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Astron. Astrophys. 362, 1109-1121 (2000)

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3. Observational results

3.1. 12CO 3-2 data

The 12CO 3-2 observations enclose an area of 4 pc[FORMULA]3.5 pc ([FORMULA]) (Fig. 1). The molecular cloud is bound by the HII region S155 in the north-west, a large arc-shaped optical emission region, and it has a linear extent of [FORMULA] pc in south-eastern direction. The CO emission peaks near the hot core region and decreases towards the south-east, showing only rather weak substructures at this resolution.

In the channel maps, however, small scale structure is much more pronounced (Fig. 2). The north-western part of the cloud near the hot core emits at -15 km s-1, which confirms its association with S155 having the same system velocity (Miller 1968, derived from [FORMULA] observations) while the south-eastern part emits at -10 km s-1. Near -14 km s-1 the cloud looks like a compact cometary globule pointing towards S155. With increasing velocity the emission region extends further south-east. The cloud appears filamentary at around -10.9 km s-1. At -9.7 km s-1 the cloud again looks compact in the south-east near ([FORMULA]/[FORMULA]). Slight indications of scanning artefacts are visible in the south-eastern part of the maps.

[FIGURE] Fig. 2. Channel map of the 12CO 3-2 emission. The velocity varies from -15.75 km s-1 to -9.12 km s-1, the channel width is 0.28 km s-1, and the contours are 2 by 3 to 23 Kkm s-1.

The observed spectra show close to Gaussian line shapes (Fig. 3); neither line wings nor self absorption features due to cold foreground material show up prominently. Only a few spectra show shoulders, probably due to more than one velocity component; the average line width (FWHM) is 2.8 km s-1 and varies marginally throughout the cloud. Fig. 3 shows spectra at the four representative positions marked in Fig. 1.

[FIGURE] Fig. 3. Spectra of the different transitions measured in Cepheus B at the 4 positions selected for the radiation transfer analysis (marked in Fig. 1). All spectra are smoothed to a spatial resolution of [FORMULA]; the 12CO 3-2 and 13CO 3-2 spectra are error beam corrected (Appendix B). (The additional noise, seen in the two 12CO 2-1 spectra at [FORMULA] and [FORMULA], was caused by a temporal technical even-odd problem of the AOS, which is now solved.)

3.2. Further CO data

We also conducted OTF mapping in 12CO 2-1, 13CO 3-2 and 2-1, and C18O 3-2 and 2-1 (Fig. 4; note: the 13CO 3-2 and C18O 3-2, 2-1 maps are spatially less extended). Each transition samples different regimes of temperatures and densities, described e.g. by its critical density needed for thermalization and its energy of the upper level above the ground state (Table 2).

[FIGURE] Fig. 4a-c. Greyscales show the velocity integrated ratios of the 12CO, 13CO and C18O 3-2/2-1 intensities a-c . The ranges of integration are -18 to -8 (12CO), -15 to -8 (13CO) and -14.5 to -10.5 (C18O). The 3-2 data were smoothed to [FORMULA], the resolution of the 2-1 data. The ratio maps are blanked at those positions where the intensities of one transition are less than [FORMULA]. Contours show integrated intensities of 12CO 3-2 (4(6)94 Kkm s-1, a), 13CO 3-2 (2.4(2)12.4 Kkm s-1, b) and C18O 3-2 (0.6(0.7)3.4 Kkm s-1, c). All 3-2 contour overlays are in their original resolution on a [FORMULA]-scale. The markers denote the same positions as in Fig. 4.


Table 2. Parameters of the observed transitions; ref.: Lovas 1988, Flower & Launay 1985, Schröder et al. 1991.

The distribution of velocity integrated brightness temperatures ([FORMULA]) naturally differs between the different isotopomers (contour maps in Fig. 4), given the isotopomeric abundance ratios ([12CO]/[H2]=[FORMULA], Frerking et al. 1982; [12CO]/[13CO]=67, Langer & Penzias 1990; and [12CO]/[C18O]=470, Frerking et al. 1982), and hence optical depth differences between the various lines. The 13CO 3-2 map peaks near the [FORMULA] position (the position of strongest CO 1-0 emission, Sargent et al. 1977) at a distance of 0.7 pc to the hot core, and the C18O map peaks still further east at [FORMULA] at a projected distance of 1.7 pc. Both rarer species show an east-west oriented ridge of emission at [FORMULA]. The map of 13CO shows another ridge of emission extending from the peak of C18O emission in south-eastern direction. Thus, the general tendency is that the rarer the isotopomers, the more does their emission peak towards the projected central position of the cloud and away from the HII region/molecular cloud interface. The spatial variation in integrated line intensities, e.g. at a scale of 0.5 pc perpendicular to the east-west orientated ridge, is much stronger in C18O than in 13CO, which in turn shows a higher contrast than 12CO. The hot core region is not pronounced in the rarer isotopomers.

From the distribution of integrated intensity ratios 3-2/2-1 (greyscale maps in Fig. 4) one can distinguish at least three regions: the interface region to S155 near the hot core showing high line ratios of 1.5 to 2 in 12CO and slightly lower in the other isotopomers; the bulk of the cloud, characterized by ratios of about 1 in all isotopomers and orientated along the east-west ridge; the north-eastern edge of the 12CO map exhibiting similarly high 12CO 3-2/2-1 line ratios as the hot core region, although with lower integrated intensities. The integrated intensity ratios


are strongly correlated with the integrated intensities of [FORMULA] and [FORMULA], respectively.

The [FORMULA] ratio (dashed curve in Fig. 5a) varies between [FORMULA], for high values of [FORMULA], and more than 8 for low values of the integrated 13CO intensity (the interstellar abundance ratio is [FORMULA]). This variation and fluctuation is consistent with varying optical depths of the 13CO isotopomer and optically thick 12CO emission. Considering LTE with [FORMULA] the 13CO opacity, given by [FORMULA][FORMULA]/[FORMULA], varies between 0.4 in the center of the cloud where intensities are high, and 0.03 at the cloud edges.

[FIGURE] Fig. 5. a  Integrated intensity of [FORMULA] 2-1) against [FORMULA] 2-1), the solid line shows the best fit of a 2. order polynomial. The dashed line shows the intensity ratio derived from the fit for integrated intensities larger than [FORMULA] (2.9 Kkm s-1 for [FORMULA] 2-1) and 0.9 Kkm s-1 for [FORMULA] 2-1)). b  The same for [FORMULA] and [FORMULA] (1[FORMULA] for [FORMULA](C18O 2-1) equals 0.2 Kkm s-1).

The integrated intensity ratio [FORMULA] (dashed curve in Fig. 5b), derived from the fit, variies between [FORMULA] for high integrated C18O intensities and more than 22 at the cloud edges where C18O intensities and column densities are low. In contrast to the former plot, most positions show a [FORMULA] ratio that is higher than the abundance ratio which is 7 in this case. Clumpy cloud models, isotope-selective photodissociation and/or chemical fractionation are possible approaches to explain this common phenomenon. Sect. 5 will compare and discuss our findings with similar results from the literature.

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© European Southern Observatory (ESO) 2000

Online publication: October 30, 2000