e = 3.2 × 10 −8 , are consistent with recent determinations in other objects.