NGC 281-W has been recently studied by Henning et al. (1994) as well as by Megeath & Wilson (1997), who have made detailed (2-1), (1-0), and C34S(3-2) maps of the clump studied by us, using the IRAM 30-m telescope. Our maps in the C34S(3-2) line are very similar to those obtained by Megeath and Wilson in the same transition, although our map is just sufficient to cover the C34S clump. From their measurements, Megeath and Wilson derive a clump mass of at least 210 and an H2 column density of 5 1022 cm-2. Henning et al. (1994) detected a 1.3 mm continuum source coincident with the IRAS source. They concluded that the main contributor to this was a compact component of temperature 45 K and angular size . They estimated the peak H2 column density to be 2 1023 cm-2 or 4 times larger than the value from . This does not agree with our beam averaged estimate based upon 13CO of 2 1022 cm-2, obtained assuming a 13CO abundance relative to H2 of 1.1 10-6 and a temperature of 29 K as done by Megeath & Wilson (1997); note that the latter is consistent with the of 13CO(2-1) (see Table 7). The column density from 13CO agrees instead with the value derived by Megeath and Wilson from . One can also estimate the column density from C34S, assuming spherical symmetry and calculating the H2 volume density with the LVG program mentioned in Sect. 4.5. This gives an H2 density of 5 105-1 106 cm-3, which implies a source averaged H2 column density of (5-9) 1023 cm-2, where we have used a distance of 3.1 kpc for the sake of comparison with the results of Megeath and Wilson. This value compares well with that derived from the 1.3 mm continuum, which suggests that C34S is indeed confined to the densest core associated with the H2O maser and the FIR source seen by IRAS.
Henning et al. (1994) also mapped the source in CO(3-2) and found evidence of an E-W offset between the peaks of blue and red shifted emission (their Fig. 5). Snell et al. (1990) had previously detected the outflow in CO(1-0) using the FCRAO system and found no clear evidence for bipolarity. They derived an age of 2 105 yrs and a swept up mass of 11 for the flow. Our data do not help much to clarify the situation. From Fig. 1a one sees that the line profiles towards NGC 281-W are quite complex: while the CS and 13CO lines peak at the bulk velocity given by C34S, the HCO+ and HCN lines peak at lower velocities than C34S. Moreover, another component is clearly visible in the red wing of the 13CO, HCO+, and CS transitions at km s-1. Finally, broad line wings are evident even in the C34S(3-2) line. Notwithstanding such a complexity of the line profile, the spatial distribution of the gas at different velocities looks quite the same, as the emission arises mostly from a region surrounding the H2O masers. The sole exception is given by the gas associated with the line wings, namely from -42.0 to -36 km s-1 and from -26 to -20 km s-1, which seem to trace a bipolar outflow approximately centred on the H2O maser and extending in the NE-SW direction. This is shown in Fig. 12 for the HCO+(1-0) line. In conclusion, it is quite difficult to decouple the different components which overlap in space and - at least in part - in velocity: high angular resolution images might help to identify the main centres of activity in the region and to describe their interaction with the surrounding environment.
|Figure 6: Maps towards NGC 281-W of the integrated bulk emission in the CH3OH(3-2) (top panel) and C34S(3-2) (bottom) transitions. Thick contours correspond to 50% of the maximum in each map. The HPBW is . The filled triangles indicate the positions of the H2O maser spots, the the observed positions. The values of the contour levels are given in Table 6|
In conclusion, one can speculate that in comparison with the "quiescent'' southern clump, the northern clump is in a more advanced phase of the star formation process, characterised by various signposts of activity such as the H2O masers and a small scale molecular outflow.
