Astron. Astrophys. 362, 1109-1121 (2000)

2. Observations

The observations of Cepheus B were carried out with the 3m KOSMA submillimeter telescope at Gornergrat, Switzerland (Winnewisser et al. 1986, 1990; Kramer et al. 1998, 2000). On-the-fly maps of the 2-1 and 3-2 low-J transitions of CO and its isotopomers ¹²CO, ¹³CO, and C¹⁸O were conducted in 5 nights between October 14 and 21 1998, the C¹⁸O 3-2 data were taken in 3 nights in March 1999. We used the Cologne dual-channel 230/345 GHz SIS receiver (Graf et al. 1998), which has tunerless waveguide mixers (Haas et al. 1997) and is mounted at one of the two Nasmyth ports. DSB receiver noise temperatures were about 120 K. As backends we used the Cologne medium and variable resolution acousto optical spectrometer (Schieder et al. 1989) with bandwidths of 1 GHz and a channel spacing of 160 kHz (330 kHz) at 230 (345) GHz. The observational parameters are summarized in Table 1.

Table 1. Observing parameters: line frequency, beam efficiency , beam width, observing mode, grid spacing, number of observed positions, mean atmospheric zenith opacity , rms of spectra , channel width , and observing period

All data were taken in a newly implemented, efficient on-the-fly (OTF) observing mode, the details of which are discussed in Appendix A. After observing the OFF-position and after moving to the source, the telescope keeps moving in right-ascension or in declination at a constant speed across the source while taking data at a constant rate. For the data presented here, we decided to restrict the total time for such OFF-ON-cycles to 2.5-3 minutes. The ON-data were integrated over 4 sec each in spatial steps of , corresponding to Nyquist sampling the KOSMA 3m-telescope beam at the highest observing frequency, 345 GHz. The source was covered 5 to 10 times and single fields were combined to cover the total area. First order (in a few cases second order) baselines were subtracted from the data. All data analysis was done using the GILDAS package of IRAM and the Observatoire de Grenoble.

In addition, we observed the [CI ] - transition at two positions in position switching mode (PSW, Table 1). A dual-channel receiver working at 492/810 GHz (Stutzki et al. 1997) was used to observe the lower [CI ] transition, SSB receiver temperature at 492 GHz were K. The higher receiver and system temperature at 810 GHz did not allow to detect the - transition. The variable resolution AOS was used with 685 kHz channel spacing (Table 1). The CI spectra suffered partly from baseline ripples, we therefore subtracted fifth order baselines from the original spectra.

Measurements on Jupiter were deconvolved to derive the HPBW of the antenna pattern (Table 1); main beam efficiencies were derived using disk brightness temperatures compiled by Griffin et al. (1986). The forward efficiency derived from skydips is 90%.

The sky transmission was estimated by measuring the radiation temperature of the blank sky at the elevation of the astronomical source. Analogous to the standard chopper wheel calibration, we then corrected the intensities to the antenna temperature () scale. The ¹³CO 3-2 data at 330 GHz and the CI line at 492 GHz were corrected for atmospheric sideband imbalance derived from standard atmospheric models (Grossman 1989).

Line intensities presented in this paper are on the main beam temperature scale, (Downes 1989). ¹²CO 3-2 and ¹³CO 3-2 data of summer 1998 are corrected for the influence of an underlying error beam (Eqs. B.1, B.2, B) due to a poorer surface accuracy at this period, which affected the 345 GHz efficiencies far more than the 230 GHz efficiencies. The calibration accuracy is estimated to about 15%.

The (0,0) position of our maps is = , = . The OFF-position lies to the west and was checked to be free of CO emission. An optical pointing camera, mounted parallel to the radio axis of the telescope, is used during weekly pointing sessions to derive new telescope pointing constants and to correct for local pointing offsets. During day time, radio pointing was checked on Jupiter. The pointing accuracy was better than .

© European Southern Observatory (ESO) 2000

Online publication: October 30, 2000
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