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Astron. Astrophys. 342, 809-822 (1999)

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1. Introduction

G34.26+0.15 is a complex of three HII regions 3.1 kpc from the Sun. Two of the H II regions are unresolved but the third is a spectacular cometary H II region [FORMULA] wide with a clearly defined tail stretching for [FORMULA] (Gaume et al. 1994). The whole complex is embedded in a molecular gas cloud approximately 3.5 [FORMULA] 4 pc with [FORMULA] [FORMULA] 6 [FORMULA] 103 cm-3 (Heaton et al. 1985). Further molecular line observations (Martin-Pintado et al. 1985, Andersson 1985, Henkel et al. 1987, Matthews et al. 1987) inferred the presence of a hierarchical density structure within the cloud; a warm (225 K) compact core within the cloud surrounding in turn a more dense "ultracompact" core. Interferometric observations of G34.26+0.15 (Andersson & Garay 1986, Heaton, Little & Bishop 1989, Carral & Welch 1992) reveal an ultracompact (0.06 [FORMULA] 0.02 pc) core with [FORMULA] [FORMULA] 4 [FORMULA] 107 cm-3 embedded in a compact (0.21 [FORMULA] 0.05 pc) core of density 6 [FORMULA] 105 cm-3. The ultracompact core is located on the eastern edge of the cometary H II region and the interferometric observations of Heaton, Little & Bishop (1989) and Carral & Welch (1992) show that the compact core appears to be wrapped around the head of the cometary H II region (see Fig. 1).

[FIGURE] Fig. 1. Contour maps of G34.26+0.15 in molecular line emission and continuum. The black contours show the 2 cm continuum emission observed with the VLA by Gaume et al. (1994) and the grey contours show the HCO+ J = 1[FORMULA]0 line emission observed with the BIMA-Hat Creek mm array by Carral & Welch (1992). The position used for the spectral survey of Paper I is marked with a white cross and the position of the halo spectral ssurvey described in this paper is marked by a black cross.

The dense core associated with G34.26+0.15 bears a resemblance to the Orion Hot Core. They both share high abundances of saturated species, are dense compact clumps of gas, and are warm (100-200 K). These objects are known as "hot cores" (Walmsley & Schilke 1993) and are thought to be clumps of gas left over from the formation of the massive star powering the H II region. The heat source responsible for warming the core is not yet fully understood. It may be an embedded source within the core itself (Wyrowski & Walmsley 1996, Olmi et al. 1996), arise from dynamical heating by molecular outflows or be simply due to radiation from the nearby massive star. Calculations of the energy balance in hot cores (Cesaroni et al. 1994) and radiative transfer modelling (Kaufman, Hollenbach & Tielens 1998) indicate that the former possibility is the most likely, however it is probable that all three processes are involved.

The relatively high gas temperature has a dramatic effect on the chemistry of the hot core. The switch-on of the heat source, be it an embedded protostar or a newly formed high mass star, evaporates ice mantles accreted on dust grains during the cold collapse phase of the cloud. The injection of the mantle molecules enriches the hot core chemistry. The high abundances of saturated species offer indirect evidence for grain mantle injection, as the relative mobilities of atoms within the grain mantles lead to hydrogenation being the most favourable grain surface chemical process. Observation and modelling of the chemistry within hot cores provides constraints upon the species injected into the gas phase, leading to a greater understanding of the chemical processes that occur in grain mantles.

In order to investigate the chemistry of hot cores Macdonald et al. (1996), hereafter Paper I, performed a spectral line survey of the hot core associated with G34.26+0.15. The survey covered the frequency range of 330-360 GHz, detecting 338 molecular lines which were subsequently identified as originating from at least 35 different chemical species. In conjunction with the spectral line survey of the hot core a detailed chemical model of G34.26+0.15 was developed (Millar et al. 1997, hereafter Paper II). Previous models of hot core chemistry (Millar et al. 1991, Charnley et al. 1992, Caselli et al. 1993, Charnley & Millar 1994, MacKay 1995 and Charnley et al. 1995) use a single point approach, i.e. the chemistry of the cloud evolves under conditions of constant density and temperature. The model of Paper II uses a structural model of the embedding cloud derived from molecular line observations (Heaton et al. 1989, 1993) to evaluate the physical conditions in a number of discrete concentric shells. The chemical evolution of each shell is followed by a reaction network containing 2184 separate reactions and the results from each shell are integrated along the line of sight to provide a radial chemical model of G34.26+0.15 as a function of time.

Observations of hot cores sample a column of gas through the cloud, encompassing both halo and core, averaging the column densities across the beam. The model of Paper II allows the chemical nature of each part of the structural model (halo, ultracompact and compact cores) to be analysed separately but has been constrained mostly by observations of the cores. We have undertaken a spectral line survey of the halo of G34.26+0.15 in the frequency range 330-360 GHz. We aim to characterise the cold gas phase chemistry of the halo, improve the predictions of the model of Paper II and test the predictions of the structural model against different molecular tracers. Our observations are described in Sect. 2 and the data reduction and analysis procedures are described in Sects. 3 and 4 respectively. The results of the radial chemical models are given in Sect. 5 and we discuss their implications and what can be done to enhance hot core chemical models in Sects. 6 and 7.

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© European Southern Observatory (ESO) 1999

Online publication: February 23, 1999