4 Discussion

The results of the survey show that the molecular gas associated with G5.89 does not have such a complex chemistry as other massive YSOs (G34.26+0.15 and Orion-KL). Roughly an eighth of the number of lines seen toward Orion-KL (Schilke et al. 1997b) and a half of the number of lines detected toward G34.26+0.15 (Macdonald et al. 1996) were seen in this survey. 19 species were detected, mostly sulphur-bearing, as opposed to the 35 species seen in the G34.26 and Orion-KL surveys. No lines of heavy organic species (e.g. CH₃OCH₃, HCOOCH₃, CH₃CH₂CN) were detected. We note that $\lambda$ = 1.3 mm lines of HCOOCH₃ and CH₂CHCN were detected toward G5.89 by Acord et al. (1997), the prevalent lines of these species seen in the 330-360 GHz range toward Orion-KL and G34.26+0.15 were not detected in this survey. This may be due to the sensitivity limit of this survey, the lines of these species detected in G34.26 are often weak with peak $T_{\rm A}^{*}$ $\sim \!0.5$ K (compared to the rms noise of this survey of 0.1 K at 1.25 MHz resolution). However there are several lines in the frequency range of the survey that exhibited strong emission from G34.26 (peak $T_{\rm A}^{*}$ $\ge$ 0.8 K) and are noticeably absent in this survey. More sensitive observations targeted toward these species may be necessary to confirm that their absence is due to chemical processes.

The temperature of the gas associated with G5.89 is 60-80 K, as shown by the rotation diagrams of CH₃CN, CH₃CCH and HC₃N. This is reasonably consistent with the temperature of the molecular envelope (90 K) derived from NH₃ by Gomez et al. (1991). SO₂ and ³⁴SO₂ appear to probe hotter gas with temperatures of 110-150 K. ³⁴SO₂ has a higher rotation temperature which is consistent with a temperature gradient increasing toward the centre of the gas. The column densities of most species are in the range 10¹⁴-10¹⁵ cm^-2. The temperature and density of many of the species in this survey are much less than the corresponding species in the surveys of G34.26 (Macdonald et al. 1996) and Orion-KL (Schilke et al. 1997b). The most striking difference is for CH₃OH, which in G34.26 and Orion-KL has a rotation temperature of 340 and 190 K respectively but a rotation temperature of only 65 K in G5.89.

Inspection of the line profiles indicates that the chemistry of the molecular gas proceeds in two very different physical regimes: that in the molecular envelope surrounding the UC HII and that in the shocked gas of the molecular outflow. In the former the species tracing the envelope are those such as CH₃CN, CH₃CCH and HC₃N; in the latter sulphur- and silicon-bearing species with broad line profiles perhaps trace the chemistry of the outflow. Here, we will examine the broad details of the survey in order to set out the most useful approaches to investigate the chemistry of G5.89-0.39. The approaches outlined here will be elaborated upon more quantatively in Paper II.

The chemistry of the envelope could be explained in terms of hot core chemical models (e.g. Millar et al. 1997 and references therein). Hot core models follow the evolution of the gas-phase chemistry of evaporated grain-mantle ices, where the evaporation is assumed to be caused by the "switch-on'' of a nearby high-mass star. The hot core is usually extremely compact and dense with $n_{\rm H_{2}}$ around 10⁶-10⁷ cm^-3. Temperatures can range from 100-250 K and the chemistry is characterised by anomalously high abundances of saturated species (e.g. H₂S and NH₃) and large molecules such as C₂H₅OH and CH₃OCH₃. The nature of the envelope surrounding G5.89 has some parallels with hot core chemistry; there are high column densities of warm ($\sim\! 90$ K) ammonia in close association with the young O-star (or stars) powering the UC HII region. There are however some differences between the molecular envelope and the standard picture of hot cores. G5.89 has the one of the lowest abundances of ammonia as seen in the UC HII region sample of Cesaroni et al. (1992), by roughly a factor of 10. The H₂ density lies between a few 10⁴ and 10⁶ cm^-3 (Gomez et al. 1991 and Cesaroni et al. 1992). The molecular gas does not exhibit any detectable emission from large molecules, unlike the more line-rich sources from the UC HII region survey of Hatchell et al. (1998a).

