Letter to the Editor

Methyl cyanide as tracer of bow shocks in L1157-B1

¹ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy e-mail: codella@arcetri.astro.it
² INAF, Istituto di Fisica dello Spazio Interplanetario, via Fosso del Cavaliere 100, 00133 Roma, Italy
³ IRAM, 300 rue de la Piscine, 38406 Saint-Martin-d'Hères, France
⁴ Department of Physics and Astronomy, University College London, Gower Street, London WC1E6BT, UK
⁵ Observatorio Astrónomico Nacional (IGN), Alfonso XII, 28014 Madrid, Spain
⁶ LERMA, UMR 8112 du CNRS, Observatoire de Paris, 61 Av. de l'Observatoire, 75014 Paris, France
⁷ Laboratoire d'Astrophysique de l'Observatoire de Grenoble, BP 53, 38041 Grenoble Cedex, France

Received: 22 September 2009
Accepted: 20 October 2009

Abstract

Context. L1157-mm is a low-mass protostar driving an outflow which is considered the proto-type of the so-called chemically active outflows. In particular, the blue-shifted lobe B1 stands out for its rich mm-wave spectrum.

Aims. Our aim is to infer the physical conditions of the molecular gas within L1157-B1.

Methods. We carried out CH₃CN(8_K–7_K) observations at 2 mm with the IRAM Plateau de Bure Interferometer to map L1157-B1.

Results. The CH₃CN image shows a clumpy structure superimposed to the classical B1 arch-like shape, displaying a unique continuous structure tracing the propagation of a large bow shock. The CH₃CN spatial distribution well agrees with that of CH₃OH, one of the most abundant species evaporated from dust grain mantles. Thanks to the present CH₃CN observations at unprecedented spatial resolution, we determine for the first time the temperature distribution of the B1 bow shock. We obtain values between 57 and 132 K, with the highest temperature at the shock apex, thus confirming the association of CH₃CN with gas affected by the passage of a shock wave. The  column densities are 8-40  10¹² cm^-2, depending on the position, leading to CH₃CN/CH₃OH abundance ratios 0.2-1.3  10^-3, i.e. values on average smaller than those found in hot-corinos. This suggests that a significant amount of CH₃CN is formed in the gas phase: in the shocked L1157-B1 regions the CH₃CN abundance may not have reached its maximum yet.