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

5. Discussion

In Table 10 we compare the line ratios found in Cepheus B with those found in two other galactic star forming regions: M17 (Wilson et al. 1999, cf. Table 5 therein) and Orion B (Kramer et al. 1996). Like the Cepheus B observations presented here, both data sets stem from large scale observations covering not only the immediate vicinity of sites where star formation is taking place but also the distant quiescent parts of the clouds far from the cloud interfaces with HII regions. While the FUV fields do not exceed a few in Cepheus B, they are stronger by at least a factor of 10 in the NGC 2024 region within Orion B and still stronger in the interface region of M17SW (Genzel et al. 1989).

Table 10. Comparison of line ratios of integrated intensities with observations of M17 and Orion B (Cepheus B: this paper; M17: Wilson et al. 1999, Tables 2-4 therein; Orion B: Kramer et al. 1996)

Local volume densities of cm^-3 are needed to explain the observed average line ratios found in Orion B (Kramer et al. 1996), while densities of are sufficient in the case of Cepheus B at all positions, due to the lower 3-2/2-1 line ratios. The average line ratios found in M17 are consistent with densities of or more (Fig. 6 in Wilson et al. 1999).

The variation of the ¹²CO 3-2/2-1 integrated intensity ratio along different lines of sight is stronger in Cepheus B and Orion B than in the M17 observations of Wilson et al.. The ratio rises significantly above 1 near the hot core and at the north-eastern edge in Cepheus B, and in the NGC 2024/IC434 interface region of Orion B. Small clumps with densities of more than may explain the ratio of 1.3 found in the latter region. However, higher ratios, like those found in parts of Cepheus B cannot easily be explained. Models taking into account internal heating sources and/or heating by shocks may be necessary.

For all three clouds, the ¹²CO/¹³CO 3-2 and 2-1 ratios vary much stronger than the interisotopomeric ratios (see Fig. 5), indicating that variations of column densities dominate while the local densities traced by ¹²CO line ratios are more or less constant. In addition, ¹²CO/¹³CO integrated intensity ratios in general rise towards the edges of all three clouds, where the integrated intensities drop and ¹³CO is becoming optical thin.

Our results confirm the findings of other authors (e.g. Plume et al. 1999, Minchin & White 1995, White & Sandell 1995) who reported rising ¹³CO/C¹⁸O line ratio with dropping column density at the cloud edges, from a typical galactic value of at the position of highest column density to values of 50 and higher at low column densities. At the edges of molecular clouds, where optical extinction is low, ¹³CO is expected to be enriched in comparison with C¹⁸O due to isotope-selective photodissociation and/or chemical fractionation (see e.g. the PDR models of Sternberg & Dalgarno 1989). It is thus tempting to interpret the observations as a confirmation of the isotopomeric fractionation predicted by PDR models.

Zielinsky et al. (2000), however, show that this straightforward conclusion is misleading, because the line-of-sight column density traced by the rare isotopomeric CO lines and the UV extinction column density responsible for the selective photoshielding and -destruction are not necessarily correlated, in particular in the edge-on geometries commonly observed. They explain the observed correlation as a natural consequence of a clumpy cloud PDR model. In small clumps (resulting in low column densities), self shielding of ¹³CO is much more effective than that of C¹⁸O, the latter is nearly totally photodissociated which leads to high ¹³CO/C¹⁸O line ratios. On the other hand, in large clumps (resulting in high column densities) even C¹⁸O is protected to a large degree from photodissociation and the ¹³CO/C¹⁸O ratios decrease.

© European Southern Observatory (ESO) 2000

Online publication: October 30, 2000
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