Letter to the Editor

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

¹ 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 and  rotational transitions, in the low mass star-forming region L1527. We thus confirm the tentative detection of the  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 3σ 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.