The cosmic ray ionization rate in solarlike star forming regions

Abstract

The ionization fraction in molecular clouds and protostellar envelopes is an essential quantity for the understanding of their chemical evolution. An important source of ionization are cosmic ray particles that can penetrate deeply into the cloud. At a certain evolutionary stage which is not yet clearly determined young stellar objects start to emit UV and X-ray photons that carry enough energy to ionize atoms and molecules. To be able to estimate the influence of this high energetic protostellar emission on the ionization rate, it is necessary to know the permanent background ionization rate by cosmic rays. In our work, we have measured transitions of the formyl ion HCO+ and its deuterated species DCO+ in a set of thirteen sources in order to calculate the cosmic ray ionization rate. Our observations were carried out at the KOSMA telescope on the Gornergrat. Then, our data were modelled with radiative transfer codes, which did not produce satisfying results. Hence we used local thermal equilibrium methods for our calculations. Finally, we obtained an average ionization rate of 〈ζ〉 = (3.42± 2.00) · 10−17 s−1.