3 Results

3.1 Identified lines and their profiles

142 lines originating from a total of 19 species were detected and are shown in Table 5. 6 further lines could not be identified and are indicated as such by U in Table 5. Many isotopomers were detected, particularly the ³⁴SO₂ isotopomer. A list of the identified species (including isotopomers) and the total number of lines from each is given in Table 1. Approximately 50$\%$ of the identified lines originate from sulphur-bearing species. Many of these lines (e.g. CS, SO, SO₂ and to some extent C³⁴S and ³⁴SO) exhibit broad line wings suggesting that their emission originates from regions of wide velocity dispersion, perhaps from the molecular outflow. Other non sulphur-bearing species known as typical outflow tracers (CO, HCO⁺, CH₃OH) also display a wide range of velocities. Isotopomers such as ¹³CO, H¹³CO⁺ and C³⁴S have broad line wings, which indicates that the outflowing gas has a significant optical depth in these molecules. The density tracer HCN also appears to have broad line wings, although this is complicated by the presence of two image band SO₂ lines blended with the HCN 4-3 line. SiO exhibits extremely broad emission (across $\sim\! 50\,{\rm km \,s}^{-1}$), however it is again unfortunately blended with a vibrational SO₂ line in the image band.

  

Table 1: Species (including isotopomers) identified in the survey. The identification of certain species from single line detections must however be regarded with caution

In contrast to the broad lines thought to have an origin within the outflow, several species with narrow line profiles (e.g. HC₃N, CH₃CN, CH₃CCH, NO) trace a relatively quiescent region of gas, probably the molecular envelope surrounding the UC HII region (Gomez et al. 1991). Figure 2 shows a selection of high-velocity lines compared with the CH₃CCH 20(2)-19(2) line as representative of the low velocity dispersion lines. The temperature scale is normalised to highlight the differences between them. The differences between the narrow CH₃CCH line which probes the gas of the molecular envelope and the broad lines of the outflow tracers SO, SO₂ and HCO⁺ are obvious. The CH₃OH line also has a distinct red wing.

  

Figure 2: Spectra of broad lines from species known to probe outflows are compared to the CH3CCH 20(2)-19(2) line. Broad red and blue-shifted wings are clearly visible in the SO, SO2 and HCO+ lines, whereas the CH3OH 7(0)-6(0) A+ line only clearly exhibits a red-shifted wing

In contrast to other high-mass YSO sources such as G34.26+0.15 and Orion-KL, there is no observational evidence of the presence of heavy organic molecules such as ethyl cyanide (CH₃CH₂CN), methyl formate (HCOOCH₃) or dimethyl ether (CH₃OCH₃). These molecules have large numbers of emission lines detected towards the other sources and are evidence of a complex hot core chemistry, which may be absent in the molecular gas associated with G5.89-0.39.

3.2 Integrated intensities

We have also compared the total integrated intensities from each molecule, in a similar manner to that of the Orion-KL survey (Schilke et al. 1997b). It is interesting to compare the results from G5.89 with those of Orion-KL. In Orion-KL the most dominant molecule in terms of integrated intensity is SO₂ which has a total integrated intensity more than twice that of CO. In G5.89-0.39 the most dominant molecule is CO with twice the integrated intensity of SO₂. This is due to beam dilution, since G5.89-0.39 is approximately 4 times more distant than Orion KL. The same effect can be seen in other species such as CH₃CCH and CH₃CN; in Orion-KL the former is much less important for cooling the gas than the latter. However in G5.89 CH₃CCH has twice the integrated intensity of CH₃CN. This is probably due to the property of CH₃CCH of tracing more extended gas (by virtue of its lower dipole moment), making CH₃CCH less affected by beam dilution. Care must be taken in interpretations such as this though, as genuine chemical differences may affect the relative abundances of particular species between the two clouds and hence their integrated intensities.

