4. and in the mapped region
These estimates of extinctions should be compared with the directly measured visual extinctions over the mapped region (Paper I, Lada et al. 1998) which vary between 3 and 28 magnitudes (Fig. 9). In Fig. 11, we plot LVG column density against visual extinction. In the range mag, (10) line intensities and column densities rise with growing extinctions, while column densities are roughly independent of extinction for mag. The scatter in intensities and column densities generally becomes smaller with rising extinction. While the scatter in line intensities (Fig. 11a) is larger than the observational error for mag by a factor of about 4, it is comparable to the 7% calibration error for larger extinctions.
LLCB94 also derived a linear dependence of the integrated (10) intensity upon for visual extinctions of less than 15 magnitudes, at a common angular resolution of , for the extended emission of the Northern Streamer. They also found a "flattening" of line intensities at extinctions above magnitudes. Alves et al. (1998) revisited this data set and used a bi-variate least quares fitting procedure which takes into account the uncertainties in both integrated intensity and extinction. This is necessitated by the fact that the errors in mean are larger than observational and are an increasing function of . This results in the following relation for the Northern Streamer extended cloud:
Fig. 11a shows that this relation fits reasonably well the high resolution data of the core region observed with the 30m telescope.
Note that the derived column densities, indicated by dashed lines in Fig. 11b,c, are well correlated with the canonical abundance ratio for mag, although the dispersion is high. Fig. 11c shows the abundance ratio / versus . The average ratio at extinctions between 5 and 10 mag is () cm-2mag-1, in agreement with the canonical ratio (Eq. 5). A linear least squares fit to the abundance ratios for mag results in:
shown by the full line in Fig. 11c. We define the depletion factor , which is an average along the line of sight, as the canonical ratio divided by the measured ratio of /:
It rises systematically from about 1 at
to about 3 at 28 mag. In Fig. 11 we also show integrated
intensities and derived
LVG column densities, both multiplied
abundance ratio of 3.65. Note that the spatial resolutions of the
and NIR observations differ slightly.
Nevertheless, the values and slope of
versus seen in
is almost exactly matched by the
data. This is a confirmation of the
abundance of 3.65 of Wilson & Rood (1994). And, it indicates that
an optical depth effect cannot account for the fall-off of
© European Southern Observatory (ESO) 1999
Online publication: December 22, 1998