|Figure 7: Maps towards IC 1396-N and L1204-G of the integrated bulk emission in the CH3OH(3-2) transitions. Thick contours correspond to 50% of the maximum in each map. The HPBW is . The filled triangles indicate the positions of the H2O maser spots, the the observed positions. The values of the contour levels are given in Table 6|
|Figure 8: Same as Fig. 7 for the C34S(3-2) line towards AFGL5142, S233, S235B, MONR2, NGC 2264-C, and IRAS23151+5912. Thick contours correspond to 50% of the maximum in each map. The HPBW is . The filled triangles indicate the positions of the H2O maser spots, the the observed positions. The values of the contour levels are given in Table 6|
|Figure 9: Spectra of the 13CO(2-1) line towards the peak position in each map. The velocity on the abscissa is relative to the of the corresponding C34S(3-2) line. The intensity is normalised with respect to the peak of each spectrum|
The spectra of Fig. 1c clearly indicate two velocity components, corresponding to the approximate velocity ranges from -17 to -13.5 km s-1 and from -20.5 to -17 km s-1. In Fig. 15 we show the maps of the CS(3-2) line emission integrated in these velocity intervals: two molecular clumps are evident, one at the H2O maser position, the other offset to the south-west. The same pattern is seen in the other transitions, with the sole exception of C34S, which traces only the H2O maser clump, as is evident from Fig. 8: this proves that the latter clump is denser than the other, consistent with the idea that star formation is going on in it.
The C34S(2-1), (3-2), (5-4) and 13CO(2-1) observations around the highly variable H2O maser located between the S235 A and B optical nebulosities, together with near infrared broad band (J, H and K) and narrow band (H2 S(1) 10 and Br) images were discussed in Felli et al. (1997).
A highly obscured stellar cluster is present between the thermal radio components S235 A and B in the near IR observations. The colour-colour analysis shows that the cluster contains many sources with infrared excess, most probably YSOs in an early evolutionary stage.
The driving source of the H2O maser does not appear to be either the YSO inside S235 A or S235 B, but is identified with a faint near infrared member of the cluster, with a large (H-K) colour excess, located near the position of the maser. This identification is further supported by the coincidence of the maser and the near IR source with the centre of the high density and hot compact molecular core observed in C34S and 13CO. The lack of radio continuum emission from the maser/near IR source suggests that the YSO powering the maser and responsible for the near IR emission must be in a very early evolutionary stage, highly obscured even at K band and strongly self-absorbed in the radio continuum, in any case much younger than S235 A and S235 B. S235 A and S235 B lie on the sides of the molecular core, suggesting that star formation in the complex is not coeval but proceeds from the outside towards the core of the molecular cloud.
As far as the CO outflow reported by Nakano & Yoshida (1986), the blue-shifted lobe from -24 km s-1 to -20 km s-1 can be seen also in the 13CO(2-1) data (see Fig. 4 of Felli et al. 1997) and is located between the H2O maser and S235 B. However, the red-shifted lobe observed in 12CO from -13 to -6 km s-1 (which is also weaker in Nakano & Yoshida 1986) was not detected in our data. The red wing of the 12CO profile may be affected by a superposition of different velocity components south of S235 B and has been questioned by Snell et al. (1990) and by Nakano & Yoshida (1986) themselves. Consequently, the earlier suggestion that S235 B was the source of the outflow is questionable and the possibility that the blue outflow comes from the H2O maser must be retained as a plausible alternative.
An extensive molecular and IR study of the source was presented by Schreyer et al. (1997), who mapped a much larger region than in our study. They observed many different transitions, of which only the CS(3-2) and C34S(3-2) lines are in common with us: for these, our results are consistent with those of Schreyer et al. (1997). In particular, the emission peaks at the position of the H2O masers rather than at the IR source IRS1. Moreover, the secondary clump identified by Schreyer et al. to the south-east of IRS1 can be partially seen also in our C34S(3-2) map of Fig. 8, although at the very edge of it. However, the existence of this clump at a velocity of 6.2 km s-1 is clearly demonstrated by the 13CO and HCN line profiles of Fig. 1h. Unfortunately, we cannot confirm the molecular outflows seen by Schreyer et al. because for this source we do not have maps in CS and HCO+, and 13CO and HCN are not suitable for investigating the line wings. In fact, 13CO arises from a much larger region than that mapped by us, while study of HCN emission is complicated by the hyperfine structure of the line.