Hot core models may be able to explain these differences from the standard picture. We have examined the model of Millar et al. (1997) which is tailored for G34.26+0.15. The model takes account of the cloud structure as derived from the radiative transfer modelling of HCO⁺  (Heaton et al. 1993) and CO (Little et al. 1994). The cloud in which G34.26 is embedded has three hierarchical components; a cold rarefied halo approximately 3.5 pc in radius, a compact core of radius 0.1 pc and an inner dense ultracompact core of radius 0.01 pc. The compact core quite closely matches the conditions in the envelope of G5.89 (the model temperature and H₂ density are $\sim\! 80$ K and 10⁶ cm^-3 respectively). The compact core model shows that the abundances of the heavy organic molecules decrease as the core ages, falling off sharply beyond roughly 10⁵ years. The compact core model is able to reproduce the abundances observed in the survey to within a factor of 10, with the exception of CH₃CCH which is under-produced in the model by at least three orders of magnitude. The observations are consistent with a chemical evolution time of $\geq$10⁵ years, although such a direct comparison of the model which is tailored for G34.26 and the observed abundances which are dependent on the geometry of G5.89 must be viewed with caution.

This must be reconciled with the apparent youth of the UC HII region. Recent high resolution astrometric observations have revealed that the HII region is expanding at a rate consistent with a dynamical age of $\sim\!600$ years (Acord et al. 1998). However the onset of the UC HII region may have been delayed by high gas and dust densities in the early phases of star formation and the gas accreting onto the massive protostar may have undergone heating prior to stellar "switch-on'', perhaps from dynamical sources such as accretion shocks or outflow shocks. Another problem which addressed is that in hot core models the abundance of sulphur-bearing species such as H₂S and SO decreases after roughly 10³ years for H₂S and 10⁴ years for SO. The abundance of SO₂ stays enhanced until roughly 10⁶ years. The hot core model is capable of reproducing the observed SO₂ abundances of 10^-6-10^-7 at a temperature of 100 K (Hatchell et al. 1998b), however the broad line wings of SO and SO₂ lines show that there must be a connection between the outflowing gas and the sulphur species.

Shock-driven chemical models predict the enhancement of sulphur-bearing species (e.g. Pineau-des-Fôrets et al. 1993; Draine et al. 1983), particularly H₂S and SO₂. A shock passing through molecular gas will heat and compress the gas, permitting reactions with activation energy barriers to proceed. The reactions O + H$_{2} \rightarrow$ OH $\rightarrow$ H₂O and S + H₂ $\rightarrow$ SH $\rightarrow$ H₂S rapidly process the available oxygen and sulphur into H₂O and H₂S. Reactions with OH and O₂ convert the H₂S into SO and SO₂, which peak in abundance further behind the shock than H₂S. The model of Pineau-des-Fôrets et al. (1993) predicts an SO₂ abundance of $\sim\!10^{-6}$ which is consistent with that observed.

The shocks also strip the ice mantles from dust grains, by thermal processes, sputtering (Schilke et al. 1997a) and grain-grain collisions (Caselli et al. 1997). Depending on the shock speed one or more of these processes may dominate overall and at high enough shock speeds grains may be completely disrupted (e.g. J-shocks with speeds of > 40 km s^-1, Seab & Schull 1983). This disruption of grain cores or mantles injects depleted species into the gas phase, altering the chemical composition significantly. Chemical models involving grain disruption have been constructed for SiO and H₂O (e.g. Schilke et al. 1997a; Bergin et al. 1998) but not explicitly for sulphur-bearing species. Significant quantities of H₂S may be injected into the gas phase directly from grain mantles, rather than by hydrogenation of elemental sulphur.

It is also difficult to distinguish the route towards the production of sulphur-bearing species. Hot core chemistry alone can reproduce the observed abundances of sulphur-bearing species (Charnley 1997; Hatchell et al. 1998b). The chemical pathways are similar but for the formation of H₂S, which instead of being formed by hydrogenation of elemental sulphur is directly evaporated from grain ice mantles. However the survey shows that many sulphur-bearing species are likely to be located within the outflowing gas, by virtue of their broad line profiles. We intend to explore the mechanisms behind the sulphur species production further by detailed physical analysis and modelling of hot core and shock-driven chemistry in the companion paper (Paper II).