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

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

2008-05-26

arxiv.org/abs/0805.4018v1

Astronomy and Astrophysics

Astrophysics

22 pages, including 3 figures

Scientific paper

10.1086/590365

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.

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