3 Observations

The observations were carried out at the Swedish-ESO Submillimetre Telescope (SEST) in two runs: February 1993 and May 2000. During the first session we mapped B68 in the $\ensuremath{\:\!} ^{13}\ensuremath{\!\;\!}$CO $(J=1\mbox{--}0)$ and C $\ensuremath{\:\!} ^{18}\ensuremath{\!\;\!}$O $(J=1\mbox{--}0)$ lines at 110.2 and 109.8 GHz, respectively. The map grid spacing was 20 in the cloud centre and 40 in the outer parts. The receiver used during these earlier observations was a 3 mm Schottky mixer dual channel receiver. By using two mixers tuned to the same frequency we observed orthogonal polarizations simultaneously. The system temperature, $T_{\rm SYS}$, reduced to outside of the atmosphere, ranged from 300 to 400 K. In the mapping the observing time per position was typically 2 min, which resulted in an rms noise level of about 0.1 K. During the second period in May 2000 we mapped the cloud in C $\ensuremath{\:\!} ^{18}\ensuremath{\!\;\!}$O $(J=1\mbox{--}0)$ and C $\ensuremath{\:\!} ^{18}\ensuremath{\!\;\!}$O $(J=2\mbox{--}1)$ at 109.8 and 219.6 GHz simultaneously with 3 and 1.3 mm SIS receivers. The grid spacing was $20\ensuremath{^{\prime\prime}}$. The system temperatures at the two frequencies were around 160 and 260 K, and the observing time per position was typically 1 min. The rms noise level attained was typically 0.06 K at 3 mm and 0.1 K at 1.3 mm.

The half-power beam width (HPBW) of the antenna is 47 at 110 GHz and 25 at 220 GHz. The pointing and focus were checked at 3-4 hour intervals towards circumstellar SiO $(v=1,J=2\mbox{--}1)$ maser line sources, and the pointing accuracy was typically found to be 3 . The map was centred on RA 17 19 34.${\rm ^s}$6, Dec -23 46 34 (1950.0). All observations were performed in the frequency switching mode. The two mixers used at the same time were connected to a 2000 channel acousto-optical spectrometer which was split in two bands of 43 MHz each. The AOS channel width corresponds to 0.12  at 2.7 mm and 0.06  at 1.37 mm. Further details of the SEST are available at http://www.ls.eso.org/lasilla/Telescopes/SEST.

Calibration was done by the chopper wheel method. To convert the observed antenna temperatures, $T_{\rm A}^*$, to the radiation temperatures, $T_{\rm R}$, the former were divided by the assumed source-beam coupling efficiences, $\eta_{\rm C} = \int_{\rm source}P_{\rm n} {\rm d}\Omega /\Omega_{\rm A}$, where $\Omega_{\rm A}$ is the beam solid angle of the antenna and $P_{\rm n}$ the normalised beam pattern. For $\eta_{\rm C}$ we adopted the main beam efficiences of the telescope interpolated to the frequencies used, i.e. 0.71 and 0.61 at 110 and 220 GHz, respectively. Our numerical estimates for $\eta_{\rm C}$were in fact close to these values when the source was assumed to be disk-like with a radius of 100 .

Line area maps of the C $\ensuremath{\:\!} ^{18}\ensuremath{\!\;\!}$O transitions are presented in Fig. 1. Characteristic for the C $\ensuremath{\:\!} ^{18}\ensuremath{\!\;\!}$O $(J=1\mbox{--}0)$ line intensity distribution is a steep rise at the edge and a plateau at the centre of the globule. The C $\ensuremath{\:\!} ^{18}\ensuremath{\!\;\!}$O $(J=2\mbox{--}1)$ line area looks patchy with maxima at the eastern edge of the globule and the prominent "nose'' (cmp. Fig. 3) in the southeast.

Figure 1: C $\ensuremath{\:\!} ^{18}\ensuremath{\!\;\!}$O line area maps, centred on RA 17 19 34.${\rm ^s}$6, Dec -23 46 34 (1950.0). The velocity range is 3.0-3.8 km s-1 and the beam (HPBW) is indicated in the lower right corner of each map. Small dots mark the positions actually observed. a) C $\ensuremath{\:\!} ^{18}\ensuremath{\!\;\!}$O $(J=1\mbox{--}0)$ line area. Grey scales range from 0.075 to 0.450 K km s-1 in steps of 0.025 K km s-1. b) C $\ensuremath{\:\!} ^{18}\ensuremath{\!\;\!}$O $(J=2\mbox{--}1)$ line area. Grey scales range from 0.050 to 0.425 K km s-1 in steps of 0.025 K km s-1.