The C/CO Ratio Problem: Chemical Effects of Turbulence

We further discuss the suggestion that the chemistry in a photon-dominated region is coupled to that in the UV-shielded region behind it by turbulent transport processes. In addition to transport time-scales, we discuss why MHD waves/turbulence will likely cause transport instead of prohibiting it. 1. On the C/CO Ratio Problem The study of interstellar chemistry has so far focused mainly on two physical regimes; regions subject to abundant ultra-violet photons, and regions which are shielded from UV photons. The former is often referred to as photon-dominated regions (so-called PDRs) where the chemistry is dominated by photon-related reactions, and the latter is often thought to be cosmic ray-dominated regions (hereafter CDRs) where cosmic rays sustain the ion-neutral reaction scheme. The observed large C/CO ratio (∼ 0.1) from molecular clouds has been a major challenge to theoreticians working in either regimes (cf. Sorrell 1992). One common opinion in the field is that the clumpiness of a cloud allows an enhanced UV penetration into the cloud interior which will dissociate the molecules and create many small photon-dominated regions (PDRs) on the clump surfaces. One severe difficulty in this picture, however, seems to be the large observed abundances of many large organic molecules. Since large organic molecules do not survive UV photons in PDRs, the observed large organic molecules must be deep inside the clumps, which requires the co-existence of abundant neutral carbon (Chièze