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Astron. Astrophys. 338, 223-242 (1998)

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3. Observations and data reduction

The 1.3 mm continuum and molecular line observations at 1.3 and 3 mm were performed at the 15-m SEST telescope at La Silla, during March 1995 (ESO No 54.C-0343). All objects listed in Table 1 were observed in the mm continuum as well as in the 12CO(2-1) and CS (2-1) lines. The observing parameters are summarized in Table 2.


Table 2. Observational parameters

3.1. Millimetre continuum observations

For the 1.3 mm continuum observations, we used the 3He-cooled SEST facility bolometer system (Kreysa 1990). The effective beam size at this wavelength is [FORMULA] (HPBW). We followed the standard chopping beam-switching ON-ON observing procedure. The positions observed are the IRAS point source positions listed in Table 1. Chopping was provided by a focal plane chopper operating at 6 Hz with a beam separation of [FORMULA] in azimuth. Beam-switching was done by nodding the telescope after 10 sec of integration by [FORMULA] in azimuth and thus shifting the object from one beam into the other. One measurement was formed by 10 ON-ON pairs with a total integration time of 200 s. The number of measurements spent on one object depended on the achieved signal-to-noise ratio.

The atmospheric transmission was measured by skydips every two hours. Pointing was done by doing cross scans on a nearby bright quasar. The telescope pointing was found to be repeatable within [FORMULA]. Mars was used as primary flux density calibrator. We estimate the total calibration uncertainty to be [FORMULA] 20 %. The average 3 [FORMULA] detection limit of the entire survey is 40 mJy/[FORMULA]beam.

The raw data were de-spiked, averaged for each ON-ON pair, and corrected for the atmospheric extinction using the approximation of a plane-parallel atmosphere. The mean value of all measurements towards one position was averaged by using weighting factors of [FORMULA] where [FORMULA] are the standard deviations for each ON-ON pair.

3.2. CO and CS molecular line observations

The CO(J=2[FORMULA]1) observations at 230 GHz were performed with a SIS receiver. For the 3 mm band (CS(J=2[FORMULA]1) at 98 GHz), the telescope was equipped with a Schottky mixer receiver. The system temperatures during the observations were in the range 450 - 550 K at 230 GHz and 400 - 460 K at 98 GHz. As back-end, an Acousto Optical Spectrometer with a total bandwidth of 86 MHz and 2000 channels was used. The beam sizes, main beam efficiencies, and velocity resolutions are listed in Table 2.

The CO observations were done in the position switching mode with OFF positions 1 to [FORMULA] away from the ON positions. The reference positions were checked for CO emission by frequency switching observations. The CS observations were performed in the dual beam switching mode with a beam throw of [FORMULA] in azimuth. Pointing was regularly checked with nearby bright SiO sources.

The standard chopper wheel method (Penzias & Burrus 1973; Kutner & Ulich 1981) was used for intensity calibration, resulting in the antenna temperature [FORMULA] which is corrected for atmospheric attenuation and telescope losses. To get the main beam brightness temperature, [FORMULA], [FORMULA] was divided by the main beam efficiency [FORMULA]. Typical 1 [FORMULA] rms values for the CO and CS spectra are 0.11 K and 0.07 K ([FORMULA]), respectively. Data reduction was done with the CLASS software.

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© European Southern Observatory (ESO) 1998

Online publication: September 8, 1998