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Astron. Astrophys. 342, 542-550 (1999)

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Silicon chemistry in PDRs

C.M. Walmsley 1, G. Pineau des Forêts 2 and D.R. Flower 3

1 Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, I-50125 Firenze, Italy
2 Observatoire de Paris, F-92195 Meudon Principal Cedex, France
3 Physics Department, The University, Durham DH1 3LE, UK

Received 11 August 1998 / Accepted 9 November 1998

Abstract

We consider the chemistry of silicon in photo-dissociation regions (PDRs), with special attention to the available observations of the Orion Bar. We focus on the "paradox" that fine structure emission of singly ionized silicon (Si+) is known to be strong from such regions, whereas silicon in molecular form (and in particular SiO) is absent. We consider a number of possible explanations of this "paradox" and conclude that the most likely is that Si is in solid form at depths greater than roughly 3-4 magnitudes of visual extinction, relative to the ionization front, in regions such as the Orion Bar. We consider that direct photodesorption with a small yield is likely to be the process responsible for the ejection of a Si-containing mantle into the gas phase. Other possibilities, such as the Si being present as a component of grain ice mantles, are considered; we believe that they are less likely. We find that grain polar ice mantles are probably also destroyed in PDRs, through photodesorption, whereas thermal evaporation destroys the apolar component of ice mantles deep within the surrounding molecular cloud.

Key words: molecular processes – ISM: clouds – ISM: dust, extinction – ISM: H ii regions – ISM: individual objects: Orion bar – ISM: molecules

Send offprint requests to: M. Walmsley

Correspondence to: walmsley@arcetri.astro.it

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© European Southern Observatory (ESO) 1999

Online publication: February 22, 1999
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