Volume 557, September 2013
|Number of page(s)||5|
|Published online||29 August 2013|
Two observing runs were carried out in band 7 on October 21 and 22, 2012, of 80 and 100 min, respectively. With these we obtained a total of 71 min of correlations on source. The band 9 observations were performed in November 3 and 4, 2012, in four runs with durations of 133, 127, 64, and 111 min, which allowed accumulating a total of 135 min of correlations on source. For band 7, J0522-364 was observed for the RF calibration. For band 9, J0538-440 or 3 C84 were observed instead. Callisto was always observed for absolute flux calibration, but for one run in band 9 in which data in Ceres was obtained. J0607-085 and J0609-157 were observed every 10 to 20 min for gain calibration.
ALMA staff provided first calibrated data in December 2012, which were considerably improved by us. In particular, we made sure that the absorption CO lines detected at the position of the RF calibrator did not get into source data. The bandpass was calibrated by fitting cubic splines. Also, having identified different RF solutions for the different observed runs, an independent calibration was performed per track. For gain calibration, data from the two observed calibrators were considered. Later, from these first calibrated data, self-calibration was carried out by using the bright and moderately extended continuum emission of
the Red Rectangle. All those processes resulted in very significant improvement in the quality of final images.
The flux calibration was based on the Butler-JPL-Horizons 2010 model for solar system bodies. A recent upgrade in that model (including, e.g., Herschel data) suggests that the intensity presented here could be slightly overestimated, by 10% in band 7 and by 5−10% in band 9.
In band 7, the total observed frequency ranges were 330.354−330.822, 333.152−333.620, 345.106−345.574, and 345.562−346.030 GHz, to map mainly the emission of the 12CO and 13CO J = 3−2 transitions. In band 9, we observed the intervals 673.540−674.477, 677.040−677.977, 687.805−688.742, and 691.004−691.941 GHz to image the 12CO J = 6−5 line.
A WFPC2/f622w optical image was obtained from the HST archive. The optical and ALMA images were aligned by adopting the astrometry provided by the Hubble Legacy Archive, which we have checked is reliable using the 2MASS coordinates of the 11 field stars detected within the WFPC2 field, and correcting for the Red Rectangle’s proper motion (as measured by Hipparcos and listed in the SIMBAD database, δRA = −6.46 ± 2.21 and δDec = −22.74 ± 2.03 mas yr-1, van Leeuwen 2007) for a time difference of 13.3 yr between the HST and ALMA observations. The resulting alignment of the centroids, within ≲01, is found to be satisfactory, in view of the uncertainty in the stellar positions and proper movements.
© ESO, 2013
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