The observations were made with the James Clerk Maxwell Telescope (JCMT) 1 between the 8th and 14th March 1996. A position (0.3 pc) offset from the ultracompact core position was chosen to sample the halo gas. The coordinates of this position are (1950) = 18h 50m 47.25s and (1950) = . This gives an offset from the ultracompact core of in and in , or 0.3 pc at the assumed distance of G34.26 (3.1 kpc). The dimensions of the ultracompact and compact cores for comparison are 0.01 and 0.1 pc respectively. The pointing accuracy of the telescope was checked regularly against the peak continuum position of G34.26 itself. The pointing was found to be good to within . The half power beam width of the JCMT at 345 GHz is and with an angular distance of only (and pointing errors of ) from the hot core the possibility of picking up hot core gas in the edge of the beam (by pointing uncertainties or otherwise) or in the error beam ("sidelobes") must be considered. The former case is dealt with more fully in Sect. 5.2. For the latter case we note that the JCMT beam at 345 GHz is well modelled by an approximately circularly symmetric Gaussian and the peak amplitude of the sidelobes at this frequency is 6 of the main beam peak amplitude. Error beam pickup of the core gas at this position should be negligible.
It was found that beam-switching (i.e. chopping the secondary mirror from on-source to off-source) was much superior to position switching for obtaining extremely flat baselines. A chop throw of in RA was used to keep a constant reference position, with a chopping frequency of 1 Hz. was more than sufficient to avoid contamination in the reference position for all species except CO (as can be seen in Fig. 2).
To cover the frequency range of the survey the 345 GHz SIS junction receiver B3i (RxB3i) was used in conjunction with the Dutch Autocorrelation Spectrometer (DAS). The DAS was used in 760 MHz bandwidth mode and with RxB3i as a frontend produces dual sideband spectra. Dual sideband spectra comprise two frequency bands (the upper and lower sidebands) folded over one another to produce a composite spectrum. The upper and lower sidebands are separated in frequency by approximately twice the local oscillator intermediate frequency (IF), depending on the doppler correction for the source velocity. The upper sideband frequency scale is also reversed relative to the lower sideband scale. The velocity of G34.26 with respect to the Local Standard of Rest (V) was assumed to be +58 km s-1. For RxB3i the IF is 1.5 GHz and the upper and lower sidebands are separated by approximately 3 GHz. Each spectrum taken thus represents a total frequency range of 1.5 GHz and this was used to reduce the total number of spectra needed to cover the frequency range of the survey.
The spectra were all observed with the "main band" set to the lower sideband, which means that the other (upper) sideband covers a frequency range of the same width roughly 3 GHz higher in frequency. We took spectra with their central frequency incremented by 700 MHz (ensuring an overlap of 30 MHz between spectra) until the lower sideband had covered the first 2.8 GHz of the frequency range. The upper sidebands of these spectra cover the next 2.8 GHz of the frequency range with a 200 MHz gap in coverage. This block of 4 spectra thus covers a total frequency range of 5.6 GHz. The remaining parts of the frequency range were observed in the same manner. The 200 MHz gaps between the blocks of spectra were to be covered by additional spectra taken at the end of the observing run, however due to bad weather this was not achieved. The blocks of spectra (with individual spectra concatenated) are shown in Fig. 2.
Two problems inherent in dual sideband spectra are the allocation of features to a particular sideband (i.e. upper or lower) and the possible overlapping (blending) of lines from each sideband. To determine the sidebands (and hence frequencies) extra spectra with a local oscillator shift of +10 MHz were taken. In the shifted spectra lines in the upper sideband will appear to shift frequency by 20 MHz relative to lines in the lower sideband. Blended lines from both sidebands were separated by this technique whenever possible.
With the DAS in 760 MHz mode the spectral resolution is 0.756 MHz. Each spectrum was divided into channels of 0.625 MHz, although later in the data reduction process all spectra were binned to a channel width of 1.25 MHz to improve signal to noise. The standard chopper-wheel calibration method of Kutner & Ulich (1981) was used to obtain line temperatures on the scale, i.e. corrected for the atmosphere, resistive telescope losses and rearward spillover and scattering. can also be corrected for forward spillover and scattering to give the corrected receiver temperature where / and is the forward spillover and scattering efficiency (0.7 for the JCMT at 345 GHz). All line temperatures quoted in this paper are on the scale, unless otherwise indicated.
© European Southern Observatory (ESO) 1999
Online publication: February 23, 1999