Our main conclusions are that: 1) the observed break in the I(C18O) versus AV relation at AV 10 magnitudes (Fig. 11a, LLCB94) is a result of the depletion of C18O gas at high extinction, and 2) the NIR data are likely to be the most reliable tracers of mass distribution in the cases of extended nearby clouds such as IC 5146, although the NIR data are (still) limited by the number of background stars detectable at high extinctions and small scales.
We have evidence in a quiescent molecular cloud core that the [CO]/[H2] abundance ratio is a factor of at least lower than derived in studies which sample molecular cloud size scales above 0.5 pc (e.g Frerking et al. 1982, LLCB94). We attribute this to depletion onto dust grains at hydrogen densities above cm-3, at kinetic gas temperatures of about 10 K, at extinctions of more than about 10 mag, and at scales of 0.1 pc and less. Our study of towards selected positions shows that it is impossible to attribute the "low" intensities at high extinctions to optical depth effects. Moreover, our excitation analysis of the (10) and (21) transitions suggests that our abundance estimates are not very sensitive to CO excitation. Our observations measure both undepleted and depleted regions along the line-of-sight in our beam. It is thus possible that all the CO we see comes from regions with mag and the depletion could be very large in the interior core regions.
CO depletion in the quiescent dense core of IC 5146 is in agreement with NIR observations of CO ice features in dark clouds (e.g. Chiar et al. 1995). Observations of CO ices, observations of the deuterium fractionation (Caselli et al. 1998), and chemical models assuming CO ices (Bergin et al. 1995) suggest CO depletion factors of 5 or less.
The depletion which we have found, if applicable to other clouds, does not greatly affect most past mass estimates based upon or since it appears to be an effect which one only sees at high densities or small size scales (pc). We show that within the central region of pc2, underestimates masses by a factor of 2. The mass of this core region, derived from NIR extinctions, is .
Thus, is probably not a good tracer of the high density condensations which are likely to lead to the next generation of stars. We suspect that other molecular line tracers are more useful in this respect although NIR extinction measurements of the type discussed here are probably the most reliable of all. However, one cannot presently obtain NIR observations of a large sample of molecular clouds and hence it is important to establish if molecular tracers other than (NH3,CS, HCO+, etc.) have a column density distribution similar to that observed in the NIR towards IC 5146.
We note incidentally that Paper I shows that millimeter continuum emission from dust also is an inexact tracer of the density distribution in the dense core which we have studied. In fact, the maps of column density (Fig. 9) and 1.2 mm dust emission (Fig. 1 in Paper I), both smoothed to resolution, show nearly identical structure. They both peak near (,), while only showing a "plateau" near (,) where the extinction peaks.
This finding is reflected by the strong correlation between the depletion factor and the dust temperature shown in Fig. 13. The depletion factor varies exponentially with dust temperature, it rises from to 2 when the dust temperature falls from 15 to 10 K. The highest depletion factor is reached in the coldest ( K) and densest region ( mag).
We supplemented our maps by observations towards individual stars outside the mapped region. This served the purpose of testing whether the particular clump selected by us had special characteristics. There is large dispersion in the I((10)/ ratios measured towards individual stars. We attribute this to the spread of clump ages which is reflected in the spread of the degree of CO depletion and/or to substructure at scales beyond 0.05 pc or a.u..
Such small scale structure is indicated by the decomposition of the (10) data into 26 Gaussian shaped clumps with clump sizes between 0.1 and 0.02 pc and clump velocity widths between 700 and 300 ms-1. Clump masses were calculated assuming LTE, i.e. neglecting subthermal excitation, and a standard abundance ratio /[H2], i.e. neglecting depletion of CO. Clump masses lie between 0.06 and , while the ratios of virial mass over lie between 14 and 1, most clumps are nearly virialized.
© European Southern Observatory (ESO) 1999
Online publication: December 22, 1998