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Astron. Astrophys. 353, 583-597 (2000)

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2. Observations

The CO(J=[FORMULA] at 115 GHz, and J=[FORMULA] at 230 GHz) observations of TT Cyg were made between November-96 and February-98 using the IRAM Plateau de Bure interferometer, France (Guilloteau et al. 1992). Five telescopes were used in four different configurations (B1, B2, C2, and D), and we observed nine primary fields in order to cover an area of about 1[FORMULA]5[FORMULA]1[FORMULA]5. The field centres were chosen in order to cover all emission in the J=[FORMULA] line within a minimum time. The positions of the fields can be seen in the J=[FORMULA] image in Fig. 2. The data were reduced using the GILDAS software package. The maps produced in each field were combined and deconvolved simultaneously using a generalization of the CLEAN algorithm to mosaics (Gueth et al. 1995). Natural weighting has been used for all data. The resulting images are corrected for primary beam attenuation. The synthesized beams are 2[FORMULA]2[FORMULA]1[FORMULA]8 (PA=26o) and 1[FORMULA]1[FORMULA]0[FORMULA]9 (PA=33o) for the CO(J=[FORMULA]) and CO(J=[FORMULA]) data, respectively. The velocity scale is given with respect to the Local Standard of Rest (LSR).

The entire shell is covered in the J=[FORMULA] line, and the S/N-ratio is high as can be seen in Fig. 1, where all the data are shown at 1[FORMULA] resolution. In the J=[FORMULA] line the primary beam is half as wide, and only parts of the shell have been covered (we estimate that about 80% of the emitting area is covered at the systemic velocity). Thus, the mosaicing is difficult, and the final J=[FORMULA] maps should be interpreted with caution. In Fig. 2 we show for comparison the J=[FORMULA] and J=[FORMULA] emissions integrated over the velocity range -27.5[FORMULA]2[FORMULA], i.e., a 4[FORMULA] range centred close to the systemic velocity (see below). It is apparent that the S/N-ratio is much poorer in the J=[FORMULA] line. The data clearly show the geometrical thinness of the shell, as well as the emission from a CO envelope around the star, which, in a relative sense, is more prominent in the J=[FORMULA] data. Also shown in Fig. 2 are the total velocity-integrated J=[FORMULA] emission, and the 1[FORMULA] interval map centred at -38[FORMULA]. At this velocity the line-of-sight is almost in the radial direction through the shell, and the details of the brightness distribution become more apparent. Most of the discussion in this paper will be based on the J=[FORMULA] data.

[FIGURE] Fig. 1. CO(J=[FORMULA]) brightness maps in 1[FORMULA] intervals (the LSR centre velocity is shown in the upper left corner of each panel) obtained towards TT Cyg. The synthesized beam is 2[FORMULA]2[FORMULA]1[FORMULA]8 (PA=26o). The systemic velocity is estimated to be -27.3[FORMULA]

[FIGURE] Fig. 2. CO(J=[FORMULA] and J=[FORMULA]) brightness maps in a 4[FORMULA] interval centred at -27.5[FORMULA], i.e., close to the systemic velocity (upper left and right, respectively). CO(J=[FORMULA]) velocity-integrated brightness map (lower left). CO(J=[FORMULA]) brightness map in a 1[FORMULA] interval centred at -38[FORMULA] (lower right). The synthesized beams are shown in the upper left corner of each panel

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© European Southern Observatory (ESO) 2000

Online publication: December 17, 1999