Observations of SiO towards photon dominated regions

Free access article

Issue		A&A Volume 372, Number 1, June II 2001


Page(s)		291 - 301
Section		Diffuse matter in space
DOI		http://dx.doi.org/10.1051/0004-6361:20010470

A&A 372, 291-301 (2001)
DOI: 10.1051/0004-6361:20010470

Observations of SiO towards photon dominated regions

P. Schilke¹, G. Pineau des Forêts², C. M. Walmsley³ and J. Martín-Pintado⁴

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
² Observatoire de Paris, 92195 Meudon Principal Cedex, France
³ Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
⁴ Observatorio Astronómico Nacional (IGN), Apartado 1143, E28800 Alcalá de Henares (Madrid), Spain

(Received 5 October 2000 / Accepted 22 January 2001 )

Abstract
We report observations with the IRAM 30-m telescope of SiO(2-1), (3-2), and (5-4) towards the Orion Bar, S 140, NCC 2023 and NGC 7023. We detect SiO towards the Orion Bar and the S 140 photon dominated region (PDR) with an abundance of order 10^-11 relative to ${\rm H}_2$ . Towards the Orion Bar, SiO is extended and is not confined to a layer close to the ionization front. We put limits of 2- $4\times 10^{-11}$ on [SiO]/[ ${\rm H}_2$ ] towards several positions in NGC 2023 and NGC 7023. In order to interpret these data, we present model calculations of the SiO abundance in PDRs as a function of the incident radiation field and consider also the expected Si⁺ 35 $\mu{m}$ fine structure line intensity. Our data suggest that silicon is very highly depleted onto dust grains within the molecular cloud but that a fraction of the order of ten percent of the silicon comes back into the gas phase in a layer close to the ionization front. We attempted to estimate whether the observed silicon fine structure line emission could originate in fully ionized gas and conclude that in the case of the Orion Bar, it is quite likely. Finally, a by-product of our observations was the detection of the 86773.43 MHz $J=1/2\rightarrow 1/2$ $F=1\rightarrow 1$ component of the 1₀₁-0₀₀ line of HCO in all four sources confirming that this species is relatively abundant in PDRs.

Key words: ISM: abundances -- ISM: clouds -- dust:extinction -- ISM: molecules

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

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