Letter to the Editor
Deuterium fractionation in the Horsehead edge*
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: email@example.com
Accepted: 22 January 2007
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.
Key words: ISM: clouds / ISM: molecules / ISM: individual objects: Horsehead nebula / radio lines: ISM
© ESO, 2007