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Astron. Astrophys. 361, 1095-1111 (2000)

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1. Introduction

The determination of the chemical composition of the dust in dense clouds is of prime importance for the evaluation of its role in the thermal and chemical evolution of the condensations, leading eventually to collapse. Indeed, whereas in previous stages of the cloud contraction the chemistry is driven by gas phase neutral-neutral or ion-molecule chemical reactions (e.g. Turner et al. 1999; Herbst & Leung 1990; Langer & Graedel 1989), at later times, the grains in dense cores ([FORMULA] [FORMULA] 105 cm-3) will strongly interact with the gas, promoting a gas-grain chemistry (Willacy & Millar 1998; Bergin et al. 1995; Hasegawa & Herbst 1993). In this phase the refractory grains provide a surface on which gas phase molecules can stick and react, through grain surface chemistry as well as cosmic ray or UV photons induced photo-chemistry (Shalabeia et al. 1998; Charnley et al. 1992; d'Hendecourt et al. 1985; Lacy et al. 1984; Tielens & Hagen 1982). The way to derive the dust chemical composition in dense clouds is to observe strong infrared emitters which provide a background, or embedded sources, that allow us to observe the dust in absorption against the continuum (Willner et al. 1982). The best candidates for such studies are generally embedded young stellar objects which have just started to ignite in the cloud that gave birth to them. The young star then allows us to probe the parent cloud environment which displays strong absorption lines in the mid infrared associated with the vibration of molecules trapped in ice mantles lying on top of the refractory grains (d'Hendecourt 1984; Hagen et al. 1980; Allamandola et al. 1980).

These absorption features are in some cases so deep that they clearly show that the solid phase represents a major part of the clouds refractory molecular mass. Unfortunately, the continuum generated by the embedded object is not spatially extended and we can only probe along the narrow beam in front of the object as the dust features are observed in absorption in such objects.

In order to investigate the geometry of the objects as well as some gas phase molecules released in the process of solid volatiles evaporation, one can use the millimetre and sub-millimetre wavelength range. In this range, we deal with emission and absorption spectroscopy as the molecular excitation temperature is lower than, or equal to, the gas kinetic temperature. The dust thermal radiation provides some continuum emission, generally in the Rayleigh-Jeans domain, and is usually optically thin, and much weaker than the line emission. We can then map the emission from transitions to investigate the source geometry.

Among the most massive infrared objects studied for their solid phase features is the class I, late O or early B star, named RAFGL7009S. This object is, together with W33A, one of the most extinguished source in terms of ice mantle absorptions. It has been observed with the Infrared Space Observatory Short and Long Wavelengths Spectrometers (hereafter SWS and LWS) and numerous studies have been reported in the infrared (d'Hendecourt et al. 1996; Dartois et al. 1998; Dartois et al. 1999).

In this paper we present multi-wavelength (infrared and millimetre) observations of the source RAFGL7009S. These observations include single dish and interferometer observations. We begin by briefly describing the observations. We discuss in Sect. 3 the global properties of RAFGL7009S, which include its spectral energy distribution, the source continuum emission and the mean H2 density in the object as seen by single dish observations. We then present (Sect. 4) the symmetric top molecules (plus methanol) observations, recorded with the IRAM 30m telescope and, for the methyl cyanide molecule, with the Plateau de Bure Interferometer (hereafter PdBI).

In the next section (Sect. 5) we focus on deuterated species observations. We compare the gas phase observations of hydrides and their deuteride parent to solid phase observations and laboratory experiments. We discuss the implications. The case of HCN and HNC isomers is discussed in the next section (Sect. 6).

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© European Southern Observatory (ESO) 2000

Online publication: October 10, 2000