  

Table 2: Integrated intensity for each species. The integrated intensities have been evaluated using the $T_{\rm A}^{*}$ scale. Where it has not been possible to place a firm value on the total integrated intensity due to blended lines an upper limit is indicated

3.3 Rotation diagrams and lower limits to column density

The rotation diagram approach assumes optically thin gas in LTE, for which the (beam-averaged) column density (N) can be written as:  

$$N = \frac{3k}{8\pi^{3}} \frac{\int T_{\rm R} {\rm d}v} {\nu ... ...rm rot}) {\rm exp} \left(\frac{E_{\rm u}}{kT_{\rm rot}} \right)$$ (1)

where $\int T_{\rm R} {\rm d}v$ is the integrated intensity of the line, $\nu$ is the line frequency, S is the line strength, $\mu$ is the permanent electric dipole moment, $g_{\rm I}$ and $g_{\rm K}$ are the reduced nuclear spin degeneracy and the K-level degeneracy of the molecule respectively. $E_{\rm u}$ is the energy of the upper level of the line and $T_{\rm rot}$ is the rotational temperature of the molecules comprising the gas and $Q(T_{\rm rot})$is the corresponding partition function. Values for $Q(T_{\rm rot})$ were obtained by interpolating the values given in the JPL molecular line database to the appropriate temperature. Equation (1) was rearranged to an equation for a straight line and $T_{\rm rot}$ and N for each species were determined by a least-squares fit. A rotation diagram for ³³SO was not constructed owing to the lack of well-determined molecular parameters, such as the partition function. Lower limits to the column densities ($N_{\rm min}$) of the remaining species were evaluated using the minimum point of Eq. (1), which occurs at $T_{\rm rot} = E_{\rm u}/k$for linear molecules and $T_{\rm rot} = \frac{2}{3} E_{\rm u}/k$ for symmetric and asymmetric top molecules. Both methods are described more fully in Hatchell et al. (1998a).

The results of the rotation diagram and lower limit analyses are given in Tables 3 and 4. Abundances have been calculated for the rotation diagram species by assuming a spherical cloud with a radius of 0.2 pc and a number density $n(\rm H_{2})$ = 10⁴ cm^-3 (taken from Gomez et al. 1991). The abundance of ³⁴SO₂ indicates that the SO₂ abundance may be underestimated due to optical depth effects and should be closer to 3 10⁶. The rotation diagrams are shown in Fig. 3. Certain lines have been excluded from the analysis; self-absorbed and blended lines, and those lines that could not be unambiguously identified with a single species. The data from SO, ³⁴SO and H₂CS did not give a satisfactory fit to a straight line and lower limits to column density have been evaluated for these molecules.

  

Figure 3: Rotation diagrams for species with sufficient detected lines

The rotation diagrams indicate that the molecular gas associated with G5.89 has a temperature of roughly 60-70 K and a column density of $\sim\! 10^{15}$ cm^-2. SO₂ appears to probe somewhat hotter gas with temperature in excess of 100 K. The temperature for the less optically thick ³⁴SO₂ is higher than that of SO₂ which is consistent with (although not conclusive evidence for) the temperature of the molecular gas increasing towards the centre.

  

Table 3: Rotation temperatures ($T_{\rm rot}$), partition functions ($Q(T_{\rm rot})$), column densities (N) and abundances (X) relative to H₂. Abundances were calculated assuming a spherical envelope with the radius and number density $n(\rm H_{2})$ given in Gomez et al. (1991)

  

Table 4: Lower limits to column density. The partition function Q evaluated at $T_{\rm ex}$ is also given, where $T_{\rm ex} = E_{\rm u}/k$ for linear molecules and $\frac{2}{3}E_{\rm u}/k$ for symmetric and asymmetric tops. Where two lines were used to evaluate the lower limit the partition functions of both lines are given

  

Table 5: The measured line parameters of observed frequency ($\nu$(obs)), peak temperature ($T_{\rm A}^{*}$) and line width ($\Delta \nu_{1/2}$) for each detected line are listed here. Multiple detections of the same line have been included. Lines that are blended are indicated in the Notes column by blended if they are blended with a different species or h/fines if they are a mixture of two or more hyperfine components. Self-absorbed lines are also listed. Blended lines that can have one or both components extracted are indicated by sl-blend

 

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