3. Reductions and results
3.1. Continuum radiation at 1.3 and 3.45 mm
The individual spectrum pairs for either beam throw were added and total averages were formed, where the weighting was done with respect to the rms noise in the spectral scans. The resulting spectrograms testified to the overall stability of the system and of the sky: linear baseline fits resulted in offsets from the zero level by some fraction of a milli-Kelvin (mK). As the average over the whole spectral band is of this order, it is clear that any continuum emission from the disk would have to be below this level. Specifically, strict upper limits of 8 mJy at 86 GHz (3.45 mm) and of 40 mJy at 230 GHz (1.3 mm) are implied by these measurements. The latter value is in agreement with the bolometer datum obtained with the same telescope by Chini et al. (1991), viz. mJy. From the disk model described in Sect. 4 we find mJy, which is clearly also in agreement with our observations.
3.2. Molecular line emission: CO, CS and SiO
Our molecular line searches gave similarly negative results: in no case did we directly detect any signal above the limiting noise, , given in Table 1, in spite of very deep integrations. In order to enhance the recognizability of presumably very faint signals we also changed the sky frequency towards by km s-1. This procedure was demonstrably warranted as we first found an apparently solid detection of the SiO (v =0, 2-1) line after some 17 000 s of on-source integration (sic!). At 1 km s-1 resolution, the level of the rms noise is at only 7 mK in this spectrogram. Clearly, this erroneous result was due to some very low-level, about 20 km s-1 wide and centred approximately on the stellar radial velocity, irregularities of the receiver response (see: Figs. 2 and 6).
Over the observed bandwidths, this direction of the Galaxy ( = 258 4, = -30 6) is virtually void of any molecular material within some tens of arcmin surrounding , making an optimum reference position for galactic millimeter line work. The rms levels of the noise temperature, in , are for SiO (v =0, J =2-1) 6 mK, for SiO (v =2, J =2-1) 10 mK, for CS (J =2-1) 16 mK and for CO (J =2-1) 25 mK per velocity channel, . Binning the channel data into widths of about 1 km s-1, decreases the noise to the values indicated in Fig. 1. Evidently, for the transitions of our primary interest, low-excitation SiO and CO (J =2-1), the achieved sensitivity of our measurements is . In Fig. 2, the data have been binned at the presumed characteristic Keplerian velocity of the disk. For CO (2-1), this can be compared to the 15 m JCMT observations by Dent et al. (1995), who obtained at 12 km s-1 binning an upper limit a factor of three times higher ( 4.2 mK, ).
© European Southern Observatory (ESO) 1998
Online publication: June 2, 1998