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Astron. Astrophys. 354, L1-L5 (2000)

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2. Observations

The observations were done partly during the winter 1998-99 and partly during the summer of 1999 with the IRAM Plateau de Bure Interferometer. A dual SIS 3mm/1.3mm receiver was used to observe simultaneously the CO(2-1) (redshifted in the 3mm band) and the CO(5-4) (redshifted in the 1.3mm band) lines. In both cases, the 3mm receiver was connected to two units of the Correlator, providing a velocity coverage of about 850 km s-1, while 4 units were connected to the 1.3mm receiver providing a velocity coverage of about 650 km s-1.
The observations made during the winter 1998-99 assumed a redshift of [FORMULA] deduced from the H[FORMULA] line. This yields frequencies of [FORMULA] GHz for the CO(2-1) line and [FORMULA] GHz for the CO(5-4) line. Both lines were detected, but blue-shifted by about 400 km s-1 with respect to the H[FORMULA] line and thus truncated. The observations were repeated during the summer 1999 with a new frequency setup: [FORMULA] GHz and [FORMULA] GHz for the CO(2-1) and CO(5-4) lines respectively.
The observations were made in the CD configuration of the interferometer. The phase drifts were calibrated by observing the nearby quasars 1633+382 and 1732+389 every 20 minutes throughout the observations. The amplitude was calibrated with the quasars 3C273, 3C345 and 2145+067, and the compact H II region MWC349 at the beginning and/or the end of each transit. The passband of the system was calibrated using 3C273 or 2145+067. At 1.3mm, the total useful integration time on source was about 9 hours during the winter 1998/99, and 12 hours during the summer 1999. More integration time was available at 3mm because some additional data were obtained, marginal at 1.3mm but of good quality at 3mm.
The 3mm receiver was tuned in single side band, while the 1.3mm was tuned in double side band, with the CO(5-4) in the lower side band. The upper side band was then available to measure the continuum.

Three visibility tables were produced from the calibrated data: two spectral tables containing the CO(2-1) and CO(5-4) data respectively and one continuum table with the 1.3mm continuum. The spectral data were smoothed in frequency to improve their signal-to-noise ratio: the velocity resolution of the CO(2-1) data was smoothed to 50 km s-1 and that of the CO(5-4) to 75 km s-1. The UV tables were imaged, CLEANed and restored with elliptical gaussian beams of [FORMULA] ([FORMULA]) and [FORMULA] ([FORMULA]) for the 3mm and 1.3mm data respectively.

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© European Southern Observatory (ESO) 2000

Online publication: January 31, 2000