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Astron. Astrophys. 329, 827-839 (1998)

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2. Improved IRAM interferometric CO(7-6) map

Our previous CO(7-6) interferometer measurements (Alloin et al 1997) have been complemented with observations at intermediate baselines, performed on 1997 April 10 and 21 under excellent conditions. Calibrations were applied similarly to the previous data set. The combined data lead to the CLEANed integrated map restored with a [FORMULA] circular beam shown in Fig. 1a. In order to search for a velocity gradient, we have first derived the spatially integrated line profile, following the procedure already discussed in Alloin et al (1997). The new CO(7-6) line profile, shown in Fig. 2, exhibits a marked asymmetry with a steep rise on its blue side and a slower decrease on its red side. Excluding the central velocity channel (so that the split in velocity is symmetric), we have built the blue (-225,-25 km/s) and the red (+25,+225 km/s) maps displayed in Figs. 1c and 1d respectively. The difference between the red-shifted and blue-shifted CLEANed maps (Fig. 1b) establishes definitely the presence of a velocity gradient at the 8 [FORMULA] level. The implication of these results on the CO source will be discussed more thoroughly in Sect. 6.

[FIGURE] Fig. 1. Image of the Cloverleaf obtained with the IRAM telescope at Plateau de Bure. a  is the total CLEANed image, c  the CLEANed blue-shifted image, d  the CLEANed red-shifted image and b  the difference between the CLEANed red and CLEANed blue image. The CLEANed CO(7-6) maps were obtained with a natural beam of [FORMULA] at P.A. 15 deg. They have been restored with a circular [FORMULA] beam for comparison with HST data. Contour spacing is 1.35 mJy/beam for 1a, 2 mJy/beam for 1c and 1d and 3 mJy/beam for 1b, corresponding in each case to 2 [FORMULA].

[FIGURE] Fig. 2. Spectrum of the CO(7-6) line towards the Cloverleaf. Error bars are [FORMULA] 1 [FORMULA]. The thin line corresponds to the best gaussian fit to the wings and the red side with FWHM [FORMULA] 450 km/s, although it is obvious that the line profile differs from a smooth gaussian shape and shows an excess of emission on its blue side. The offset of the baseline from zero corresponds to 13 mJy of continuum flux; this is larger than the [FORMULA] mJy reported by Alloin et al (1997), owing the inclusion of shorter baselines in the new dataset and detection of a more extended continuum component. The central frequency is 226.715 Ghz.

Measurements of the spot characteristics from the CO(7-6) image have been performed (spot flux ratios, sizes and orientations) through a fitting procedure in the visibility domain, as explained in Alloin et al (1997). The final parameters are provided in Table 1 where the spot sizes are intrinsic to the image, i.e. deconvolved by the interferometer beam. Although the measurements have not been corrected for seeing effects (mean seeing estimate of the order of [FORMULA]) the spots A, B and C appear to be definitely elongated (see Fig. 3 for spot labels).


Table 1. The Cloverleaf CO(7-6) image: positional shifts, fluxes and elongations of the spots. One [FORMULA] error bars are given within brackets. When no error bar is given, the value was fixed from the fit to the -225,225 km/s image.

[FIGURE] Fig. 3. Image of the Cloverleaf obtained with the HST/WFPC2 through the F702W filter (central rest wavelength [FORMULA] 1975 Å). The spots shape is perfectly circular. The spots are labelled A, B, C, D, following Magain et al (1988).

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© European Southern Observatory (ESO) 1998

Online publication: December 16, 1997