3 Results

3.1 FIR results and derived parameters

ISOSS J20246+6540 is a faint FIR source with an upper limit on the I₁₀₀ brightness of about 0.5 MJy sr^-1 derived from the "raw'' (IRDS format) IRAS data. Comparison of ISOSS and ISSA data (Wheelock et al. 1994) was made as described by Tóth et al. (2000). An upper limit of the colour temperature of 14.5 K was estimated from the bisector slope of the I₁₇₀ vs. I₁₀₀ scatter plot. Assuming a dust temperature of 14.5 K, an average dust column density of $1.3\times 10^{-5}$ g cm^-2 was derived following Hildebrand (1983). This corresponds to an average hydrogen column density of N(HI+2H $_2)=8.6\times 10^{20}$ cm^-2 where a hydrogen-to-dust mass ratio of 110 (Launhardt & Henning 1997) was assumed.

3.2 Optical images

ISOSS J20246+6540 appears as a faint, isolated reflection cloud west of the L1155/L1157 cloud complex (Lynds 1962). When smoothed to 15 $^{\prime\prime}$ the excess diffuse surface brightness distribution of ISOSS 20246+6540 shows a "bright'' lobe at the NE and a fainter fragmented one at SE. The $\sigma_{\rm B}$ standard deviation of photographical density outside the globule on the star-removed, smoothed image was 1.3% of the average background value in the blue band image. The POSS blue band diffuse surface brightness is shown in Fig. 2a, where the lowest contour is drawn at an excess of three times $\sigma_{\rm B}$.

Figure 2: Dust and gas in ISOSS 20246+6540, reference coordinate (0, 0): RA =  $20^{\rm h}24^{\rm m}44^{\rm s}$ Dec =  $65\hbox {$^\circ $ }40\hbox {$^\prime $ }04\hbox {$^{\prime \prime }$ }$. a) DSS-2 blue band diffuse surface brightness, contours are drawn at photograpic density values of 4, 8 and 12% above the background level. b) W(12CO (1-0)), contours are from 1.5 K km s-1 with 1.5 K km s-1 steps; c)13CO (1-0), contours are from 1.0 K km s-1 with 1.0 K km s-1 steps; CO isotopomer line intensities are integrated in the velocity interval -5 km s $^{-1} \le v\rm _{LSR} \le 0$ km s-1.

The average excess surface brightness of ISOSS J20246+6540 is $\approx$8% of the background photographic density value in the DSS blue band. The same morphology is shown by the B and R surface brightness distributions, except that the SE lobe is slightly more red than the NW lobe. The lack of a dark core indicates an average visual extinction below 1.5 mag.

3.3 Molecular line results, and derived parameters

All observed lines show a LSR velocity of -2.7 km s^-1. The FWHM widths of the ¹²CO lines are around 0.4 km s^-1, and around 0.3 km s^-1 for all the other detected lines. The ¹²CO and ¹³CO integrated intensity distributions are shown in Figs. 2b and c. The bipolar shape is well seen and the CO line intensities are in accordance with the excess surface brightness distribution, the NE lobe being much brighter. The two lobes show the same radial velocity.

The ¹²CO and ¹³CO lines trace the ISM well over most of the cloud. A comparison of C¹⁸O to ¹³CO lines (both 1-0 and 2-1) at the centre of the NE lobe indicates an underabundance of C¹⁸O by a factor of 4. This effect is expected in cold clouds with moderate density, exposed to UV radiation (Glassgold et al. 1985). A more careful modelling may account for it as we show in Sect. 4.2.