Astron. Astrophys. 339, 872-879 (1998)

3. Results

3.1. Morphology

3.1.1. FCRAO observations

The 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 observations

Because 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 , barely covering the region occupied by the sources A, B and C. In Fig. 2, the CS(2-1) and CS(3-2) integrated intensity maps are given, overlayed on the radio continuum map of TOHTFG. The signal-to-noise ratio in the CS(5-4) line was generally very low, so we shall not show a map of this line.

Fig. 2. IRAM 30-m maps of the CS integrated intensity (in the range -15 to -11 km s-1) in K km s-1, toward VLA source A of Cep B. The pointlike source B is visible in the NW corner of the map, whereas source C can be seen at position . The grey scale corresponds to the CS(2-1) (top panel , beam) and CS(3-2) emission (bottom panel , beam). The small crosses represent the observed positions. The contour levels refer to the VLA-D continuum emission at 8.4 GHz, from TOHTFG, and their range is the same as in Fig. 1. Offsets are given with respect to the position of source A listed in Table 2

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 ( km s^-1), whereas at more red-shifted velocities the emission is smoothly distributed and it also begins to appear along a NW-SE direction, including the centre of source A.

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 (hereafter offsets in arcseconds with respect to the position of source A will be given) where both CS(2-1) and (3-2) reach their maximum integrated intensity, whereas the spectra on the right side of Fig. 3 were taken at position , where the maximum of CS(5-4) integrated intensity has been observed. The results of Gaussian line fits (, FWHM, ) and the line integrated intensity, , of the spectra taken at position and at the nominal position of source B are given in Table 3.

Fig. 3. IRAM 30-m spectra of CS towards two positions around source A: position (left column ), and (right column ). The intensity scale is in main-beam brightness temperature. We show in the top row CS(2-1), in the middle CS(3-2), and in the bottom CS(5-4). The CS(5-4) spectra have been smoothed to a 0.24 km s-1 velocity resolution.

Table 3. Observed (IRAM 30-m) line parameters toward source A , corresponding to the peak CS(5-4) integrated intensity, and source B, . The integrated intensity has been estimated in the range -15 to -11 km s^-1, and the errors are formal 1  values and do not take into account the calibration uncertainty.

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 , i.e. only about 13 arcsec away from the peak of source B, also shows a dip at the same velocity. These two spectra averaged together give a stronger evidence for the presence of self-absorption. Although more sensitive observations are needed to confirm it, we can exclude that source B is the cause of absorption. In fact, the brightness temperature at 3 mm, extrapolated from the observed flux at 3.6 cm, would only be 0.17 K even under the assumption of free-free emission, or much lower with the observed non-thermal spectral index, thus too small compared to the excitation temperature of CS. Most probably the absorption comes from cooler foreground gas at a slightly blue-shifted velocity with respect to the systemic velocity.

Fig. 4. IRAM 30-m spectra of CS toward VLA source B: position . We show in the top panel CS(2-1), in the middle CS(3-2), and in the bottom CS(5-4). The CS(5-4) spectrum has been smoothed to a 0.49 km s-1 velocity resolution.

3.2. Kinematics

The FWHM of all CS lines varies in the range  km s^-1, which means that at the gas kinetic temperature of  K believed to be representative of the hotspot (MWW), the CS lines are dominated by turbulent or large scale motions.

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 and , i.e. at the direct interface between the molecular cloud and the sharp side of the blister, the CS(2-1) and (3-2) spectra seem to have 2 components (separated by  km s^-1, see Fig. 3), with the strongest component being the blue-shifted one. It thus looks as if the line FWHM is higher along the molecular cloud sharp edge.

Fig. 5. Map of the velocity width, in km s-1, of CS(2-1) (IRAM 30-m data). The contour levels of the VLA map are the same as in Fig. 1.

The line-width also reaches a high value ( km s^-1) at the position of source B. At this position CS(2-1) and (3-2) have essentially the same kinematic parameters, while CS(5-4) shows some deviations: it has a wider FWHM and it is also slightly blue-shifted with respect to the lower excitation lines. The amount of the shift is 0.57 km s^-1, i.e. more than two resolution channels in the CS(5-4) smoothed spectrum, and is even more pronounced in two other CS(5-4) spectra. This result would seem to suggest that the (5-4) transition is tracing a different volume of gas at this location.

There are variations in the spatial distribution of the line 's. The most red-shifted CS(2-1) line was observed towards the centre of source A, with  km s^-1 (i.e., very close to the systemic of Cep B). Other positions with lines having red-shifted central velocities are found in the NW-W region, whereas in the S-SE region spectra are more intense at blue-shifted velocities, with  km s^-1 at position .

We also note that CS(2-1) at has a slightly non-Gaussian profile, with peak at  km s^-1. A two -line Gaussian fit with a fixed line-width of 1.1 km s^-1 leads to a blue-shifted component at  km s^-1, almost coincident with the velocity of the single line at position , and a red-shifted component at  km s^-1, almost coincident with the velocity of the red-shifted component observed in the spectra at positions and , as shown in Fig. 6.

Fig. 6. IRAM 30-m spectra of CS(2-1) towards positions (solid line), (dashed line), (dotted line).

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 0.5 km s^-1. The emission in the NW-W region probably comes from gas which is not directly at the blister skin, and whose velocity is  km s^-1, close to the systemic velocity. The gas observed towards the centre of source A has the most red-shifted velocity and an asymmetric profile suggesting that we are looking at emission which is partly produced at the molecular cloud interface with the blister, on the side receding from us, in agreement with the bright H appearance of this region. More southward, the line of sight intercepts the very interface between the ambient gas and the expanding I-front, causing the gas to be accelerated both towards and away from us and giving rise to the observed sharp edge in the blue-shifted emission and to the largest value of the FWHM.

3.3. LVG model

Information 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, , is about 6-7 K in the SE region but it increases up to 17 K for the CS(2-1) transition in the NW region. The CS optical depth is always .

Fig. 7. The LVG model results (small circles and dashed line), after a Gaussian fit to the position-averaged line profiles (triangles), in the SE (top panel), NW (middle panel), and throughout the whole (bottom panel) interface regions. was fixed at 60 K and the hydrogen density and CS abundance to velocity gradient ratio are listed in Table 4

Table 4. Results of an LVG model applied to the position-averaged spectra of CS, for  K.

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 km s^-1 pc^-1 derived from the LVG model is somewhat lower than that usually found in a variety of molecular clouds (see e.g. Linke & Goldmith 1980) and would require a low CS abundance and/or a high velocity gradient. It can be a further indication that CS is depleted in the region close to the ionization front.

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  cm^-2.

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 is the CS rotational constant, measured in degrees K, and J is the quantum number of the upper level.

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 at blue-shifted velocities, to less than 1 at more red-shifted velocities in the S-SW region, suggesting density variations along the line of sight in this direction, while it is constant in other parts.

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  (km s^-1 pc^{- 1})^-1 at  K), then the line ratio is independent of and is basically an indicator of molecular hydrogen density, with a value suggesting densities cm^-3.

The fact that CS(5-4) is observed at least towards some positions, and has an appreciable fraction ( %) of the intensity of the (2-1) line, is a further indication of the presence of high density gas.

© European Southern Observatory (ESO) 1998

Online publication: October 22, 1998
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