Free access article

A&A 478, L19-L22 (2008)
DOI: 10.1051/0004-6361:20078985

Letter

Search for anions in molecular sources: C₄H^- detection in L1527

M. Agúndez¹, J. Cernicharo¹, M. Guélin², M. Gerin³, M. C. McCarthy⁴, and P. Thaddeus<sup>4

1</sup>  Departamento de Astrofísica Molecular e Infrarroja, Instituto de Estructura de la Materia, CSIC, Serrano 121, 28006 Madrid, Spain
    e-mail: [marce;cerni]@damir.iem.csic.es
²  Institut de Radioastronomie Millimétrique, 300 rue de la Piscine, 38406 Saint Martin d'Hères, France
    e-mail: guelin@iram.fr
³  LERMA, UMR 8112, CNRS, Observatoire de Paris and École Normale Supérieure, 24 rue l'Homond, 75231 Paris, France
    e-mail: gerin@lra.ens.fr
⁴  Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
    e-mail: [mccarthy;pthaddeus]@cfa.harvard.edu

(Received 3 November 2007 / Accepted 28 November 2007)

Abstract
Aims.We present the results of a search for the negative ion C₄H^- in various dark clouds, low mass star-forming regions and photon-dominated regions (PDRs). We have also searched for C₆H^-, C₂H^- and CN^- in some of the sources.
Methods.The millimeter-wave observations were carried out with the IRAM-30 m telescope.
Results.We detect C₄H^-, through the J = 9-8 and J = 10-9 rotational transitions, in the low mass star-forming region L1527. We thus confirm the tentative detection of the J = 9-8 line recently reported toward this source. The [C₄H^-] /[C₄H] ratio found is 0.011%, which is slightly lower than the value observed in IRC +10216, 0.024%, but above the 3 upper limit we derive in TMC-1, <0.0052%. We have also derived an upper limit for the [C₆H^-] /[C₆H] ratio in the Horsehead Nebula, and for various anion-to-neutral ratios in the observed sources. These results are compared with recent chemical models.

Key words: astrochemistry -- ISM: molecules -- radio lines: ISM

© ESO 2008

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