Letter to the Editor

Deuterium fractionation in the Horsehead edge^*

¹ IRAM, 300 rue de la Piscine, 38406 Grenoble Cedex, France e-mail: [pety;hilyblan]@iram.fr
² LERMA, UMR 8112, CNRS, Observatoire de Paris and École Normale Supérieure, 24 rue Lhomond, 75231 Paris Cedex 05, France e-mail: [javier;gerin]@lra.ens.fr
³ European Space Astronomy Centre, Urb. Villafranca del Castillo, PO Box 50727, Madrid 28080, Spain e-mail: dteyssier@sciops.esa.int

Received: 22 December 2006
Accepted: 22 January 2007

Abstract

Context.Deuterium fractionation is known to enhance the [ DCO⁺] /[ HCO⁺] abundance ratio over the  elemental ratio in the cold and dense gas typically found in pre-stellar cores.

Aims.We report the first detection and mapping of very bright DCO and  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 ( 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 ( cm^-3). DCO⁺ is not detected in the warmer photodissociation front, implying a lower [ DCO⁺] /[ HCO⁺] ratio ().

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.