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Astron. Astrophys. 318, 931-946 (1997)

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6. Conclusions

From our 4.9, 14.9 and 22.5 GHz and H66 [FORMULA] (22.64 GHz) line images of W3, we conclude:

(1) In the core region, there is a marked anticorrelation between dense molecular and ionized gas.

(2) The compact H II regions have cleared away their parent molecular cloud. They are probably a few times [FORMULA] years old:
- The shell-like compact H II region W3 A shows edge-brightening with low visual extinction and marked dust depletion. Our images show that the stellar winds of the embedded stars are photoevaporating material off remaining dense molecular clumps, forming swirls and trunks near the interacting surfaces. In the SE this gas flows unimpeded into regions of lower density. From radial velocities we find that the shell of W3 A is expanding. W3 A is probably one of the oldest H II regions in the W3 core.

- W3 D is associated with IRS 10. It is a shell-like H II region ionization bound toward W3 C, with dense molecular material to the S and SE. The velocity gradient found in C18 O is also seen in the ionized gas. It indicates that W3 D is rapidly expanding toward us in the NW. The direction of the velocity gradient in the SE and S is the reverse of that found on larger scale. This is most likely caused by a "champagne flow" of ionized gas, from W3 C.

- W3 H is to the north of the core region. The C18 O and continuum data show a lack of molecular gas. W3 J and K to the south of the core are the oldest regions of W3 main. These regions have presumably dispersed their parent clouds. The exciting stars are of spectral type O9 to O6 from continuum flux densities. O5 to B3 stars have been observed in the optical.

(3) The ultracompact H II regions appear to be partially embedded in their parent clouds. Their age is estimated to be a few times [FORMULA] years.

- The ultracompact H II region W3 C has a teardrop-like shape with edge-brightening to the SW and NE. There is a double peaked H66 [FORMULA] line and a gradient in the radial velocity which may indicate a bipolar flow within W3 C and an outflow of the ionized gas toward the north.

- The shell source W3 B is located between two dense molecular condensations. W3 B is an emerging blister, edge-brightened to the NE and SW, with a expanding limb to the NW and a compressed ridge to the S. In the south, the cm continuum emission is double peaked. This may be an indication that there is a bipolar outflow associated with IRS 3a. We find [FORMULA] [FORMULA] K from our H66 [FORMULA] line data.

- The cometary ultracompact H II region W3 F is coincident with IRS 7. This source is ionized by a B0 to O9.5 ZAMS star. From the gradient in the radial velocity of the H66 [FORMULA] line we find that W3 F may be caused by a moving star bow shock to the NE with a 5 km s-1 motion relative to the ambient gas. However, the proximity of the dense molecular clump surrounding IRS 5 in the NE and the strong bipolar outflow of this source blowing toward W3 F may also be causing the edge-brightening of this region. To the SW a champagne-flow cannot be ruled out since we find [FORMULA] [FORMULA] K and high electron density, thus the thermal pressure of the H II region is high.

- W3 G is to the center in the core region, embedded in moderately dense molecular gas. No candidate for an exciting star has been associated with this H II region: extinction toward this region must be high.

(4) The hypercompact radio continuum regions W3 Ma-g and W3 Ca with radii of 1000 AU and less are detected toward dense molecular gas and strong sub-mm emission sources. They are estimated to be a few times [FORMULA] years old. From the spectral indices one cannot conclusively associate each of the radio continuum sources with a ZAMS type B star. Multi-epoch, multi-frequency observations are necessary to determine if these sources form yet another "Radio Zoo" as observed toward the Orion nebula near the Trapezium cluster.

(5) Bipolar limb-brightening detected at high resolution from the cm radio continuum toward W3 B and W3 C, and the S-shaped alignment of the hypercompact sources toward W3 M may be indicative for a direct interaction of the bipolar outflows with the ambient neutral material. This should be accounted for in models of expanding H II regions.

(6) If the W3 core is the result of a supercritical collapse caused by the expansion of W4, initial star formation may have been triggered as part of a postschock layer. From the spatial correlation and relative numbers of older and younger H II regions, the most recent star formation, however, has most probably been triggered by the expansion of the H II regions in W3 and the subsequent collapse of molecular clumps and not from the initial shock. The images show that the most important influences on the dynamics of the H II regions are the clumpy densities and kinetic temperatures of the ambient molecular clouds.

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© European Southern Observatory (ESO) 1997

Online publication: July 3, 1998