Letter to the Editor

Probable detection of H₂D in the starless core Barnard 68

¹ Leiden Observatory, PO Box 9513, 2300 RA Leiden, The Netherlands e-mail: michiel@strw.leidenuniv.nl
² INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
³ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
⁴ I. Physikalisches Institut, Universität zu Köln, Zülpicher Straße 77, 50937 Köln, Germany
⁵ Netherlands Institute for Space Research (SRON), PO Box 800, 9700 AV Groningen, The Netherlands
⁶ Max-Planck-Institut für Radioastromie, Auf dem Hügel 69, 53121 Bonn, Germany
⁷ European Southern Observatory, Karl Schwarzschild-Straße 2, 85748 Garching bei München, Germany
⁸ Present address: Centro Astronómico Hispano Alemán, Apt. 511, 04080 Almería, Spain
⁹ European Southern Observatory, Casilla 19001, Santiago 19, Chile

Received: 27 February 2006
Accepted: 4 May 2006

Abstract

Context. The presence of H₂D⁺ in dense cloud cores underlies ion-molecule reactions that strongly enhance the deuterium fractionation of many molecular species.

Aims.We determine the H₂D⁺ abundance in one starless core, Barnard 68, that has a particularly well established physical, chemical, and dynamical structure.

Methods.We observed the ortho-H₂D⁺ ground-state line 1₁₀–1₁₁, the N₂H⁺ –3 line, and the H¹³CO⁺ 4–3 line with the APEX telescope.

Results.We report the probable detection of the o-H₂D⁺ line at an intensity  K and exclusively thermal line width, and find only upper limits to the N₂H⁺ 4–3 and H¹³CO⁺ 4–3 intensities.

Conclusions.Within the uncertainties in the chemical reaction rates and the collisional excitation rates, chemical model calculations and excitation simulations reproduce the observed intensities and that of o-H₂D⁺ in particular.