Abundances relative to H₂ of HF and HCl (including H³⁵Cl and H³⁷Cl) as a function of radius in IRC +10216. Abundances plotted as solid lines have a constant value in the inner envelope, derived from a fit to the observed lines (see Figs. 1 and 2), and decrease in the outer envelope due to photodissociation, as computed with a photochemical model. The region of the envelope sampled by the observed lines of HF and HCl (see text) extends up to 3 × 10¹⁵ cm (light shaded region). Dashed lines correspond to abundances computed by thermochemical equilibrium, which only holds up to  ~3 R _∗  (dark shaded region).