Deuterium fractionation in the Horsehead edge

Free access article

Issue		A&A Volume 464, Number 3, March IV 2007


Page(s)		L41 - L44
Section		Letters
DOI		http://dx.doi.org/10.1051/0004-6361:20067009

A&A 464, L41-L44 (2007)
DOI: 10.1051/0004-6361:20067009

Letter

Deuterium fractionation in the Horsehead edge

J. Pety^{1, 2}, J. R. Goicoechea², P. Hily-Blant¹, M. Gerin², and D. Teyssier³

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 $\rm D/H \sim 10^{-5}$ 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⁺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 ~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 $(\geq$ 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 ( $\geq 2 \times 10^5$ 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.

Key words: ISM: clouds -- ISM: molecules -- ISM: individual objects: Horsehead nebula -- radio lines: ISM

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