  Deuterium fractionation is known to enhance the [DCO+]/[HCO+] abundance ratio
over the D/H elemental ratio of about 1e-5 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 4 K 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 about 0.1 pc 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 (10-20 K) and dense (>= 2x10^5
cm-3). DCO+ is not detected in the warmer photodissociation front, implying a
lower [DCO+]/[HCO+] ratio (< 1e-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 20 K, in good agreement with DCO+ excitation
calculations.
