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Astron. Astrophys. 342, 257-270 (1999)

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4. [FORMULA] and [FORMULA] in the mapped region

These estimates of extinctions should be compared with the directly measured visual extinctions [FORMULA] over the mapped region (Paper I, Lada et al. 1998) which vary between 3 and 28 magnitudes (Fig. 9). In Fig. 11, we plot [FORMULA] LVG column density against visual extinction. In the range [FORMULA][FORMULA] mag, [FORMULA](1[FORMULA]0) line intensities and [FORMULA] column densities rise with growing extinctions, while column densities are roughly independent of extinction for [FORMULA][FORMULA] 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 [FORMULA][FORMULA] mag by a factor of about 4, it is comparable to the 7% calibration error for larger extinctions.

[FIGURE] Fig. 10. The positions of 94 background stars from LLCB94 showing extinctions [FORMULA] of more than 10 mag in the IC 5146 Northern Streamer region overlayed on a map of the distribution of mean visual extinction, [FORMULA], observed by LLCB94. Contours are 5 to 30 by 3 mag. We conducted pointed observations of [FORMULA](1[FORMULA]0) and (2[FORMULA]1) towards each of these stars to compare molecular column densities and extinctions [FORMULA]. In addition, the positions of all 5 candidate protostars taken from the compilation of Dobashi et al. 1993 are marked by dots. The rectangle marks the mapped region.

[FIGURE] Fig. 11a-c. Dependence of the [FORMULA] LVG column density [FORMULA] upon the visual extinction [FORMULA] determined from the NIR data. Each point (o) of the plot corresponds to one position of the [FORMULA] grid of the mapped data smoothed to [FORMULA] resolution - the resolution of the smoothed NIR observations. Panel a shows integrated [FORMULA](1[FORMULA]0) intensities. The observational error is [FORMULA]. The full line corresponds to the I-[FORMULA] relation (Eq. 6) derived by Alves et al. (1998) from the LLCB94 data. Panel b shows [FORMULA] and panel c the abundance ratio [FORMULA]/[FORMULA]. The dashed lines correspond to the canonical abundance ratio (Eq. 5). The full line c is the result of a linear least squares fit to the abundances at all points with [FORMULA] [FORMULA] mag. (Eq. 7). The thick circles and errorbars correspond to the mean and rms of the abundances after binning into intervals of 5 magnitudes. The full points ([FORMULA]) correspond to [FORMULA] integrated intensities and [FORMULA] column densities multiplied by the abundance ratio [FORMULA].

LLCB94 also derived a linear dependence of the integrated [FORMULA](1[FORMULA]0) intensity upon [FORMULA] for visual extinctions of less than 15 magnitudes, at a common angular resolution of [FORMULA], for the extended emission of the Northern Streamer. They also found a "flattening" of [FORMULA] line intensities at extinctions above [FORMULA] 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 [FORMULA] are larger than observational and are an increasing function of [FORMULA]. This results in the following relation for the Northern Streamer extended cloud:

[EQUATION]

Fig. 11a shows that this relation fits reasonably well the high resolution [FORMULA] 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 [FORMULA][FORMULA] mag, although the dispersion is high. Fig. 11c shows the abundance ratio [FORMULA]/[FORMULA] versus [FORMULA]. The average ratio at extinctions between 5 and 10 mag is ([FORMULA]) [FORMULA]cm-2mag-1, in agreement with the canonical ratio (Eq. 5). A linear least squares fit to the abundance ratios for [FORMULA][FORMULA] mag results in:

[EQUATION]

shown by the full line in Fig. 11c. We define the depletion factor [FORMULA], which is an average along the line of sight, as the canonical ratio divided by the measured ratio of [FORMULA]/[FORMULA]:

[EQUATION]

It rises systematically from about 1 at [FORMULA][FORMULA] mag to about 3 at 28 mag. In Fig. 11 we also show integrated [FORMULA] intensities and derived [FORMULA] LVG column densities, both multiplied by the [[FORMULA]]/[[FORMULA]] abundance ratio of 3.65. Note that the spatial resolutions of the [FORMULA] and NIR observations differ slightly. Nevertheless, the values and slope of [FORMULA]/[FORMULA] versus [FORMULA] seen in [FORMULA] is almost exactly matched by the [FORMULA] 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 [FORMULA]/[FORMULA] at high [FORMULA].
Instead, we interpret this as an indication of CO depletion on dust grains within the dense core regions (Sect. 6).

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© European Southern Observatory (ESO) 1999

Online publication: December 22, 1998
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