Astron. Astrophys. 334, 646-658 (1998)

2. Observations

The observations of the and transitions of and the transition of ²⁸ SiO, ²⁹ SiO and ³⁰ SiO were carried out at the Swedish-ESO Submillimetre Telescope (SEST) at La Silla (Chile). All C¹⁸ O() data were taken at the James Clark Maxwell Telescope (JCMT) on Mauna Kea, (Hawaii), where we also obtained some ²⁸ SiO() and () measurements for calibration purposes. Finally, the transition of ²⁸ SiO was measured at the Heinrich Hertz Telescope (HHT) on Mt. Graham (Arizona). Table 1 summarizes the observations, including line frequencies, telescope parameters, receivers and spectrometers used and typical system temperatures. A telescope beamsize of corresponds to 1.9 pc at a distance of the Galactic center of 8.5 kpc.

Table 1. Summary of the observations

Typically, the telescope pointing was checked every three hours. At SEST, two strong SiO masers close to the Galactic center, AH Sco and VX Sgr, were measured. At the JCMT, the pointing was done using five-point continuum maps on NGC 6334 I. At the HHT, continuum cross scans on Sgr B2 were used. At SEST and JCMT, the pointing accuracy was always better than . At the HHT, the accuracy was always better than and usually .

At all telescopes, calibration was done with the standard chopper wheel method, giving temperatures on the scale. These were converted to , the main beam brightness temperature (see, e.g., Downes 1989 and Rohlfs & Wilson 1996 for definitions), using the main beam efficiencies, , listed in Table 1.

To account for structure in our extended sources, we mapped the beam used to measure the () line in the () and () transitions. Due to time restrictions, this was not possible for the SiO() line observed at the HHT. From maps obtained at the JCMT for two sources in SiO(), the intensity in a beam is higher by 8% and 15% than in and beams, respectively. This is caused by the structure of the individual sources, and the numbers are, therefore, uncertain. We will, however, use these as correction factors when comparing HHT SiO() data to measurements obtained with larger beamsizes.

To adjust the calibration scales of the three telescopes, we have observed a number of sources in the 1.3 mm band which is accessible to all instruments (see Table 1). For (), the integrated intensities at the 15 m-telescopes SEST and JCMT agree to better than 10%, i.e. to within the uncertainty of the calibration of the individual measurement. The SiO() JCMT maps, convolved to the larger size of the HHT beam, also give intensities that are consistent to within 10% with the HHT data. Thus, the -scales are in excellent agreement, and we are confident that our complete data set has been placed on a compatible intensity scale.

We carried out all observations employing position switching. This was necessary given the large angular extent of the sources, especially in . Offset positions were typically chosen (SiO) or () away from the source, perpendicular to the Galactic plane. This observing mode causes baselines that are not always perfectly flat. Thus, polynomial baselines of order were subtracted from the data. Baselines were determined over a region of from the line center. For the JCMT data, which were observed in exceptionally good weather ( throughout), linear baselines were usually sufficient.

For the final analysis, all spectra were smoothed to a velocity resolution of .

© European Southern Observatory (ESO) 1998

Online publication: May 15, 1998

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