Deuterated molecules as a probe of ionization fraction in dense interstellar clouds

The ionization degree x(e) (= n(e)/n(H2), with n(e) and n(H2) the electron and H2 number density, respectively) plays a key role in the chemical and dynamical evolution of interstellar clouds. Gas phase ion–molecule reactions are major chemical routes to the formation of interstellar molecules. The time scale for ambipolar diffusion of neutrals across field lines is proportional to the ionization degree, which therefore is a crucial parameter in determining the initial conditions which precede the collapse to form a star. A direct measure of x(e) is hindered by the difficulty of observing H+3 and H3O , two of the most abundant molecular ions, and atomic species with low ionization potentials, such as atomic carbon and metals, which may be the main repositories of positive charge. Deuterium fractionation in molecular ions, in particular HCO, has been extensively used to estimate the degree of ionization in molecular clouds. This paper reviews recent work on ionization degree in homogeneous clouds. We will show that the N(DCO)/N(HCO) column density ratio furnishes a measurement of x(e) only in regions where CO is not significantly depleted, thus in the outer skirts of dense cloud cores. To probe x(e) deep inside the clouds, one has to gauge deuterium enhancement in molecular ions with parent species not affected by depletion (e.g. N2H ), and rely on chemical models which take into account the cloud density structure. Unlike N(DCO)/N(HCO), the N(N2D )/N(N2H ) column density ratio is predicted to considerably increase with core evolution (and/or the amount of CO depletion), reaching large values (>∼ 0.2) in cloud cores on the verge of forming a star.