2007A&A...464L..41P

Deuterium fractionation is known to enhance the [DCO⁺]/[HCO⁺] abundance ratio over the D/H ∼10^–5 elemental ratio in the cold and dense gas typically found in pre-stellar cores. We report the first detection and mapping of very bright DCO⁺ J=3-2 and J=2-1 lines (3 and 4K respectively) towards the Horsehead photodissociation region (PDR) observed with the IRAM-30m telescope. The DCO⁺ emission peaks close to the illuminated warm edge of the nebula (<50'' or ∼0.1pc away). Detailed nonlocal, non-LTE excitation and radiative transfer analyses have been used to determine the prevailing physical conditions and to estimate the DCO⁺ and H¹³CO⁺ abundances from their line intensities. A large [DCO⁺]/[HCO⁺] abundance ratio (≥0.02) is inferred at the DCO⁺ emission peak, a condensation shielded from the illuminating far-UV radiation field where the gas must be cold (10-20K) and dense (≥2x10⁵cm^–3). DCO⁺ is not detected in the warmer photodissociation front, implying a lower [DCO⁺]/[HCO⁺] ratio (<10^–3). According to our gas phase chemical predictions, such a high deuterium fractionation of HCO⁺ can only be explained if the gas temperature is below 20K, in good agreement with DCO⁺ excitation calculations.