Our Pico Veleta observations as well as Plateau de Bure interferometer (PdBI) maps of the molecular gas and near infrared images of the same region, were presented in Cesaroni et al. (1997a).
All lines peak at km s-1 and the lower density tracers like 13CO or HCO+ present broad wings extending up to km s-1 from the peak. Even the higher density tracer C34S, despite the much poorer S/N, shows strong wings, although only up to km s-1 from the peak. On this basis, five velocity ranges were defined:
The bulk emission is best studied through the high density tracers such as the CH3OH(3-2), CH3CN(8-7), and C34S(3-2) transitions. In all cases the emission originates from the same region with size , centred on the H2O maser spots: this is a clear indication of the existence of a molecular clump in which the IRAS source and the H2O masers are embedded.
In order to map the line wings, one has to use the lines with the best S/N, such as CS(3-2) and HCO+(1-0) (see Fig. 4 of Cesaroni et al. 1997a). The outer wings seem to trace a NW-SE velocity shift, with the blue- and red-shifted emission coming respectively from NW and SE. The surprising result is that the blue- and red-shifted emission in the inner wings shows an orientation completely reversed with respect to the NW-SE axis above: although the direction is the same, the velocity increases from SE to NW.
Including the interferometric observations made with the PdBI, the main results are the following:
|Figure 10: Plots of the line brightness temperature ratios (3-2)/(2-1) and (5-4)/(3-2) for the C34S lines of the sources in our sample (bottom panel) and in the Cesaroni et al. (1992) study (top panel). The filled points represent the ratios computed with an LVG model for a temperature of 50 K (circles) and 100 K (triangles) and H2 density of 105, 3 105, 106, 3 106, and 107 cm-3, going from the bottom to the top of the figure|
|Figure 11: Boltzmann plots from the CH3OH(3-2) and (5-4) lines. The column densities are source averaged assuming the deconvolved angular diameters given in Table 11 for CH3OH. The straight lines represent least square fits to the data. The derived rotation temperatures and column densities are given in Table 12|
|Figure 12: Map of the HCO+(1-0) line emission in NGC 281-W integrated from -42 to -36 km s-1 (full contours) and from -26 to -20 km s-1 (dashed). The contour levels range from 0.7 to 1.9 by 0.3 K km s-1. The triangles indicate the position of the H2O maser spots|
|Figure 13: Map of the CS(3-2) line emission in AFGL5142 integrated from -7 to -3 km s-1 (full contours) and from -3 to 1 km s-1 (dashed). The contour levels range from 7 to 32 by 5 K km s-1. The triangles indicate the position of the H2O maser spots|
|Figure 14: Map of the HCO+(1-0) line emission in AFGL5142 integrated from -19.5 to -7 km s-1 (full contours) and from 1 to 15 km s-1 (dashed). The contour levels range from 2 to 5.6 by 0.6 K km s-1. The triangles indicate the position of the H2O maser spots|
|Figure 15: Map of the CS(3-2) line emission in S233 integrated from -20.5 to -17 km s-1 (full contours) and from -17 to -13.5 km s-1 (dashed). The contour levels range from 5 to 25 by 5 K km s-1. The triangles indicate the position of the H2O maser spots|
|Figure 16: Map of the CS(3-2) line emission in IC 1396-N integrated from -7.25 to -1.25 km s-1 (full contours) and from 2.75 to 8.75 km s-1 (dashed). The contour levels range from 3 to 13 by 2 K km s-1. The triangles indicate the position of the H2O maser spots|
|Figure 17: Map of the HCO+(1-0) line emission in IRAS23151 +5912 integrated from -60 to -55 km s-1 (full contours) and from -55 to 48 km s-1 (dashed). The contour levels range from 0.8 to 4.3 by 0.5 K km s-1. The triangles indicate the position of the H2O maser spots|
|Figure 18: Top panel: full width at zero intensity of the HCN(1-0) (filled points) and HCO+(1-0) (empty) lines as a function of the FWZI of the 12CO(1-0) line (from Table 2). Middle panel: same as above as a function of the luminosity of the IRAS source. Bottom panel: FWZI of the HCN(1-0) line versus the FWZI of the HCO+(1-0) line|
|Figure 19: Ratio of the integrated intensities under the HCO+ (1-0) and HCN(1-0) lines as a function of the ratio between the 60 and 12 m IRAS fluxes (top panel), the ratio between the CH3CN(8-7) K=0+1 integrated intensity and the 100 m IRAS flux (middle), and the luminosity of the IRAS source (bottom)|
In summary, IRAS20126+4104 represents a rare beautiful example of a disk-outflow system originating from a young early type massive (proto)star still in an evolutionary phase prior to the development of an UC HII region.
