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Astron. Astrophys. 357, 75-83 (2000)

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2. Observations

The observations in the southern sky were carried out with the 15m antenna of ESO-SEST in La Silla (Chile), during 6-10 November 1999. We used two SIS receivers at 3 mm and 2 mm simultaneously, to observe the HCO+(1-0) and CS(3-2) lines at 89.188523 and 146.969033 GHz respectively. Although CS is less abundant, it would have been interesting to have both molecules in case of detection. The HPBW were 57" and 34" and the main-beam efficiencies were [FORMULA]=0.75 and 0.66 respectively at the two mentioned frequencies. The backends were accousto-optic spectrometers (AOS) both at high-resolution (HRS) and low-resolution (LRS). The corresponding channel spacings (velocity resolutions) were 0.144 (0.268) and 0.087 (0.163) km s-1 at HCO+ and CS for HRS and 2.32 (4.7) and 1.4 (2.8) km s-1 respectively for LRS. The number of channels were 1000 and 1440 for high and low resolution backends respectively, so that the velocity coverage was 140 and 3352 km s-1 at high and low resolution for the HCO+ line, and 85 and 2057 km s-1 for CS respectively. Since all backends were centered of the expected HVC velocity, there was no problem detecting any galactic line around V=0 km s-1 with the low-resolution, but it was most of the time outside of the range at high-resolution. We have retrieved the HCO+ absorptions already reported by Lucas & Liszt (1996) with the IRAM interferometer. In cases where dilution in large velocity channels made detection problematic, we shifted the high-resolution backend to zero velocity to verify the detection. Pointing was corrected regularly (every 2 hours) using known SiO masers (and the 3 mm receiver retuned accordingly). The weather was clear throughout the run, and the typical system temperatures were 180 K for both the 3 and 2 mm ranges. The observing procedure was dual beam switching at high frequency (6 Hz) between same elevation positions in the sky (beam throw of 2' 27"), to eliminate atmospheric variations. Each source was observed on average for 2 hours, reaching about 3.5 mK of rms noise in channels of 1.4 MHz (4.7 km s-1 at HCO+).

Because the width of the absorption lines may be expected to be small (even though the HCO+ absorption lines are the wider mm lines, and therefore more favorable for detection), a concern should be noted that line profiles may be diluted in the low resolution spectrograph. The technique proved to be valuable, allowing us to retrieve the HCO+(1-0) absorptions already detected by LL96 for galactic clouds, near zero velocity (see below). We also wished to check that the HCO+(1-0) absorption was not hampered by emission. In one of the sources (2251+158) we observed an offset position (with a beam throw of 12' for the beam-switch) to check for emission. Emission was not detected at the same signal-to-noise at which the absorption was clearly present.

The observations in the northern sky were carried out with the IRAM interferometer in Plateau de Bure (France), during July, October and November 1999. The interferometer data were made with the standard D configuration (see Guilloteau et al. 1992). The array comprised 4 15-m telescopes. The receivers were 3-mm SIS, giving a typical system temperature of 150-200 K. One of the source, 3C 454.3, was used itself as a phase reference, while bandpass and amplitude calibrations were done using also sources such as 3C 273, MWC 349, 1823+568, 2145+067. The data reduction was made with the standard CLIC software. The synthesized beams are of the order of 6[FORMULA]8 arcsec. The auto-correlator was used in three overlapping configurations of bandwidth 160, 80 and 40 MHz and a respective resolution of 2.5, 0.625 and 0.156 MHz, giving a maximum velocity resolution of 0.5 km s-1. The largest bandwidth observed is 540 km s-1.

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© European Southern Observatory (ESO) 2000

Online publication: May 3, 2000