Hot cores provide a unique opportunity to investigate the composition of grain mantles via observations of species in the gas phase. As the density increases during collapse to form a core, molecules freeze out onto dust grains and then undergo processing on the grains. As the cores heat, temperatures become high enough to evaporate molecules from ices, injecting them into the gas phase and leading to a rich chemistry. High abundances of saturated molecules are believed to be a hallmark of grain mantle evaporation as species are hydrogenated on grains. Unlike gas phase chemistry, the chemistry that occurs in the grain ices is poorly understood. Molecules are difficult to detect and distinguish in the solid ice phase, and any species with an abundance less than 1% of solid H2O is essentially undetectable. In hot cores in the gas phase, densities are high enough for large numbers of species to be detected, and we are able to constrain the composition of the ice mantles by looking at molecular abundances of the evaporated molecules from grains.
Another motivation for a survey of molecular gas associated with HII regions is to look at the effects of the environment on the morphology and size of the ionised region. The molecular cores in our sample were discovered by Churchwell et al. (1990) while investigating the environment of UC HII regions. UC HII regions come with several different types of morphology, classified as cometary (e.g. G34.26, G29.96), core-halo (G31.41), spherical or unresolved (G10.47), shell (G5.89) and irregular (G45.45). A unifying explanation for the expansion of UC HII regions, which appear smaller than one would naïvely expect from their lifetimes, and the creation of these different morphologies, has not been settled on. In our molecular line survey we obtain measures of the density and the kinematics of the molecular environment of the HII region. In Sect. 4.2 we compare these to requirements and predictions of the HII expansion models.
Whether the hot cores are heated solely by interaction with the neighbouring HII regions, or internally by secondary star formation, their chemical composition will reflect their history. Molecules released from the grain ices are processed in the gas phase, changing their abundance ratios as time progresses. Differences in abundance ratios between sources may be evidence of evolution. By comparison with chemical models, such as the model for G34.26 (Millar et al. 1997) we can estimate the ages of the hot cores. This approach has been successfully used to rank three sites of high mass star formation in W3 in order of age (Helmich et al. 1994). We consider the chemical evidence for evolutionary differences between the hot cores in our survey in a subsequent paper (Hatchell et al. 1998, in preparation).
Our sample of fourteen hot cores (see Table 1) was chosen to consist of cores known to contain hot gas because they have been observed in emission from high excitation lines of ammonia ((4,4) and (5,5), Cesaroni et al. 1992) and methyl cyanide (Olmi et al. 1993). The sample also includes the archetypal hot core source G34.26. We have previously published an extended molecular line survey of G34.26 and compared it with a detailed chemical model (Macdonald et al. 1996; Millar et al. 1997). Subsamples of our selection have been observed in the dense gas tracers CS and C34S (Cesaroni et al. 1991; Plume et al. 1992; Churchwell et al. 1992). Enhanced abundances of deuterium have been observed in G34.26, G10.47, G31.41, G29.96 and G9.62, supporting the argument that material is being evaporated from ices as deuterated molecules form preferentially in cold conditions (Jacq et al. 1990; Gensheimer et al. 1996; Rodgers & Millar 1996). Most of the hot core/UC HII complexes have associated H2O masers, which are strongly associated with recent star formation (Churchwell et al. 1990; Hofner & Churchwell 1996; Codella et al. 1997), and some (if not all) have outflow activity (Cesaroni et al. 1991; Shepherd & Churchwell 1996; Hunter et al. 1997; Olmi et al. 1996a; Hofner et al. 1996; Shepherd et al. 1997).
We choose frequency bands that include transitions of many different molecules. We include two sets of high excitation transitions of CH3CN (methyl cyanide) as a temperature tracer in hot, dense gas. CH3CN is a symmetric top molecule with groups of transitions closely spaced in frequency but involving widely spaced energy levels. It is used as a temperature probe because the lines within a group can be observed together, thus removing the need for various calibration factors to be taken into account when comparing line ratios. We include many lines of CH3OH (methanol) which span a large range of excitation energies and trace both hot and cold gas. C17O and C18O are observed as tracers of column density. Other molecules with multiple transitions which can be used for temperature and density analysis are CH3CCH (methyl acetylene) and HCOOCH3 (methyl formate). Observations include a number of sulphur species which are good tracers of chemical evolution.
Following a description of the observations made (Sect. 2), we present the results of the analysis molecule-by-molecule (Sect. 3.2), first briefly describing the techniques used. In Sect. 4 we draw together the results from individual species to make general statements about the physical conditions and structure, the implications for UC HII models, and the chemistry and evolution of the molecular gas. Finally, Sect. 5 summarises our conclusions.
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