The IC 1396-N (or IC 1396E) globule has been studied in detail by Serabyn et al. (1993) using molecular line data taken with the Effelsberg 100-m, the IRAM 30-m, and the CSO 10.4-m telescopes. In particular, they obtained maps in CS(3-2) which well compare with ours, although the region mapped by them is larger. They determined the mass of the globule to be 480 within an outer radius of 0.9 pc and with a density distribution varying as r-1.75. From the ammonia measurements, they determined the temperature to be 20-23 K. With this model, they predicted an intensity of 1.6-1.9 K for C34S(3-2) with the 30-m or 35% larger than our observed value of 1.3 K. They find the wing emission (-7.25 to -1.25 km s-1 and 2.75 to 8.75 km s-1) to be localised around the water maser source whereas the bulk emission (i.e. from -1.25 to 2.75 km s-1) is more extended. Indeed our results are quite consistent with this view. However, in their Fig. 5 Serabyn et al. present the CS(3-2) line emission integrated under both line wings: if one instead compares the emission in the blue wing with that in the red, an offset in the E-W direction between the two is found. This is shown in Fig. 16, where the same velocity intervals as in Fig. 5 of Serabyn et al. (1993) have been used. Note that such an offset and orientation is also found in an interferometric map of the HCO+(1-0) emission (Testi, private communication). One possibility is that the structure outlined in Fig. 16 is a bipolar flow, although it should be noted that only the blue lobe is peaked at the centre position, namely where the bulk CS(3-2) emission peaks. Whatever the interpretation, the structure of the molecular cloud in this source is not as simple as assumed by Serabyn et al. (1993). Although to a first approximation one can make the hypothesis of a single homogeneous clump, in practice the line profiles shown in Fig. 1j demonstrate that sub-structures must exist, and temperature and density gradients must play an important role - as witnessed e.g. by the self-absorption features in the HCO+ line.
In this source the H2O masers are slightly offset from the peak of the C34S emission (see Fig. 8), although likely embedded in the C34S clump. By inspection of Fig. 1l, one sees that two velocity components are detected in the HCO+(1-0) and, perhaps, HCN(1-0) lines. Indeed, the region mapped in these transitions looks more extended than that seen in CS or 13CO (see Fig. 3g). The complexity of the molecular cloud is better evidenced by separating the contribution of the two velocity components in the HCO+ line. This is done in Fig. 17 where the distribution of the HCO+ line emission integrated from -60 to -55 km s-1 is compared with that from -55 to -48 km s-1. Figure 17 seems to outline two clumps, neither of which can be safely associated with the H2O maser spots. Under this respect IRAS23151+5912 differs from all the other sources. Clearly, the situation is more complex in this case: a tentative explanation may be that the H2O masers are located in a high density region at the interface between the two clumps. Any further speculation will require higher angular resolution data.
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