The depletion of NO in pre-protostellar cores ^*

¹ Physics Department, The University, Durham DH1 3LE, UK
² IRAM, 300 rue de la Piscine, 38406 Saint Martin d'Hères, France
³ IAS (UMR 8617 du CNRS), Université de Paris-Sud, 91405 Orsay, France
⁴ LERMA (UMR 8112 du CNRS), Observatoire de Paris, 61 Avenue de l'Observatoire, 75014 Paris, France
⁵ INAF, Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi 5, 50125 Firenze, Italy

Received: 28 July 2006
Accepted: 24 October 2006

Abstract

Aims.Understanding the depletion of heavy elements is a fundamental step towards determining the structure of pre-protostellar cores just prior to collapse. We study the dependence of the NO abundance on position in the pre-protostellar cores L1544 and L183.

Methods.We observed the 150 GHz and 250 GHz transitions of NO and the 93 GHz transitions of N₂H⁺  towards L1544 and L183 using the IRAM 30 m telescope. We compare the variation of the NO column density with position in these objects with the H column density derived from dust emission measurements.

Results.We find that NO behaves differently from N₂H⁺ and appears to be partially depleted in the high density core of L1544. Other oxygen-containing compounds are also likely to be partially depleted in dense-core nuclei. The principal conclusions are that: the prestellar core L1544 is likely to be “carbon-rich”; the nitrogen chemistry did not reach equilibrium prior to gravitational collapse, and nitrogen is initially (at densities of the order of 10⁴ cm^-3) mainly in atomic form; the grain sticking probabilities of atomic C, N and, probably, O are significantly smaller than unity.