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Astron. Astrophys. 339, 872-879 (1998) 3. Results3.1. Morphology3.1.1. FCRAO observationsThe spatial distribution of the CS(2-1) emission shown in Fig. 1 resembles very closely the CO(3-2) map of MWW around the position of the hotspot . Our overlay clearly shows that source A has formed between the edge of the elephant trunk and another minor clump located NW of the HII region, and coincident with the RIDGE and source B. One can also note that the peak of the integrated intensity is not at the interface with the ionization front, but is located more SE and actually the elephant trunk has two distinct peaks. As in the CO(3-2) map of MWW, the figure clearly shows the steep drop-off in emission at the molecular cloud edge. The bulk of the CS(2-1) emission is at velocities of -11.5 to -14.5 km s-1, slightly blue-shifted relative to the CO(1-0) emission (Sargent 1977), and over a smaller velocity range than that observed by MWW in CO(3-2), who detected emission from -10 to -16 km s-1. An interesting feature is that most of the emission from the NW clump is concentrated at velocities -13 to -14.5 km s-1. The less clearly defined edge of the clump at these velocities may also be a further indication of disruption of the cloud edge, as suggested by MWW. 3.1.2. IRAM 30-m observationsBecause of the weakness of the lines, CS was mapped by sampling at
HPBW intervals (at 3 mm), for a total of 12 points in a map
Fig. 2 clearly shows a noticeable anticorrelation between the ionized gas of the blister and the molecular gas in the hotspot . There is a sharp increase in integrated intensity (and, as we shall see, also in column density) to the south of the sharpest side of source A, proving that the blister HII region has cleared of molecular gas the ionized region and that the sharp side of the blister faces a dense wall of molecular material in the hotspot , just at the edge of the elephant trunk. Both CS(2-1) and CS(3-2) channel maps show that the strongest
emission comes from the S-SE region, where the sharp edge of the
molecular cloud is very clearly defined. The sharpness of the emission
is particularly evident for blue-shifted velocities
( The spectra of the CS(2-1), (3-2) and (5-4) lines towards two
different positions are presented in Fig. 3 to show the relative
strength of the emission. The spectra on the left side were taken at
position
Table 3. Observed (IRAM 30-m) line parameters toward source A In Fig. 4 we also present spectra of the CS lines observed
towards VLA source B. The most interesting feature is the dip in the
CS(2-1) line, at a velocity of -13.6 km s-1,
which could be the result of some moderate self-absorption. A spectrum
taken at position
3.2. KinematicsThe FWHM of all CS lines varies in the range
It is of interest to consider the kinematic relation between the
ionization front of source A and the molecular cloud. This can be done
by overlaying a map of the CS line-width with one of the continuum
emission, as in Fig. 5. The FWHM of CS(2-1) has a minimum at the
centre position and increases S and SE of the HII
region. At positions
The line-width also reaches a high value
( There are variations in the spatial distribution of the line
We also note that CS(2-1) at
We therefore suggest that the expansion of the HII
region and its associated shocks have accelerated the molecular gas
along the line of sight both toward and away from us by as much as
3.3. LVG modelInformation on local density is provided by the relative intensity of the three CS rotational transitions. From the observed line temperatures we can derive average densities along the line of sight using a "Large Velocity Gradient" (or LVG) model, assuming uniform density and temperature. It is important to note that because of the limited number of observed points in the CS maps obtained at the 30 m telescope, and also because of the fact that the CS(5-4) transition is both undersampled and very weak, we considered inappropriate to attempt any spatial resampling of the CS(5-4) (and also 3-2) map to the same angular resolution as in the (2-1) map. On the other hand, we did not have an a priori model of the source. All things considered, we then decided to proceed in the following way: we selected spectra in the SE and NW regions, and made a Gaussian fit to their average to compare the resulting peak line temperature with the output of the LVG models. We also selected those positions throughout the whole CS(5-4) map with the best RMS noise, and averaged all CS transitions at these same positions. In the LVG models we assumed a temperature of 60 K, a unity
filling factor for the position-averaged spectra, and adopted the
CS-H2 collision rates of Green & Chapman 1978 (M.
Walmsley, private communication). In this way we avoid the
introduction of any arbitrary hypothesis on the source structure, as
it would be implied by adjusting the filling factor. The results for
the averaged spectra are shown in Fig. 7, and are summarized in
Table 4. The excitation temperature,
Table 4. Results of an LVG model applied to the position-averaged spectra of CS, for The LVG results seem to suggest the presence of a density gradient between the SE and NW regions of the 30 m maps, although we caution the reader about possible residual beam-coupling effects in the three wavelength bands observed. These effects should be less important when the average characteristics of the interface region are considered, as in the case of Fig. 7, at the bottom. The CS abundance to velocity gradient ratio of
Assuming uniform excitation conditions across the line profile, and uniform hydrogen density in the region of emission, then the CS column densities can be written as: where a Gaussian line profile has been taken (Linke & Goldmith
1980). The column density is in units of cm-2, the
fractional abundance per unit velocity gradient is in units of
(km s-1 pc-1)-1, the
molecular hydrogen density is in cm-3, and the line width
in km s-1. Using the values of Table 4 we derive
CS column densities of a few times
These results are in good agreement with the approximated estimates that can be obtained assuming that the optical depth of the CS transitions is only moderately thick and that the beam filling factor is unity, and then using the following formula: where Fig. 2 shows that much less molecular gas is present along the line of sight, when we scan the region progressively from the SE to the NW, with a strong anticorrelation between radio continuum/optical emission and molecular emission. If, for the sake of simplicity, we also assume that the excitation temperature of the gas is uniform throughout the observed region, then we see that Fig. 2 is also a representation of the spatial distribution of column density. In the framework of the LVG approximation each velocity within the
line profile can be interpreted as a phisically independent region of
the cloud. Then, variation of density along the line of sight can show
up as variations in the intensity ratio of two lines. The CS(3-2) to
CS(2-1) intensity ratio monotonically decreases from
The constancy of the CS line intensity ratio towards the centre and
in the NW-W region, together with low optical depths, would require
that these CS emitting regions have nearly uniform density along the
line of sight. Furthermore, if the optical depths are less than one
(e.g., with a CS abundance to velocity gradient ratio
The fact that CS(5-4) is observed at least towards some positions,
and has an appreciable fraction ( ![]() ![]() ![]() ![]() © European Southern Observatory (ESO) 1998 Online publication: October 22, 1998 ![]() |