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Astron. Astrophys. 320, 575-579 (1997)

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2. Observations

Observations by the CS [FORMULA] 1-0 and [FORMULA] 2-1 transitions at 48.991 and 97.981 GHz, respectively, were made with Nobeyama Millimeter Array during the Dec. 1988 - April 1989 session using five antennas with 2 configurations in each CS line (20 base lines each). The SIS receivers at 49 and 98 GHz were operated in the double side band mode and the system temperatures at these frequencies were about 300-400 K. The side-band separation was made with a phase switching by 90 degree. The 1024 channel, digital FFT spectro-correlator with a band width of 320 MHz was used, giving the velocity resolution of 1.9 and 0.9 km s-1 per channel for the [FORMULA] 1-0 and 2-1 transitions, respectively. The radio source, BL Lac (2200+420), was used as a flux and phase calibrator. The flux density of the calibrator at each frequency was obtained by observing Venus and Mars at each configuration. The flux densities of 3.7 and 2.2 Jy at 49 and 98 GHz, respectively, were adopted through the observing session. The band pass calibrations were made by observing 3C84 after every CRL 2688 observation. The data were reduced using the Nobeyama package of the AIPS system and CLEANed maps were made. The resulting synthesized beam was nearly circular with a full width of a half maximum of [FORMULA] at 49 GHz and [FORMULA] at 98 GHz. The continuum was detected only at 98 GHz and the total flux density of the continuum was 0.15 Jy at 98 GHz, which is consistent with previous measurements (Knapp et al. 1994).

Fig. 1 shows integrated intensity maps for the [FORMULA] 1-0 (left pannel) and [FORMULA] 2-1 (right pannel) lines. Emission is extended by about [FORMULA]. They are approximately circular for both lines, but slight deviation from the circular symmetry can be seen in the central part in the right pannel (the J =2-1 line). The elongated structure can be seen in the [FORMULA] 2-1 map. The direction of elongation seems to be misoriented from the direction of the optical bipolar axis which is shown as the broken line in figure 1. It is possible that the complex structure seen in the [FORMULA] 2-1 map is smeared with the larger beam in the [FORMULA] 1-0 map and becomes unrecognizable.

[FIGURE] Fig. 1. The integrated-intensity maps of the CS [FORMULA] 1-0 (right pannel) and [FORMULA] 2-1 (left pannel) lines for CRL 2688. The CS emission is integrated in the velocity range between [FORMULA] -80 and -15 km s-1. Contour leves are drawn in every 20 percent of the peak value. The broken lines shows a direction of the optical bipolar axis. The gray ellipse near the bottom-right corner shows a beam shape. The cross mark shows the peak of emission ([FORMULA] 21h00m19.8s, [FORMULA], 1950).

Figs. 2 and 3 show the resulting channel maps of the CS [FORMULA] 1-0 and 2-1 lines of CRL 2688. The [FORMULA] 1-0 maps (Fig. 2) with a [FORMULA] resolution show that emission is almost peaked at the center, i.e., at the mid point between the optical bipolar lobes. In contrast, the [FORMULA] 2-1 maps (Fig. 3) with a [FORMULA] resolution show rather complicated structure. The total flux densities of the [FORMULA] 1-0 and 2-1 lines at the peak (calculated from the map) are 1.1 and 4.5 Jy, respectively.

[FIGURE] Fig. 2. The velocity-channel maps of the [FORMULA] 1-0 line for CRL 2688. Contour leves are drawn in every of 90 mJy/beam (one sigma) above the lowest contour level (three sigma). The bottom-right three panels, (c'), (i'), and (o'), are the channel maps otained from the model calculations. The cross mark indicate the peak position of emission
[FIGURE] Fig. 3. The velocity-channel maps of the [FORMULA] 2-1 line for CRL 2688. The contour levels are drawn in every 50mJy/beam (one sigma) above the lowest contour level (three sigma). The bottom-right three panels, (g'), (k'), and (n'), are the channel maps obtained from the model calculations.

The most distinctive features in the [FORMULA] 2-1 channel maps are two concentrations of emission at the North-East and South-West sides of the core. The two peaks of emission are separated by about [FORMULA]. They are closely oriented to the direction of the optical bipolar axis (NNE to SSW), but look slightly misaligned to the bipolar axis. Another notable feature is a lateral elongation which is oriented approximately in the direction of the optical dark lane; this feature can be seen in the panels (j), (l), and (m) in Fig. 3.

The line profile which is integrated over the source is shown in Fig. 4. The high velocity component at [FORMULA] -70 to -55 km s-1 seems to be detected, though it is quite difficult to recognize it in the indivisual channel maps. The peak flux density of 4.5 Jy of the [FORMULA] 2-1 line is consistent with the previous single-dish observation of [FORMULA] K of this transition with the NRAO 11-m telescope (Zuckerman et al. 1976) and the missing flux is estimated to be less than 20% in the present interferometric observations.

[FIGURE] Fig. 4. Synthesized spectra of the CS [FORMULA] 1-0 and [FORMULA] 2-1 lines for CRL 2688. The top thick line shows the [FORMULA] 2-1 line and the lower thin line the [FORMULA] 1-0 line. Broken lines show the spectra calculated from the model.
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© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998
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