Hydrogen cyanide and isocyanide in prestellar cores^⋆

¹ Institut de Ciències de l’Espai (CSIC–IEEC), Campus UAB, Facultat de Ciències, Torre C5–parell 2 a, 08193 Bellaterra, Spain
e-mail: padovani@ieec.uab.es
² INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
³ Dublin Institute of Advanced Studies, 31 Fitzwilliam Place, Dublin, Ireland
⁴ Observatorio Astronómico Nacional (IGN), Alfonso XII 3, 28014 Madrid, Spain
⁵ Université Joseph Fourier et CNRS, Institut de Planétologie et d’Astrophysique, 38041 Grenoble Cedex 9, France
⁶ IAS (UMR 8617), Université de Paris-Sud, 91405 Orsay, France
⁷ LERMA (UMR 8112), Observatoire de Paris, 61 avenue de l’Observatoire, 75014 Paris, France

Received: 25 April 2011
Accepted: 19 August 2011

Abstract

Aims. We studied the abundance of HCN, H¹³CN, and HN¹³C in a sample of prestellar cores, in order to search for species associated with high density gas.

Methods. We used the IRAM 30 m radiotelescope to observe along the major and the minor axes of L1498, L1521E, and TMC 2, three cores chosen on the basis of their CO depletion properties. We mapped the J = 1 → 0 transition of HCN, H¹³CN, and HN¹³C towards the source sample plus the J = 1 → 0 transition of N₂H⁺ and the J = 2 → 1 transition of C¹⁸O in TMC 2. We used two different radiative transfer codes, making use of recent collisional rate calculations, in order to determine more accurately the excitation temperature, leading to a more exact evaluation of the column densities and abundances.

Results. We find that the optical depths of both H¹³CN(1−0) and HN¹³C(1−0) are non-negligible, allowing us to estimate excitation temperatures for these transitions in many positions in the three sources. The observed excitation temperatures are consistent with recent computations of the collisional rates for these species and they correlate with hydrogen column density inferred from dust emission. We conclude that HCN and HNC are relatively abundant in the high density zone, n(H₂) ~ 10⁵ cm^-3, where CO is depleted. The relative abundance [HNC]/[HCN] differs from unity by at most 30% consistent with chemical expectations. The three hyperfine satellites of HCN(1−0) are optically thick in the regions mapped, but the profiles become increasingly skewed to the blue (L1498 and TMC 2) or red (L1521E) with increasing optical depth suggesting absorption by foreground layers.