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Volume 418, Number 3, May II 2004
Page(s) 1035 - 1043
Section Interstellar and circumstellar matter
DOI http://dx.doi.org/10.1051/0004-6361:20035718

A&A 418, 1035-1043 (2004)
DOI: 10.1051/0004-6361:20035718

Complete depletion in prestellar cores

C. M. Walmsley1, D. R. Flower2 and G. Pineau des Forêts3, 4

1  INAF, Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi 5, 50125 Firenze, Italy
2  Physics Department, The University, Durham DH1 3LE, UK
3  IAS, Université de Paris-Sud, 92405 Orsay Cedex, France
4  LUTH, Observatoire de Paris, 92195 Meudon Cedex, France

(Received 20 November 2003 / Accepted 9 February 2004)

We have carried out calculations of ionization equilibrium and deuterium fractionation for conditions appropriate to a completely depleted, low mass pre-protostellar core, where heavy elements such as C, N, and O have vanished from the gas phase and are incorporated in ice mantles frozen on dust grain surfaces. We put particular emphasis on the interpretation of recent observations of H 2D + towards the centre of the prestellar core L 1544 (Caselli et al. 2003) and also compute the ambipolar diffusion timescale. We consider explicitly the ortho and para forms of H 2, H 3+, and H 2D +. Our results show that the ionization degree under such conditions depends sensitively on the grain size distribution or, more precisely, on the mean grain surface area per hydrogen nucleus. Depending upon this parameter and upon density, the major ion may be H +, H 3+, or D 3+. We show that the abundance of ortho-H 2D + observed towards L 1544 can be explained satisfactorily in terms of a complete depletion model and that this species is, as a consequence, an important tracer of the kinematics of prestellar cores.

Key words: submillimeter -- molecular processes -- radio lines: ISM

Offprint request: C. M. Walmsley, walmsley@arcetri.astro.it

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