Deuterium fractionation in the Horsehead edge

Abstract

Context. 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. Alms. 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-30 m telescope. The DCO+ emission peaks close to the illuminated warm edge of the nebula (<50″ or ∼0.1 pc away). Methods. 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. Results. 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 (10-20 K) and dense (≥2 × 105 cm-3). DCO+ is not detected in the warmer photodissociation front, implying a lower [DCO +]/[HCO+] ratio (<10-3). Conclusions. According to our gas phase chemical predictions, such a high deuterium fractionation of HCO+ can only be explained if the gas temperature is below 20 K, in good agreement with DCO+ excitation calculations. © ESO 2007.