Simultaneous Determination of the Cosmic Ray Ionization Rate and Fractional Ionization in DR21(OH)

TL;DR

This paper introduces a new observational method to simultaneously determine the cosmic ray ionization rate and ionization fraction in dense molecular clouds, demonstrated on the star-forming region DR21(OH).

Contribution

It presents a novel approach combining chemical reaction networks with molecular line observations to directly measure ionization parameters in dark clouds.

Findings

Measured cosmic ray ionization rate: 3.1x10^(-18) s^(-1)

Determined ionization fraction: 3.2x10^(-8)

Derived isotopic ratios consistent with previous studies

Abstract

We present a new method for the simultaneous calculation of the cosmic ray ionization rate, zeta(H2), and the ionization fraction, chi(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 H3+ 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 H3+. We apply this technique to the case of the star-forming region DR21(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(H3+) and chi(e) directly from observations, and estimate zeta(H2) accordingly. Our results, zeta(H2)=3.1x10^(-18) 1/s and chi(e)=3.2x10^(-8), are consistent with recent determinations in other objects.