next previous
Up: Physical and chemical

2. Observations

Observation time was granted for a total of tex2html_wrap_inline3719 shifts spread over a number of observing runs between January 1992 to November 1994 on the 15 m JCMT on Mauna Kea, Hawaii. The program was originally set up to observe only specific lines, but was changed after the first successful run to a complete spectral scan of the three sources. The approach of systematic stepping through the 345 GHz window with 500 MHz bandwidth was therefore adopted only after the first run. In contrast with other surveys, no redundancy was built in, mainly because of time limitations. For more than half of the allocated time, no observations at 345 GHz or higher frequencies were possible due to weather conditions. As a result, many frequency settings in the 230 GHz window were observed as well during the bad weather periods.

At 345 GHz, the facility receiver B3i (Cunningham et al. 1992) was used in all runs, whereas at 230 GHz the Schottky receiver A1 was employed in January 1992, and the SIS-receiver A2 (for characteristics see Davies et al. 1992) in all subsequent observing runs. All three receivers provide an instantaneous bandwidth of 500 MHz. The intermediate frequency of 1.5 GHz for receivers B3i and A2 results in a separation of the two sidebands of 3 GHz, whereas for A1 the difference is 7.88 GHz. The line crowding is such that there is little ambiguity in the assignment of the side band, and line identification is aided by the slightly different tex2html_wrap_inline3721 of the three sources. From repeated observations of the same line in different settings and side bands, the side band ratio was found to be close to unity in most runs, except in November 1993, when a potential problem was identified after the run by the JCMT staff. Although only a few measurements were done at this time, the spectra for W 3(tex2html_wrap_inline3723) were re-observed in order to determine the side-band ratio. The difference turned out to be small, so no corrections were made.

The JCMT beam is 15tex2html_wrap3779 at 345 GHz and 21tex2html_wrap3781 at 230 GHz. Pointing was checked every tex2html_wrap_inline3729 on the continuum of the nearby compact HII region W 3(OH), and was found to be within 3tex2html_wrap3783 in most cases and somewhat larger under the worst observing conditions. In most observations, a +180tex2html_wrap3785 beam-switch in the azimuth direction was used. Experiments show that this is sufficient for the higher-lying transitions of almost all molecules (see Paper I). Only for tex2html_wrap_inline3735 a larger switch must be employed. Some of the CO 3-2 profiles toward W 3(tex2html_wrap_inline3739) and W 3 IRS4 clearly suffer from artificial ``absorption'' due to emission at the off position.

Calibration was done in the standard way, using the chopper-wheel method (see Kutner & Ulich 1981). In general, the absolute calibration is uncertain by tex2html_wrap_inline3741, which is confirmed from repeated observations of the same lines. Occasional problems did occur, however, especially during warming up of the cold load in August 1993. Because the program was spread out over many observing sessions, it cannot claim the high internal accuracy of some other surveys. However, in some crucial cases it is believed that the internal accuracy is better than 10%, while in the worst cases it should be within 50%.

As described in Paper I, a significant dip due to a mismatch in the mixer was found in the B3i spectra taken in January 1992. This dip was effectively removed by a flatfield provided by the JCMT staff. It also influenced the calibration since the whole bandwidth was used as a total power detector in those data. After 1993 the dip was removed by a channel-by-channel calibration of the backend, so that no further flat fielding was necessary.

As the backend, the facility's 2048 channel acousto-optical spectrometer (AOSC) was used in 1992. It provides a 500 MHz bandwidth and a channel spacing of 250 kHz, with an effective resolution of 2 channels corresponding to 0.65 and tex2html_wrap_inline3743 at 230 and 345 GHz respectively. The Digital Autocorrelation Spectrometer (DAS) built in Dwingeloo, The Netherlands was used after 1992. The DAS offers the possibility of observing at different bandwidths of 125, 250, 500 and 920 MHz (receiver permitting). In the survey, the 500 MHz bandwidth mode has mostly been used (tex2html_wrap_inline3745 resolution at 345 GHz), with occasional higher resolution settings at 125 and 250 MHz bandwidth. Since the DAS is calibrated channel-by-channel, not only the dip in the B3i spectra is removed, but also the uncertainties in the calibration of lines at the edges of the band. The availability of the DAS in the later runs greatly improved the detection and reliability of the strengths of weak lines.

Integration times of 30 min (tex2html_wrap_inline3747) were chosen for each frequency setting on each source. With typical system temperatures of tex2html_wrap_inline3749 at 345 GHz, this results in a tex2html_wrap_inline3751 noise level in tex2html_wrap_inline3753 of tex2html_wrap_inline3755 per resolution element (1 resolution element equals 2 channels in 500 MHz bandwidth mode) and tex2html_wrap_inline3757 per resolution element at 230 GHz (tex2html_wrap_inline3759). For comparison, the rms in the 345 GHz Orion line survey of Groesbeck (1994) and Schilke et al. (1996) is 80 mK, whereas the 230 GHz survey of Sutton et al. (1985) and Blake et al. (1986) had an rms of tex2html_wrap_inline3761 The strongest CO, tex2html_wrap_inline3763 and HCN lines in this survey are a factor of tex2html_wrap_inline3765 weaker than those in Orion; thus, within a factor of 2, the chemistry is probed in comparable detail.

The beam-efficiency for extended objects varied considerably over the three years of observation. The 345 GHz efficiency was measured to be 0.45 in February 1994 when adjustments were being made to the dish surface, whereas it was 0.60 in June and November 1994 when the adjustments had been completed. The efficiency at 230 GHz varied from 0.5 in August 1992 to 0.72 in June and November 1994. A complete list of efficiencies used is given in Table 1 (click here). These are the values for small sources as determined from observations of planets. For the W 3 sources, it may not always be justified to use the efficiency for extended objects rather than the value for point sources, but generally the difference between the two values is smaller than the uncertainties in calibration between different runs. In addition, some molecules may be more extended than other species, and there is no systematic method to correct for this. Only a single position was observed for the three sources.


Observing run

tex2html_wrap_inline3767 tex2html_wrap_inline3769 tex2html_wrap_inline3771 # of
RxA1 RxA2 RxB3isettings
230 GHz 230 GHz 345 GHz
(21'') (21'') (15'')

January 1992

0.7 - 0.6 23
August 1992 - 0.5 - 12
December 1992 - 0.63 0.53 4
July 1993 - 0.63 0.53 15
August 1993 - 0.63 0.53 4
November 1993 - 0.63 0.53 2
December 1993 - 0.63 0.53 3
February 1994 - 0.53 0.45 11
June 1994 - 0.72 0.6 16
November 1994 - 0.72 0.6 1

Table 1: Beam efficiencies, number of settings


next previous
Up: Physical and chemical

Copyright by the European Southern Observatory (ESO)